i pulled one of my heads today and found that my intake valves were in fact hitting the pistons. the only thing broken as a result was the pushrods. It looks like maybe even .010" less and the valves would have cleared. What I think I will do is grind the valve reliefs down .075 or so, anyone have tips on this? how much can be ground safely?

my cam/engine specs are in the sig.

 

piston

 

valve:

 

does anyone know the angle at which the valves are to the face of the piston? i think i'm gonna rig up a jig of sorts so the reliefs are ground evenly

 

Before you notch more outta your pistons, did you degree your camshaft in. An advanced camshaft will make the intakes more vulnerable to contacting the piston.

Are all the pistons like that?

 

yeah, all the pistons are like that. i'm not too familiar with degreeing the cam shaft, what is it exactly? i put the crank gear on 0°, put the dot straight up and then put the cam gear on with the dot straight down. I don't think the chain's off by a tooth cuz the engine was running too well for that much advance although I have thought of that because other people are running the exact same setup with no problems.

 

Man that sucks I would have the piston valve releif fly cut by your local machine shop and see if you have anyone you know that will know how to degree a cam to help you accomplish that. You can get a boost in power in certin rpm's by degreeing the cam in retarded will give you more top end power and advancing the cam will give you a little more power in the lower rpm range by shifting the rpm band slightly. Talk to a local compitent machine shop that can coach you on your decisions on things like this and have them do the fly cut on the pistons so they will know what your situation is and can help you more efficently.

Good luck.

 

Do you have stock pistons? Also did you have a machine shop deck the heads or the block any? The hot cam doesn't really have enough lift to hit the piston and I have never heard of this kind of problem with the hotcam.

 

It looks to me like he already has "forged" valve reliefs in the piston tops..

But seriously, check the cam timing and measure the compressed thickness of the head gaskets you removed. If you are using something less than 0.039" gaskets, you are pushing the edge.

Cam specs?

Rocker ratio?

Were the heads milled or block decked?

Were new rods or pistons installed?

I know some people don't like the "silly putty" clearance method, but if you don't trust your math and want a final check for absolute certainty, it's a viable test.

 

Here is a couple of sites that show degreeing a cam and checking valve to piston clearance.

http://www.moparts.com/ubb/ubb/Forum11/HTML/000016.html

http://www.moparts.com/ubb/ubb/Forum11/HTML/000017.html


http://www.cranecams.com/camvtfaq.htm



Goto Harbour freights. For about $25 you can get a magnetic stand and dial indicator that work great.

http://www.harborfreight.com/cpi/cta...itemnumber=623


http://www.harborfreight.com/cpi/cta...temnumber=5646

 

Silly putty ggooooood

Doing your own math bad

Knowing for a fact the piston to valve clearance: priceless

Seriously next time use silly puddy and bolt the head down not all the way tight and put all your rocker assy on and turn the engine over and take the heads back off and measure the puddy and you should have at least .08" of clearance anything else is pushing it.

 

Gonna jump in for UT here, I've been working on this motor with him. It's a stock alum headed L98 from a '91 vette. Totally virgin block, not decked, stock pistons etc. He's running the lt4 hotcam which is .492 lift with his rockers (1.5) and a .039 head gasket. He had .050 taken off of the guides and spring cup spacers, and visual inspection shows no interference with the valve guides and no coil bind in the springs.


I'm going to make sure he degrees in the cam, I personally installed the timing set and I'm pretty sure I did it right, dot to dot, but who knows, maybe this is one of the flukes where the cam is ground wrong.

 

You need a minimum of .100" clearance for valve to pistons. The silly putty/modeling clay method is the best way to check it. It requires assembling and disassembling the engine a few times and also requires solid lifters when checking.

Since it's obvious you don't have enough clearance theres only a couple of things you can do. Reduce the amount of lift on the cam or have the pistons flycut. Flycutting pistons means removing them from the block and con rods and having them installed on a special machine that cuts the valve reliefs more. It might be cheaper to just buy new pistons with deep valve reliefs.

I bought LS6 BBC forged pistons instead of good racing pistons and had to have them all flycut for clearance. My final clearance is .093" on the intake and .139" on the exhaust.

I'm pretty sure the first engine I blew up last year was because of valves hitting pistons. When the head of an intake valve broke off it caused some major damage.

 

i've saw a boat load of SBCs and checked a lot of valve to piston clearance and have never saw any clearance problems untill you get to over .6 lift, and closer to .65 with flat tops w/valve reliefs. i'd degree the cam, make sure the rockers are or were adjusted correctly and dummy the whole thing up and check it again, i use play dough and no gasket for extra safety factor.

 

Doing your own math (if you know how) GOOD...

Trying to cut a piece of crap clay that was pulled off (thus distorted) then cross sectioned with a knife (more distortion) then try to measure a soft pliable solid with calipers BAD....

Finding out that I started dumping all my cooling fluid at the 330' mark and all the way down the track PRICELESS LAUGHTER (a few years later that is, dumped an exhaust valve from the piston hitting it).




If your close, clay is not accurate for the above mentioned reasons.

I will only check valve clearance with an indicator and stand. Goof proof.

 

With all the intake valves contacting the piston, I'll bet that you'll find the cam is advanced. I'd even guess at ~10° advance. The "timing dots" may line up, but that does not insure that cam timing is correct.

You'll either need a solid lifter or valve clearance checking springs to check valve to piston clearance.

