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Clearances and oil pressure


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Still waiting on my engine's final autopsy and getting different stories/theories every week

In short I rapidly dropped oil pressure on a fresh 406C stroker resulting in engine noise and a $400 towing bill

Decided to send it to my machinist a couple of long weeks ago and his initial diagnosis after removing a couple of bearing caps was the thrust bearing was wiped out due to the crankshaft being pushed forwards

This led to "a" conclusion it must be my transmission having excessive charge pressure or a misalignment issue which is certainly a possibility but the fact there were no witness marks on the flexplate or convertor I was not convinced

Having excessive pressure in the trans would also cause excessively harsh shifts which was certainly not the case at all

Regardless I took the trans to a local shop and he can't see any evidence of this so far

 

Get another call from my machinist to say they have removed the crank/pistons etc and after measuring the wear on the thrust, end play and the fact that all the bearings have prematurely failed/worn he no longer blames the trans but thinks the clearances might have been too tight

I told him my oil pressure was consistently 80-85 psi cold, 75-80 hot revving with approx 50 psi hot idle which to me is not excessive for a new engine

I don't remember my exact clearances but checked them with plasti-gauge (I know not perfect!) and they were certainly within spec at about 0.0025-0.0030 mains and 0.0020-0.0025 rods

The crankshaft turned freely even with pistons installed and although plasti-gauge is not ideal even at the tighter side of the above clearances I am far from convinced it was the clearances causing this failure

The oil was always clean even when I stopped the vehicle and wasn't until I got home the dirty oil finally mixed in

Machinist is yet to check oil pump, galleries and cam bearings so could get another story next week

 

My "theory" is something caused a drop in oil pressure such as a blocked oil gallery or an internal leak such as a slipped cam bearing or oil pump issue which led to the bearing failure and major scuffing in no 4 cylinder with minor scuffing in the rest

 

Would be interesting to hear from members what oil pressures they have had on a fresh engine especially with new crank, rods etc

Oh and you're welcome to add to the theory list, I could use a laugh right now

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The numbers you stated are in the general ballpark. What was the failure mode causing you to be towed? What do the bearings look like? What were the ring gaps set at? Picture are usually helpful. Chuck

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What does the cam and lifters look like? Did you use zinc additive in your oil?

When I worked in race shop you opened up the clearances for couple reasons. The flow of oil cooled the bearings for car that run on track for 500 mile races. The other was pure friction reduction. What we saw with too much oil pressure was that the bearings eroded looked just like when water washes out a section of soil. Most people are not looking at engines that are ran at max for 500 miles and do not see this. A 1/4 mile engine will probably never make it 500 miles. We ran pumps with higher volume not necessarily high pressure. 

We never ground the cranks to the factory numbers. We would get the new bearings and install them and mike the inside diameter of the bearing. The boss would then tell the crank grinder what diameter to grind each journal. They would vary sometime .001" from the spec. number but usually a few tenths of a thousand of an inch. 

I would double check the clearance given for the pistons and check with the mfg. for the proper clearance. Some forged pistons need extra clearance due to expansion being more than a cast piston. 

Did you grind your ring end gaps or just put the rings on and slap the pistons in? Rings might have been butting ends and caused scuffing of the walls.

I think one of the members had an issue with the journals on his new crankshaft not being aligned with the bores of the cylinders. Do you have pics of the bottom of the engine when you assembled the first time? When he bolted the rods to the crank the small end of the rod was not centered with piston. 

Also check the clearance on the lifters. Being a wet lifter galley if there is excess clearance there all the oil leaks out around the lifters starving the bearings. 

The position of the front cam bearing is also important. 

Did your main bearings have oil grove all the way around or 1/2 way. My old boss preferred the 1/2 way around. 

For sure you do not want to go back together with the engine and do the same way as last time. Will probably get the same results. 

I am suspect of end gap on rings being cylinder issue then the resulting crap gets into the oil and causes other issues. Or piston clearance issue.

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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The numbers you stated are in the general ballpark. What was the failure mode causing you to be towed? What do the bearings look like? What were the ring gaps set at? Picture are usually helpful. Chuck

G'day Chuck

 

Sorry no pictures yet as block etc at the machine shop 50 miles away

 

Story in short I was driving along the highway 50-60 MPH all going well and constantly checking the gauges

Heard a noise like loose tappets then looked at the gauges, oil pressure 40 and slowly dropping

Had to drive a couple of miles, very slowly, to find a parking bay and shut down, checked for leaks, none, checked oil, clean as a whistle and full, so clean I had trouble seeing the oil on the dipstick. Checked it several times, same.

