valve to piston clearance

[Omie01]

I have an interesting predicament, I want to check my valve to piston clearance on my build, I have the pedestal mount roller rockers, heads have been shaved .025, and block zero decked, with flat top single relief pistons. Stock rods. My heads are the late 73 small valve 4bbl heads. (2.04-1.70) I need to figure out pushrod length and valve geometry, but before I do that, I want to check valve to piston clearance, how do I do this without the right pushrods? Does anybody know for sure if a zero decked block, and .025 shaved heads will still clear piston with .513in lift, and .519ex and 1:73 ratio rockers? 351c. Oh, and .041 head gasket crush thickness. I don't have cam card in front of me, but I think these numbers are correct. I know the cam is the XE262h from comp.

 

[RacerX]

play dough.

Put some on top of piston where valves would touch. put head on, assemble the 1 cylinder pushrods and rocker arms. Turn engine over with socket and rachet 2 full turns. Remove head and check play dough squish. You can cut the play dough away to measure the squish with the imprint of the valves. I suspect it'll be fine.

 

[jbojo]

Using clay is the method that I think most people use. For determining push rod length get an adjustable one to determine the correct length that you'll need. You'll want the roller wear occurring in equal amounts on both sides of the center of the valve stem tip. If you are using push rod guides you'll also want to get hardened rods. Search You Tube as they have good videos in the areas that you are asking about. I always like watching a video showing how to do this.

Edit: I found this video that's pretty good.

 

[Omie01]

Good info, but the dilemma is the fact I don't know what pushrod length I need, and cant find out until I can safely rotate motor, but can't rotate motor until I know that valves won't hit piston. Or valves have the necessary .090- .100 clearance they are supposed to have.

 

[TommyK]

Good info, but the dilemma is the fact I don't know what pushrod length I need, and cant find out until I can safely rotate motor, but can't rotate motor until I know that valves won't hit piston. Or valves have the necessary .090- .100 clearance they are supposed to have.

As jbojo suggested get an adjustable length pushrod (you are going to need one anyway). Determine your pushrod length and then use it to check PTV clearance.

You are only going to rotate the engine by hand so you won't damage anything.

While you should definitely check it, I doubt you will have an issue with that small a cam and small valve heads.

 

[red351]

that's a small cam no real need to check clearance until .580 in lift or more with even .025 shaved off heads.

 

[Don C]

The push rod length won't change the valve/piston clearance on properly adjusted rocker arms/lifters. The length of the push rod does not change how far the lifters go up and down nor do they change the ratio of the rocker arms, which are the only things that can change how far the valves move.

As jbojo said, the correct push rod length is determined by having a centered wear pattern on the valve stem.

Here's an article on determining push rod lengths:

http://www.hotrod.com/how-to/engine/ctrp-0609-pushrod-length/

 

[Jeff73Mach1]

Clearance won't be a problem with that combination. You can go through the process to check, but I feel confident you have a large margin of safety. To test you will need a solid lifter or lightweight check springs (Neither of which are worth the trouble in my opinion) Start with a stock length rod for a cleveland and check your contact pattern. If you can't get the adjustment right, then you will need to consider nonstandard pushrod lengths

 

[c9zx]

These two articles may be helpful. Note the discussions of checking springs, lifter preload, and effective rocker ratio due to it moving in an arc. http://sbftech.com/index.php?topic=57.0

http://www.thedirtforum.com/pushrodlengths.htm

Chuck

 

[Omie01]

Those are good articles, love the idea of feeler gauge under rocker tip! I already have the test springs and adjustable pushrod, just have to get to that point now!! Got hung up on the oil pick-up tube hitting the windage tray stud!! Grrrr, my little black cloud again!! Thanks for all the info guys!!!

Here is a pic of my engine with parts mocked up!!

 

[Jeff73Mach1]

Perhaps we could start a clearance guess thread!

I'll go first, I think you will have .235 clearance to the valves at the closest point.

Since you have everything, go through the process, it will make you feel better. I find it easiest to cut the squished clay with a plastic razor blade and then peel off half and measure the other half. I also suggest doing it without any head gaskets-you can add the crushed thickness to your numbers easily enough

 

[TommyK]

Those are good articles, love the idea of feeler gauge under rocker tip! I already have the test springs and adjustable pushrod, just have to get to that point now!! Got hung up on the oil pick-up tube hitting the windage tray stud!! Grrrr, my little black cloud again!! Thanks for all the info guys!!!

Here is a pic of my engine with parts mocked up!!

That looks great!

 

[Omie01]

Jeff, you think that much? Well honestly some of these steps I want to take simply for the education too. I still want to build a 4 bolt, close chamber 351 in the near future, and a buddy of mine wants to build a fire breathing chevy engine soon too, so some things we want to do simply to have the experience. Our winters are long, so usually no hurry to get things together!!

 

[Jeff73Mach1]

I actually think it will be more! With my engine zero decked and with .641 lift and 2.21 intake valves I still had over .1 inch clearance A lot has to do with cam profile and timing, so it is always good to check, but I think you'll have ample clearance

 

[Omie01]

Wow!! That thing must be a fire breather!!! What kind of HP are you making? Torque?

 

[Jeff73Mach1]

It is. Power levels are extremely high-Best estimate is close to 600 hp and over 500 ft pounds of torque. We haven't dynoed it yet as the exhaust is too small and it has eaten my clutch, but when it warms up I'll fix all of that and take it to the drag strip