Please do not use clay for clearance checking, please...!!!!!

 

I dont understand the big problem with using modeling putty/clay for clearance checking. Sure, you fragged your engine and youre blaming it on the clay. If anything, when you cut or pull off the clay it will become THINNER and hence will be telling you that you have LESS clearance than is actually there, not more. So, wheres the problem? It just adds extra clearance you may not need if you believe the measurements. And the times I used both the differences I got between the two was negligible anyway.

In any case its a sure fire method that requires little effort or tools, and as ede said, most of us arent going to see a problem unless something is totally wrong.

 

Just for the record, a hotcam on a stock L98 vette motor is about twice the cam it needs. I'd recommend solving two problems by going with a milder cam.

 

Thanks for all the help guys. I won't be able to work on it til this weekend and first thing I'll do is check dot alignment, if that's right then I'll put the crank gear on the 4° retarded setting and check clearance. If it's still too close then I'll probably get a degree wheel and see how much the cam is off.

If the cam is let's say 8° too far advanced can I just leave the crank gear at 0 and just skip a tooth on the chain to get the cam back to 0? How advanced should this cam be?

 

Oh, now we are going to run a hotcam RETARDED on a stock TPI motor?

Seriously, this is the wrong approach. Either throw on a superram and some AFR's to go with the cam, or put in a milder cam. Retarding the cam is just going to make it perform worse.

 

Vroom,

read the ENTIRE thread before posting replies. If you had then you'd know that the consensus is that the cam might be too far advanced and that's why I'd be retarding it.

Second, I'm running a carb. (in my sig. in the first post)

third, if the cam was ground too advanced and I retard the cam timing that excess amount that it was ground too far it'll be back to where it should have been if it was ground correctly (x-x=0). So if the cam should be 4° advanced but is actually 8 and I retard the cam timing 4° by rotating the crank gear then I'll be back to 4 advanced, where I should be.

 

I understand that. Don't just move it 4* back though, degree it so you can be sure.

And still, even with a carb and a free flowing manifold, the hotcam is way too much for stock L98 aluminum heads. You'd be better served by more mods, or less cam.

These days it seems like everybodies suggestion whenever a cam comes up is a hotcam for every motor. It's not a bad cam, but you need a bottom end built to spin and an induction system built to flow to use it.

 

UT, I agree with vroom, you should just get a degree wheel and degree the cam in so you're not just guessing on the timing. Trial an error is not the right way to do engines.

Vroom, I really dont think the hotcam is too much for his motor. I've built up an accurate model of the engine in dyno2000 (I know its not exact but its great for comparing cams) and its really a pretty good choise. The hp peak is right around 5500 rpm and the torque curve is nice and flat all the way down to 2000 rpm. I think the torque peak is at around 3500. Plus, I'm sure if UT had a few grand to throw at the issue he'd have some afrs and a forged rotating assembly, but he doesn't, so for the time being, the hotcam is a decent match.

 

As I pulled off the clay, I was "rolling" it off which I believed that it compressed the clay.

Plus can you say that you accurately measured a soft pliable solid with the pointy ends of a caliper? I can't.

With a dial indicator, you can repeat measurements, with clay (at least for me) you can't repeat numbers.

I was shooting for 0.070" with the intake and 0.100" exhaust. With clay, I was guestimating that my clearance was better that this. With the dial indicator, I found out that my clearance was under this, and that a 0.020" shim took care of my clearance problems.

 

Thats my whole point. If for instance you compressed the clay, its now thinner. You put the 'pointy' caliper on there and compress the clay some more. Each case, the clay is becoming thinner than original and telling you that you have less clearance between the valve and piston, not more. If you compressed the clay lets say .050" and believed a measurement of .060" of the clay after compression and you wanted .100" of clearance, you'd be looking to add another .040" somehow that you in actuality dont need since the measured thickness and compressed amount is .110", more than the .100" you want.

Maybe I'm just more careful, but I wasnt seeing any large discrepancies in my measurements. To me, less than +/-.005" is pretty minor for most engines. If you want to be closer than that then you should be checking every single valve because they are all going to be different.

I'm also confused what a shim has to do with valve to piston clearance, unless you are talking about head gasket shim.

 

If you use a dial caliper, you can leave the clay in place and poke the stem of the caliper through the clay until it hits aluminum (standard blind-hole depth measurement).

And actually, Ede has the best idea - Play Doh. Real modelling clay is just that - clay. It is an abrasive mineral that you really don't want in your engine. Play-Doh is basically very thick bread dough with pretty colors. It won't be nearly as damaging as a mineral based clay.

 

Yes, I was talking about a head gasket shim.

I was "rolling" the clay, that is lift one side and pulling it toward itself, which probably compressed it on itself making it thicker. And I also was using play-doh as Vader suggested (no little rocks in there).

With the calipers, I did not measure the actual thickness of the clay (play-doh), I eye balled it with calipers knowing that the calipers would compress the clay.

About five other valves contacted the piston like on UT engine, but with no damage.

I also clayed every piston because everything was so close.

For me clay doesn't work with a 0.620" lift cam, 13.5:1 pop-up pistons and a piston that comes 0.100" out of the deck. Maybe it is me or my method, but I will NEVER clay an engine again.

I can get +/-0.0005, (that is 1/2 a thousandths) repeatablilty with the dial indicator method.


If claying works for you, so be it.