piston to head clerance

hello
can anyone tell me what minimum piston to head clerance i need , blown hemi ,dart billet heads , iron block ,brooks 88 rods, arias pistions .
we have plenty of piston to valve clerance but the pistons seem very close to the head , around 60thou only

cheers

 

60 thou is about absolute minimum, at high rpm's the pistons will touch the heads. 80 thou or so is about right.

 

thank you ken

 

Piston to head

How do you check piston to head clearance?

 

Measure how far piston is down in the hole, add in head gasket thickness.

if the piston is .023 down from flush, and you have a .043 gasket, you are at .066 P to H clearance.

 

My combo is a BAE stage 5 hemi. J&E domed pistons. 451" 4.375 bore x 3.750stroke. .070 head gasket.

 

With no pushrods in the cylinder you are checking , install a dial indicator through the spark plug hole and bring the piston up to TDC , now zero the indicator . Remove the rod cap and slowly push the piston/rod assembly up until it hits the head and read your indicator .

 

Ken,

So wouldnt that just about eliminate the 40 thou head gasket? Not to many guys build the piston further then 20 thou in the hole

 

clearance

Just the way I do I do it. Not right or wrong. I check everything without head gasket. First to see if the piston hits the head. If it picks the head up, I put feeler gauges under it till I find the correct thickness. That tells me what is zero. If I have clearance I put a piece of modeling clay on the dome and turn it around. Pull the head off, and check thickness. But I do like the dial indicator method. Some people may not have that or the parts to mount it. I made metal plates to mount on the Alum block and heads so the magnetic base will stick. Good luck. Don

 

Hi Zero, not really sure I understand the issue. I use modelling clay to test clearances, I put a strip about 1/8 inch thick on each side of the piston, put the gasket and the head on, then rotate the motor, take the head off and measure the thickness of the clay. The least I have gotten away with is .060 inches.

I do also like the suggestion by DQUES though, sounds a little more accurate.

 

Clearances

. Don't need a used compressed gasket and don't have to torque the head to compress gasket on the o-ring. No variables. Just zero.. I have done it this way for a long time. Nothing wrong with the other ways people do it. I do like the dial indicator. Going to do it on the motor I am doing. Thanks Don

 

The initial question was about piston to head clearance only, but as I see it, both the piston clearance and the valve clearance have to be measured and taken into consideration equally. I am seeing here that most are agreeing that over .060" P to H clearance seems to be the minimum, by consensus. But you can have that and even more clearance than that and "jamb" a valve into the top of the piston because valve clearance wasn't measured. I may be straying here from the subject but "both" need to be considered when setting up the clearances of the engine.

In this post the piston to valve clearance was already checked. Just trying to throw this comment in here so others won't forget both measurements are absolutely essential!

I hope this post didn't detract from the original
.

 

Piston to Valve not good

Thanks for all the great responses to my original question on how to check piston to head. I checked p to h with dial indicator through the spark plug hole, .150 - good to go.

I degreed in the cam and rotated the motor over to 10 degrees atdc to check intake valve to piston.....NOT GOOD only .042 with zero lash, .026 lash would still only be .066, I'd like to see .100 with the very min .085

Would it be safe to fly cut the piston .040 in the intake pocket? The .040 would give me a total of .106 clearance. It's a J&E 4.375 shelf pison that BAE carries, it has .315 thickness in the pocket now so with .040 removed it would be .275, is that sufficient?

Engine combo
BAE billet -.500 short deck
4.375 bore
3.750 stroke
6.900 rod length
.071 head gasket
Stage 5 Heads
Red stands
Crane BAE Pro Mod Cam

Piston to Exhaust valve at 10 btdc was .150 - good to go.

Thanks in advance!
Marc

 

I'm surprised your intake clearance is so close. The exhaust clearance (.150) is OK, but I wouldn't go any less than that. I have found that any less than 0.125 clearance for the intake valve is too closee; the good thing is that the piston and the intake valve are going in the same direction, so there shouldn't be a collision. As far as cutting the piston, I think you should be able to run safely with .25 piston thickness.

Double check your cam timing, something seems a bit off here, especially since you have so much piston to head clearance. I have basically the same combination, but with .090 piston to head clearance, I have .150 intake clearance and over .200 exhaust clearance.

 

Piston to valve clearance should be checked 10 degrees before top dead center in increments of 2 degrees until you get to 10 degrees after top dead center, the critical clearance is before top dead center on intake, as the valve is following the piston after tdc I go with .090 intake and .110 on ex with a screw blower, .110 in and 130 ex with a roots. Make sure you have good valve springs, set to .060 before coil bind.

 

Just curious, Are you using a test spring on the head or the actual spring you will be running? A test spring will give you incorrect numbers.
Ken

 

I'm using a test spring....if I use a real tripple spring how would I compress it to get a reading?

Thanks.

 

You would have to get Travis the chimp over there to push it down P/S thats the right thing to do, a lite spring, I think wizzart sells them.

 

Why? Just curious.

 

Well the correct way is to use the actual valve spring....a quick call to BAE confirms that and the numbers don't lie! Basicially you install the valve spring, set the lash, turn over to 10btdc or 10atdc whichever valve your checking intake or exhaust, set up the gauge on the retainer and screw in the adjuster until it stops (contacts the piston) and Whaaalaw!

Using the soft springs my readings came out at .042 - using the actual valve springs it came out to .120 - right where it should be!

Thanks for all the info.

Marc