Question: valve to piston clearance

[chris]

I know that the rule of thumb is .080 on the intake and .100 on the exhaust. Has anybody run it much tighter? in checking mine, I used a .030 shim instead of using a head gasket and with the valve lash set at 0 came up .026 on the intake and exhaust. I have both 1.9 and 2.0 head gaskets which i have been told crush to approx .039. My valve lash is .026 am i correct in my calculations that i only have .061 clearance? The cam has an LS of 106 degrees and i advanced it 2 degrees to 108. This added a few thousanths tothe IN and took a few away from the EX. I have already had it at a machine shop and it was supposed to have been checked and corrected but as far as I can see all he did was make to eyebrow bigger to accept the larger valves (1.84) i put in. That was after it being there a year. I am hoping for a solution that I can do myself and maybe I can drive it this summer!!! ARe there any thicker head gaskets available and has anyone used them? Any help we be appreciated. thanks, chris

[wrench459]

If you dial in the cam at 106 and advance the cam two degrees wont you be at 104?

No there's no way the racers will push the limits to the extreme..

BTW I like .100 on the intake and .080 on the exhaust just personal preference as a rule of thumb for a street motor.

[RallyBob]

Originally Posted by chris I know that the rule of thumb is .080 on the intake and .100 on the exhaust. Has anybody run it much tighter?

I have, but really don't recommend it. One over-rev and your day is done. I've gone as tight as .060" on the intakes, but with that tolerance you're one bad valve adjustment away from interfering.

The cam has an LS of 106 degrees and i advanced it 2 degrees to 108.

That's 2 degrees retarded then. Advancing it to 104° will likely cause piston-to-intake valve interference if you are already that close. IIRC you are running the roller cam, right? Roller cams are SOOO much more aggressive on the opening and closing sides that this would not have been an issue (piston clearance) with a flat-tappet cam. But then again that's what makes a roller cam fun!

ARe there any thicker head gaskets available and has anyone used them?

They are fairly common in Europe, but not here. Elring and Reinz I think make thicker 'racing' gaskets, but be prepared to grab your ankles and brace yourself when you hear the price! $100-$300 is the norm depending on the bore diameter and gasket thickness.

[GTJIM]

A thicker head gasket just reduces the 'squish' or 'quench' between the head and pistol top flats and encourages detonation. Not good!

Looks to me like your only real option is to dig the valve notches deeper into the piston tops - taking care to check that there is enough thickness there to allow this. Should be about .250" of metal (aluminium) left under the thinnest point (where the valve notch is deepest).

HTH

[chris]

Sorry for my dyslexia on the intake and exhaust numbers and I DID retard the cam 2 degrees. Yes it is a roller cam which was not ground to the specs I needed. I also talked to Cam Techniques, and he said the cam could be reground but he was not to keen on it because he did not know how deep the heat treet is. I guess my options are to deepen the valve reliefs or ditch the cam for a solid lifter or spend big $$ for the thick gasket that is going to lower the comp ratio. thanks for all your help, chris

[Paul]

I watched someone who was in a pinch use a Dremel with an Aluminum cutter and a very steady hand deepen the valve notches before. Then he checked them on digital scale and they were still amazingly close to each other weight wise.... Just a thought.

[RallyBob]

A few options that you have:

*Thicker head gasket (but costs a bunch of $$$, especially if you ever need to replace it again, plus waiting time)
*Pull off the head, recess the valves in the seats a bit (costs money, reduces compression, reduces low-lift airflow)
*Pull engine apart, send out pistons to have valve notches deepened (cost money and time)
*Have camshaft reground (costs money, questionable heat-treat?)
*Buy Isky valve notching tool and do it yourself (costs money to buy tool, you must know exactly where to use it [crank location relative to valve timing])

So no matter how you look at it, it'll cost you something. However it's certainly not insurmountable.

Bob

[wrench459]

Another point to think about when running close fits.
S/S valves only.. that way when they kiss the pistons they'll bend and not drop the face into the piston

[chris]

Thanks for going over all the options for me. I am going to check out a new machinist this weekend. I will either recess the valves or have the pistons fly cut again. I let you know how I make out. thanks Chris