opinions on bronze valve guides

[Omie01]

This is kind of a re-visit on the subject, but I was just talking to another engine builder this week, and he is of the opinion that valves will eventually stick in an engine that has cast iron heads and bronze valve guides. He says that the heat expansion rate is different between iron and bronze and this causes bronze guides to swell in on the valve stem eventually seizing them. Has this happened to anybody? My other engine builder disagrees and says its all in the clearances. Keep in mind, the builder that claims this usually only builds super horsepower/supercharged/nitrous engines. I'm only concerned because I had phosphorous bronze guides installed in my 4v heads, and if this IS a problem I have to do something about it now. Any input would be great!! Thanks!

 

[piper62j]

All the years in the industry, I've never heard of a problem with bronze guides.. It's all about clearance. If the machinist is good and knows his stuff, he will compensate for any expansion differences between the steel and bronze expansion rate.. This includes knowing what size to bore the guide hole to in the head so the guide doesn't fall out and cause damage.

 

[Don C]

While it's true that bronze expands or lengthens more than cast iron, for a given temperature range, we're looking at a fairly thin section of material (0.20" +/-). The additional expansion of each wall of a bronze guide will likely be less than 0.0005". This additional expansion is taken into consideration when setting the clearances.

 

[4Vforever]

I've never heard anything so crazy, I've done many, many cylinder heads over the years with bronze guides and bronze liners (K-Lines) and never ever had such issues. As has been previously said, if the clearances are honed correctly to manufactorers specifications, then why should there be a problem. Many big horsepower engines running cast iron heads over the years, haven't seemed to have had an issue using bronze guides unless clearances were an issue. If this was a problem, then why would aftermarket companies such as World Products (Dart) have bronze guides installed in their cast iron performance cylinder heads. As I've said, this statement makes no sense at all, plus I'd have more faith in running bronze guides over cast iron ones in a high performance application.

 

[Omie01]

Thanks gents, I called the guy that installed my bronze guides again today and he says he will loose no sleep over it, he knows he allowed for clearance and has never had a problem either so I will breathe a little easier!!

 

[basstrix]

Bronze guides have been used in cast iron heads for a long time w/o issue.

CTE for bronze: 10E-6 in/in-R

CTE for cast iron: 6E-6 in/in-R

Consider the guide/head are heated to 300F above ambient (= ~370F) the difference in expansion for a 1/2" OD guide is .0006". Realistically, the majority of the guide/head should not go above 200F...in this case, the differential expansion is .00026".

Now consider the elastic modulus of each material to see which one is going to expand/compress:

Bronze @200F: 13.7E6 psi

Cast Iron @200F: 13.2E6 psi

In this case, bronze wins the battle (barely) and will actually force the cast iron to expand slightly. It's a very small difference and I wouldn't be concerned. The point here is that the ID of the guide is not going to get smaller due to thermal expansion.

The next step would be to evaluate the cast iron to confirm that it does not get strained (stretched) beyond the elastic range (which would result in a loose guide in the head).

 

[Omie01]

WOW! That is some good info there!!