Flush Cutting Rb26 Valve Guides?

[R32N00B]

So I recently obtained an expensive short block (tomei) by luck. I could never afford one outside of this opportunity.

Im now using my rb26 head on this block and Ive read about alot of people having issues with the stock guides breaking off and killing pistons/heads (quench)

Normally i wouldnt be so concerned but this motor is special and I really have one shot at it.

Ive considered flush cutting the factory valve guides not only for better flow but mainly to avoid the broken guide issue. obviously they only break where they protrude into the bowl so cutting them flush would negate that from happening. But im wondering if this is good practice or acceptable. Its a street car but wont see alot of duty. I know the guides will prolly wear quicker but is this something i should avoid all together with an RB head? I tried looking online and it looks like practically NO ONE in the RB world is flush cutting the guides so Im concerned there is a reason why..... any help you can give would be GREATLY appreciated.

I recently had my RB26 fully built and the builder dropped the ball on me and i ended up with a destroyed crank, block, bearings, and couple of toasted rods in about 1000 miles..... Murphies law is out to get me.

Edited October 7, 2011 by R32N00B

[zebra]

I would have new bronze guides fitted.

Seems to be the exhaust guides that break...and they are approaching or even over 20 years old now

[kitto]

+1 to fitting bronze valve guides. Ill be doing this on my build.

Iconel exhaust valves are a good idea aswell.

[R32N00B]

money is extremely tight right now...... plus all my valves are already lapped in to perfection

if i do guides thats a ton more labor to press out and press in than hone/ream them all AND than do a whole valve job to boot..... we're talking over a grand

[kitto]

Doing things properly aint cheap. But its your engine...

You answered your question in a way, you found out stock guides are prone to failing. Shouldnt that tell you to replace them. Not just do a dodgy and cut them flush?

[jet.26r]

at the very least id get a head reconditioned new springs and guides and make sure all valves are still seated right.

[GTRNUR]

If the valve guides are within tolerence then new guides aren't needed. The new guides should only be fitted if the existing ones are worn beyond acceptable tolerances. If they arent broke dont fix them.

A good machine shop that is capable of properly inspecting the head can measure to see of the valve movement is within manufacturer specification (0.0007-0.002" intake and 0.0016 - 0.0028" exhaust at 15mm lift).

Cut back valve guides is normally a race only thing, and in my opinion more of a back yarder race only thing at that. Less valve guide supporting the valve causes more lateral valve moment, which means less consistent seat wear and therefore worse seat sealing over time. The cut back guide will also wear faster. You see cut back valve guides in heads where the guy that did the porting wasnt able to properly remove the valve guides first. A properly ported head would have all the guides pressed out and replaced either before or after the porting process depending on how the porting is done.

I think you will find that most valve stem failures occur because of the tune being wrong, which in turn causes a material failure. Its not a rich/lean issue, rather its more related to high EGT's. So E85 and alcohol tuned engines will have more issues on average than an unleadded fuel tune.

High EGT's cause the bronze alloy of the guide that is exposed to high temps to expand at a faster rate than the rest of the valve guide. The aluminium around the unexposed part of the guide acts as a big heat sync, drawing away heat from the guide. So you get a thermally induced stress at the point where it is exposed to the exhaust gas stream. This is where it cracks. Moments later the cracked part off part of the valve guide can fall down onto the head of the exhaust valve. Finally it gets hammered enough in all of a few seconds to come apart. Then it all goes pear shaped from there resulting in spun rod bearings, piston and head damage, turbine damaage etc.

Richer tunes keep EGT's down to a point, but once you reach a certain power level and you want to hold that level for a sustained length of time (not just for 15 seconds on a dyno), water injection is about the only thing that will keep EGT's down.

If the head you are using has been used on a radically tuned engine before and the guides werent replaced, that could be a reason to consider swapping them even if they are within tolerance.

[R32N00B]

thanks thats what i needed to hear..... im already gonna be checking the guide play this weekend.... so if its good im gonna go for it....

[R32N00B]

also i understand what you are saying about the valves taking more lateral force than usual and in turn wearing the guides faster.... ive considered this as well.... but i find the rb26 a good candidate for such a modification. you see the rb26 head is cam over bucket (as you obv know) and in a setup like this.... the bucket absorbs a great majority of the lateral force... the valve sees very little of the lateral forces of the cam going around.... the bucket/bore see it all. and whats left is mostly straight up and down motion for the valve.....

now if we were talking about an SR20 head or any other "rocker style" head i would be inclined to say that its a bad candidate for such a mod....

Im not saying its right or wrong just contributing to the discussion.....

Edited October 7, 2011 by R32N00B