Got vehicle home and perhaps from getting on/off the tilt tray the oil had mixed so it was now a dark color on the dipstick

 

What I'm having trouble comprehending is that if the bearing clearances were too tight there should have been some wear from the beginning and there would have been some discoloration in the oil much earlier on especially after the dyno testing

Ring gaps were set at where they should be but don't quite remember exact measurement, I did file a little too much off a couple but that wouldn't cause any problems

In my opinion it is more likely an oil starvation issue considering all bearings including thrust surfaces are worn, number 4 cylinder badly scuffed perhaps as one of the last to receive oil

 

Machinist is yet to examine the oil pump, galleries and camshaft so will have to lose some more sleep waiting

Luckily the crank can be saved and the block will only need one sleeve at this stage

Hopefully I have more news in a week or so

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Hi David

 

Cam/lifters looked fine but yet to remove cam, machinist will do this

Pistons were clearanced by the machine shop and he is certainly aware of the need for extra clearance for forged pistons

Ring gaps were filed to spec although I did file a little too much off a couple of compression rings but this wouldn't cause any issue

Machinist reckons piston/cylinder wear due to oil starvation

Pistons perfectly centred, crank end float spot on and rod side clearances spot on

Not sure on mains, ACL Race series I think 1/2 grooved?

Machinist reckons cam all good but hasn't reported on cam bearings yet, lifter bores look good but will double check this and front cam bearing

 

I have assembled half a dozen Ford V8s always checking bearing clearances with plasti-gauge and never had an issue

Plasti-gauge while not very accurate will still show a pattern and from memory there was nothing out of the ordinary

Unfortunately with age my memory not so good and don't remember exact clearances but would expect with all new components they would have been slightly on the tight side. Everything rotated freely 

I have my doubts that a slightly tighter clearance would have caused a sudden failure but I could be wrong

Perhaps it's the chicken or the egg question - did the oil pressure drop first causing the damage or the other way around?

Thanks for your input, will post more info if/when it comes to light and hopefully get a better night's sleep!

 

 

 

 

Carolina_Mountain_MustangsWhat does the cam and lifters look like? Did you use zinc additive in your oil?

When I worked in race shop you opened up the clearances for couple reasons. The flow of oil cooled the bearings for car that run on track for 500 mile races. The other was pure friction reduction. What we saw with too much oil pressure was that the bearings eroded looked just like when water washes out a section of soil. Most people are not looking at engines that are ran at max for 500 miles and do not see this. A 1/4 mile engine will probably never make it 500 miles. We ran pumps with higher volume not necessarily high pressure. 

We never ground the cranks to the factory numbers. We would get the new bearings and install them and mike the inside diameter of the bearing. The boss would then tell the crank grinder what diameter to grind each journal. They would vary sometime .001" from the spec. number but usually a few tenths of a thousand of an inch. 

I would double check the clearance given for the pistons and check with the mfg. for the proper clearance. Some forged pistons need extra clearance due to expansion being more than a cast piston. 

Did you grind your ring end gaps or just put the rings on and slap the pistons in? Rings might have been butting ends and caused scuffing of the walls.

I think one of the members had an issue with the journals on his new crankshaft not being aligned with the bores of the cylinders. Do you have pics of the bottom of the engine when you assembled the first time? When he bolted the rods to the crank the small end of the rod was not centered with piston. 

Also check the clearance on the lifters. Being a wet lifter galley if there is excess clearance there all the oil leaks out around the lifters starving the bearings. 

The position of the front cam bearing is also important. 

Did your main bearings have oil grove all the way around or 1/2 way. My old boss preferred the 1/2 way around. 

For sure you do not want to go back together with the engine and do the same way as last time. Will probably get the same results. 

I am suspect of end gap on rings being cylinder issue then the resulting crap gets into the oil and causes other issues. Or piston clearance issue.

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Oz, This sort of failure is puzzling. I wish I had some helpful insight to offer. The only thing I can suggest is start checking every tolerance, especially the big end of the rods. I say that because I lost a new engine on the dyno a couple of years ago. It showed good oil pressure and coolant temp, it just wasn't making the power it should have been. We shut it down and disassembled. Bearings damaged and crank throws scarred. It turned out that all 8 (new) rods had gone out of round on the big end effectively removing clearances. I hope you get it sorted quickly. Keep us updated. Chuck

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Were any oiling system modifications made? Were you using a high volume oil pump?

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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Still nothing conclusive and machinist still holding to the clearances theory and maybe he's right?

