Piston to head clearance

trainguy

trainguy

Well-known member
Is .035 enough piston to head clearance.just ran into clearance problem with Egge pistons with 8BA heads on a 49 merc.I have recon toured the chambers and used alum foil balls to check and they measure .035 clearance is that enough? Phil
 
My brother rebuilt a Merc 255 back in the seventies and it knocked when he first started it up with the new pistons. Turned out it was one cylinder that they had supplied him with the wrong piston.
 
I used a new 4" grinding wheel on edge and a 4" sanding disc both working toward a line scribed using the head gasket,with a little patience it came out pretty good.I also used a die grinder to smoothe the edged and where the disc didn't fit. Phil
 
Yes the .035 should be enough, that's what the Chebbys use. Make sure they are all the same. I have one engine that's running 37 to 40 tho for the past 12 years and my 280 is running 40.
 
Thanks for the responses.I read that there were problems with the shape of the dome on Egge pistons.I removed the troublsome alum heads from Hi Boy and am replacing them with 8ba heads and found the were just kissing the replacment heads.The valve pockets also needed to be deepened.I hope that it is all worth the effort.Phil
 
If this is a street engine(low RPM) and if the rods have already taken a "set" to maximum length, and if the .035 is with a fully compressed(used and torqued) gasket then .035 will be ok. This is on the borderline. The long, skinny, Flathead rods "grow" much more than a 5.7" heavier beam Chevy rods. If the rods are NOS or if you are pushing 5000+ RPM .035 is not adequate.
 
Rod streach was a problem back in the day, especially with the heavy pistons we used. Mostly Jahns. When I wrote my book I suggested .100" thinking if our racing engines. We've come along way since with lighter pistons and "H" beam rods. Most street engines seldom see 4 grand let alone 5. I've been using Egge pistons for most of my builds and with.050" piston to head clearance never ad a problem with the pistons hitting the heads at any RPM a missed shift could reach. Lately I've droppent that clearance to .040". But you have to measure. I use a /050" shim gasket for checking, or a used gasket that has already been crushed. then you have to check for the top and sides of the piston. Next, if your not running a stock cam, you may have to clearance for it before you start on the pistons.
There's more to building an engine than just buy ibg the parts.
 
Just some bench racing thoughts on rod stretch here: It would seem that the weakest part of the rod (as far as stretch goes) would be the big end, not the beam. At the top of the exhaust stroke, as the piston is coming to a stop and changing direction, all the stress is on the rod cap (or lower insert). This tends to distort the bore of the big end of the rod, pulling it into an egg shape. A few thousandths' worth of distortion and the sides of the big end will pull in and wipe out the oil film, and the rod self-destructs. But this doesn't happen (usually). It just seems to me that the rod bore would distort before the beam of the rod could stretch. Therefore, I suspect that the real culprit when a piston kisses a head is crankshaft deflection from centrifugal force, not rod stretch. (After all, it's been said that a crankshaft suffers from deflection by merely laying on a floor!)
Any comments?
 
Everything streaches and distorts, but these are very small amounts. These are what clearances are for.
 
In a competition engine with 21A rods the rod ALWAYS breaks at the small end just below the wrist pin. This happens at around 7000 RPM after the 4th quarter mile run with NOS rods. Therefore the rods get changed before the 5th run.

Even if the rod growth is only a couple of thousands per inch we could easily be seeing .015 clearance loss because of the length. I used NOS 29A rods in a build that received some dyno flogging and was surprised when I measured the rods during an inspection and found almost .020 C to C length gain. That engine had light weight Forgedtrue pistons and .090 tool steel pins all of which have much less weight than an average engine.
 
trainguy said:
I now have .040-.050 with new "Best"gaskets torqued to 40ft lbs. now onto the rh head.,Phil
Click to expand...
You need to do the tinfoil test again now, checking the edge of the chamber. Milling a head alters the shape of the chamber, and the edge could kiss the pistons. I drew the gasket outline on mine (which was just kissing after a 50thou mill) and softened the edges with a flap wheel. Works brilliantly. 40lbs is light for an iron head.
 
Can't really say about rod strech in a flattie my guess is it almost nonexistent. I would put the head on without a gasket and see if you can turn it over. If you can; put the gasket on and run it! FYI, I have run 11,000+ in my blown alcohol hemi's with .050 piston to head so my guess is you'll be fine... if it turns over without a gasket! g
 
Can someone tell us what the various head gaskets end up at thickness wise.
I intend to use a Best gasket Graphtite set. I am using a set of new french rods with a set of Ross pistons. I dont have a used gasket to work with so being able to calculate it will help.
Thanks in advance.
 
I know this thread is a few months old, but what do you use to transfer the contact area of the piston to the head? I've tried grease but it is hard to get an even coat. I searched other threads and saw someone used lipstick but I couldn't see the contact on the head.

I have EAB heads and have contact on the 3 5/16" pistons with no gasket. I put .025" inch feeler gauges under each side of the head and still get a bump so I suspect I need some trimming.
 
You must log in or register to reply here.
Back
Top