valve-piston clearance

[bigbrotherjohn]

I just measured the clearance between the valves and pistons on my 912 engine as part of a rebuild. What is the clearance supposed to be? In Duane Spencers book it says the clearance must be more than 3mm. Using the method from the book I got about 2mm on the exhaust valves. What could be possible reasons for this, I have put in aftermarket big bore cylinders. I also measured the deck height and there was plenty of room. Thanks

[jpamental]

I've read that with big-bore kits you typically need to shim the cylinders - maybe a 1mm shim is what you should be looking for? I'm guessing the cylinders just stick up farther.

Jason

[jashropshire]

bigbrotherjohn,

What did your deck height measure out to be? - jeff

[bigbrotherjohn]

where could I get these shims and how much would the change my compression ratio. Also is this completely neccesary. 3mm seems like alot and I know that the 911 are fine with 2mm.
And for Jeff, the deck height was just about 1.6mm on all four pistons.

[jashropshire]

I'm going through a similar process. That's why I asked the deck height question. Was curious. I installed a set of big bore pistons with the standard .25 mm copper sealing ring that came with my gasket set. I measured the deck height using the solder crush method. My deck height is at 1.65mm. Everything I read says it should be between 1.25mm and 1.50mm. I am concerned about compression ratio being too low as a result. I have cc'd the combustion chambers, and at 61cc's, it would appear that my compression ratio will be much lower than stock. So, that's a concern as well. Am interested in what you learn. Thanks

Jeff

[jpamental]

Search past posts here and on http://www.bbs912.org - there's been a lot of discussion about this by some really knowledgable folks. I think the issue of 3mm is related to stretch at high rpm's - apparently much more than you'd think... (but I have no firsthand knowledge)

Jason

[bigbrotherjohn]

I would think that the 3mm is recommended because of the posibility of valve float. Would this even matter if I don't plan on going over 6000rpm. Also, jason, what are you searching for because I haven't been able to find much information in the archives.

[bigbrotherjohn]

Also would cutting deeper reliefs into the piston tops be a possibility. My C.R. is already low and I don't want to shim any more. The reliefs in my aftermarket pistons are not as deep as in the stock pistons, would Shasta pistons solve this problem? Thanks.

[bigbrotherjohn]

Jeff I sent you a PM. I tryed to Email you but it wouldn't go through.

[jpamental]

Here's a link to a good thread:
http://www.912bbs.org/index.php?a=thread&t=9252

You can search that board on 'piston clearance' or something like that...

Good luck!

Jason