Chuck - you're right it is a difficult one and even the machinist is not overly confident but will be checking everything

 

Both mains including thrusts and the rod bearings failed which to me is oil starvation and perhaps if the clearances were a little tight that would certainly have contributed to the bearings early demise

 

One thing that gives a hint of oil starvation is the oil filter when removed had very little oil in it

 

TommyK - No oil mods, not necessary for a mild build and standard volume Mellings pump 85 psi cold, 75 psi at revs and 45 psi hot idle

 

Jay - Oil pickup to pan clearance was spot on - can't remember but maybe 3/8 inch and machinist did also check this

 

Anyway I have 2 choices to fix this

One was to sleeve no 4 cylinder, new piston, grind crank, new bearings, rings etc

Second option was to put all new pistons boring it to .030 which would not need a sleeve but this option would cost about $500 more and I would rather keep my block at .020 o/s to leave room for future rebuilds so will go with the first option

Only issue is will have to wait a couple of weeks for a piston from the USA

I only wish my memory was good enough to remember the exact clearances I came up with, next time I will record everything!

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Did you or the machinist measure the volume of oil that remained in the oil pan?

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Did you or the machinist measure the volume of oil that remained in the oil pan?

 

Don - I drained the oil before removing the engine, dipstick was on the full mark and about 6 litres came out with a little remaining in the pan and filter

Oil was contaminated no doubt mostly due to number 4 cylinder and there was a small amount of oil in this cylinder

 

I suppose if the clearances were too tight it would cause the bearings to overheat and since this was the first drive over 10 miles distance they finally gave up with the loss in oil pressure scouring the cylinders

 

Unfortunately there is no way to confirm what exact clearances were on the bearings as they are all shot

This is a mistake on my behalf and although I've had success in the past with plasti-gauge I won't be using it again except perhaps to check how accurate it is once I get my short block back

 

Hopefully will have it back on the road in a month or so and it will be a nerve wracking experience driving it for a while without a 100% conclusive diagnosis

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I believe you are getting the failure thoughts in reverse, think about this.

The cylinders start to seize which causes excessive pressure and force to be applied to the bearings which causes the crank to beat them out. Withe the bearings beating out oil pressure starts to fall. Rod bearing should show the most wear. As the pistons start to stick in the cylinders it pounds the bearings. Mains not as much just rotation but would also see much more force. You should have noticed the extra drag and would have to give it more gas to overcome the friction. 

If you had ran until a piston actually seized that is when you break a rod and boom there goes the side of the block usually. 

Some older engines had a hole drilled in the upper side of the big end of the rod to squirt oil up on the piston skirt and wrist pin to help oil the upper side but a Cleveland stock rod does not have that. Does the aftermarket rods have hole or not? If they do were they turned the right way?

You have to figure out what happened or same thing will happen if you go back together the same way.

If you could turn the engine by hand the bearings were not too tight and would not over heat without long period of high rpm. 

I want to build mine this winter if I can get the Yates heads but might use the 70 cc if I cannot. Would like to know what the failure was on your engine for sure.

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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I agree with David. I dont think the root cause was your bearings. Although Plastigage is not that accurate i dont think it will be off by 5 thousands to create a tight bearing issue. Based on your Platigage measurements even if off it would have worked with slightly higher pressure but not to the point of destruction.

 

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1971 M-Code Mach 1 w/Ram Air, 408 stroker, 285/291 0.558" roller cam, Blue Thunder intake, TKO600, Hooker headers with electric cut-offs, FiTech EFI w/ RobBMC PowerSurge pump

Strange center section with Truetrac, 3.5 gear and 31 spline axles, 4-wheel disc brakes

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I also agree it may not be the bearing clearances that caused this but this is all the machinist can come up with so far

He reckons number 4 cylinder/piston (wasn't seized just badly scuffed all the way around) was due to the lack of oil pressure

Cam bearings not bad but will be replaced

I will get all the old parts back and post some pics in a few weeks

We've pretty much eliminated 95% of all possible causes so far but will be persistent in getting an answer

I can only hope he finds something conclusive and I don't have to pre-book the tow truck next time!

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Going back to the oil pick up, was the tube checked for cracks and I'm assuming it was still firmly attached to the oil pump?

 

Still trying to come up with a reason for oil starvation.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I'm hoping the machinist has already checked this

I could not see any cracks and pick up was tight with correct alignment/clearance

I'm certainly no expert but would expect if clearances were the sole cause it would have been a more gradual progression with some contamination in the oil especially after the dyno session but this was a sudden drop in oil pressure

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  • 3 weeks later...

Did I miss something here? Did the person who ASSEMBLED this engine check the thrust clearance?  I think the original posting says that the thrust was wiped out, and has anyone asked any questions about the thrust clearance? I do this ( engine assembly ), for a living, and on my worksheets, EVERY engine gets the thrust bearing clearance checked prior to the crank and rods being assembled into the block. Off the top of my head, I think the window is something like .004"-.008" thrust clearance, again just off the top of my head, but whatever it is, WAS THIS CHECKED? It is totally not an un-common occurance, especially when using an aftermarket crank, for this measurement to be too tight. the thrust bearing sides ( #3 bearing )is lightly flat plate sanded to bring the clearance into the window, and, it is done on the flange side facing away from the flywheel, leaving the most bearing material where all the thrust pressure will be, on the flywheel side. This is not as critical on an automatic car, as it is on a stick shift, which puts a heavy loading on the thrust bearing flange. Another thing to be aware of is, checking the crank's endplay by just laying it in the block's main saddle inserts isn't accurate, you MUST install the main thrust bearing cap just finger tight, set the lightly lubed bearings by rapping ( I use a dead blow mallet and aluminum bar ) the crank to the rear, then forward, and torqueing the thrust bearing cap, and dial indicate the crank snout for endplay.

You may have had some other issue as well, but to wipe out a new thrust...….bets are on improper thrust bearing clearances.

Shown is a 427 SOHC Ford at .005" thrust check, and a 250 Ferrarri at .004" thrust check.IMG-3343.jpg

 

IMG-3344.jpg

 

IMG-3412.jpg

 

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How is the finish on the crank journals? Friend of mine had 50 miles on a rebuilt engine when it started losing oil pressure. The machine shop put to rough of a finish on the crank when they ground it causing a similar failure to what you are describing.

Steve

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I wish I had all the answers, believe me I do!

 

Spike - the machinist probably removed the centre main cap first and assumed the worn thrust was the main issue however after further investigation both mains and rod bearings were prematurely worn so his diagnosis moved from a thrust failure to bearing failure due to tight tolerances which lead to a cylinder(no 4) failure with minor scuffing on the other cylinders, obviously due to lack of oil pressure

 

When I assembled the engine the thrust clearance was the first thing I checked and it was certainly within specifications as were the bearings

The question is what specifications did I use?

 

After many nights with little sleep trying to get my failing memory to work I realised I may have got my clearances from a book by George Reid called - 351 Clevelands How To Build For Max Performance so I had a look and there is a whole page with most clearances/torque specs

 

Upon looking at the recommended bearing clearances it had approx .0009 - .0015 which I now know is not for performance builds but OEM stock specs

I had a vague memory of how wide the plasti-gauge was and sure enough it was about .0015 or possibly less

The thrust clearance would have been within OEM specs but shouldn't be an issue however if I did have my rod/main bearing clearances at OEM specs then this could cause a problem with a performance build

 

IA Rider - it was a brand new Scat crank and was checked by the machinist, journals looked perfect to my untrained eye

 

Anyway I've gone with boring it to .030 over, new set of SRP pistons (ouch!!) and the crank is being ground to suit new bearings with clearances at .002 rods .0025 mains

 

Hopefully have the block back by the end of the week and on the road following week but it will be a nervous first few hundred miles

 

Oh and lucky I got my C4 trans checked out, nothing wrong with how I put it together but the front band and a few clutch plates show some feathering which is probably due to old(new) parts getting moisture in them. This could have caused some issues so getting them replaced

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I know that this is splitting hairs but how all the gear heads around here do it.

We put the bearings in the rods and block and torque them to spec. The we measure the actual bearing with an inside micrometer and grind the crank to fit the bearing. We never just grind it .010" or .020" under standard dia. Yes it is usually only a few tenths of a thousandth but it is right. 

When we raced small engines in carts we set the clearance by using a roller burnisher and you could size the rods, wrist pin holes right to a .0001" easily. Roller burnisher for diameter of rods and crank mains would be expensive. 

Like you my short term memory is gone, lol. Didn't go back and read it all. Are you sure that you got all the oil passages spotless clean? A little bit of grit in there is a bad thing.

When they did the cash for clunkers deal they poured the crushed glass in the oil. Was talking with guy at local dealer he said most Japanese cars would seize in less than a minute. He had a Ford 300 6 cylinder run for 30 min. before it seized.

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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I know that this is splitting hairs but how all the gear heads around here do it.

We put the bearings in the rods and block and torque them to spec. The we measure the actual bearing with an inside micrometer and grind the crank to fit the bearing. We never just grind it .010" or .020" under standard dia. Yes it is usually only a few tenths of a thousandth but it is right. 

When we raced small engines in carts we set the clearance by using a roller burnisher and you could size the rods, wrist pin holes right to a .0001" easily. Roller burnisher for diameter of rods and crank mains would be expensive. 

Like you my short term memory is gone, lol. Didn't go back and read it all. Are you sure that you got all the oil passages spotless clean? A little bit of grit in there is a bad thing.

When they did the cash for clunkers deal they poured the crushed glass in the oil. Was talking with guy at local dealer he said most Japanese cars would seize in less than a minute. He had a Ford 300 6 cylinder run for 30 min. before it seized.

As the crank was brand new there was no need to grind it

The mains tunnels were align honed, block zero decked, fully balanced, new rods etc. Machinist supplied pistons, rings, bearings etc

Not sure exactly what the machinist measured but as I wanted to assemble the block myself I just used plasti-gauge to check it and unfortunately I must have used the OEM clearances.

If I had of gone with "performance" clearances then it would have been just a matter of purchasing ACL H series bearings which I think are .001 oversized then mix and match until acceptable clearances achieved

 

All good in hindsight to speculate, and boy oh boy have I done that, but the fact remains is I failed to get the right clearances from the start

The blocked was thoroughly cleaned including using rifle brushes to clean the oil galleries, flushed several times and blown out with compressed air

I can only hope it was just tight clearances that caused this but having no proof leaves me wondering, fingers will be crossed!

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"When they did the cash for clunkers deal they poured the crushed glass in the oil. Was talking with guy at local dealer he said most Japanese cars would seize in less than a minute. He had a Ford 300 6 cylinder run for 30 min. before it seized."  One of the most stupid things I've seen our somewhat mistake prone government do. How bored your friends have been to put ground glass in the engines to see how long they would run. Chuck

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"When they did the cash for clunkers deal they poured the crushed glass in the oil. Was talking with guy at local dealer he said most Japanese cars would seize in less than a minute. He had a Ford 300 6 cylinder run for 30 min. before it seized."  One of the most stupid things I've seen our somewhat mistake prone government do. How bored your friends have been to put ground glass in the engines to see how long they would run. Chuck

That was not them being bored. That was the government requirement. When you traded a clunker in they kept the car for three days if memory is right. They spray painted C F K on the side of the car. The wise government idea was to boost car sales and get the clunkers off the road. 

The glass in the engine was required by the rules. None of the drive trains could be sold out of the salvage lots. It was so stupid. Killed the used car market for a couple years because there were no good used cars they all got crushed. They could sell some of the sheet metal but engines all had to be trashed. 

That was an Obama idea.

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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OzCoupe,

If you have the option with your assembler, don't shy away from the larger bearing clearances. In my Clevite books, which we use to enter bearing clearance windows to a build's paperwork, the window may very well read something like .0009"-.0029" for a given journal. That doesn't ever mean that just because the low number reads .0009" ( read that 9/10ths of a thousandth of an inch ), and it's technically "in the window", that anyone should EVER set, or allow a rod or bearing clearance to be so tight. Common SAE thinking is to give at least one thousandths oil clearance for every inch of journal diameter, as in .002" MIN for a 2" journal, again, this is a standard MINIMUM. There are lots of great running loose engines out there, but no great running tight engines, so go to the high side on your oil clearances, and you'll be happy.

The old jingle we all learned in basic auto shop for ring gap was "....Three to four ( thousandths) for inch of bore...", meaning for a 4 inch bore, you want MINIMUM .012 to .016" top and second compression ring gaps . This is, again, an SAE minimum, depending on ring manufacturer and ring materials used, they may very well have a ring gap information sheet calling out other gaps for an intended use ( I.E. stock, supercharged, alky, etc ), and using a Hypereautectic piston requires an even larger top ring gap, due to the rings's close proximity to the heat of combustion. Lots to know.

If you bought a Scat kit, crank, rods, pistons, bearings, you are likely in good shape. as Scat generally has the rod side clearances opened up as well, but you will still ( and always) need to check the ring gaps. And, not to beat this dead horse, always check the thrust ( crank endplay).

None of this is difficult, maybe scary for someone who doesn't do it all the time, but I hope you do great with your current build. Let us know how things work out.

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As has been said on here many times, new does not mean good. Did you mike each journal twice, at 90°, to make sure they were all the correct size and round? Did you plasti-gauge every journal? Everything absolutely clean during assembly? It doesn't take a very big piece of debris under a bearing to wind up with zero bearing clearance. Main bearing caps on in the same position as when line honed?

 

There are so many variables that can cause an engine failure you may never know what caused it, if nothing obvious can be found.

 

Even something weird like a short oil pump drive shaft that drives the oil pump when bottomed out in the pump, but works it's way up into the distributor shaft and out of the oil pump might never be found. Not even sure this could happen, I would have to take some measurements.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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