How low can I go?

[chargd72]

Can I use a head gasket with a .025 crush on a set of closed chamber SB heads with a repro 340 cam (.464/.464 lift)?  I'm concerned with valve clearance and how reliable it might be with the higher compression.  Or should I just go with a fel-pro .039?  Thanks guys.

These are the ones I'm considering.
http://cgi.ebay.com/ebaymotors/MOPAR-340-360-273-318-HEAD-GASKETS-025-thin-PAIR-/380232531133?pt=Motors_Car_Truck_Parts_Accessories&hash=item58879ffcbd

[elacruze]

The only way to know your valve clearance with certainty is to measure it. Stick the heads on with no head gasket at all, torque them to something easy like 10 ft-lbs. Have the valves installed with light checking springs if you have them (hair curler springs used to be good, hard to find now) or just grease the stems to keep them from falling out. Then use a dial indicator to check the distance to the piston at TDC. This is a lot more accurate than clay modeling, and you're certain not to dink the pistons by rolling the motor through. Once you know your distance, just add the thickness of the head gasket (the valve angle off center isn't significant) and you'll have your running clearance.
With the bigger valves it could happen that the piston pockets are deep enough, but too small around the OD. If that happens to me on the 318 I'll just address it carefully with the carbide cutter.

I'm building my BB motor with aluminum heads and .027" quench. That's pretty tight, to go .025" I'd say you need to be sure that everything in the lower end is in perfect order. At that clearance you'll almost certainly find witness marking on the pistons and quench area when you disassemble. Worst case is at RPM it will contact significantly which is bad for everything. .025 is a realistic minimum for any (automotive) engine in any situation.

To go a step farther, most people don't understand the purpose of 'quench'. The quench area creates a great deal of combustion chamber turbulence during compression, which improves the efficiency of combustion and allows less timing advance, and lessens the chance of detonation. That's wonderful, but the textbook definition of 'quench distance' tells us that there is a minimum space through which a flame may travel-at atmospheric pressure and at rest, that distance is about .040", and that is why many people set their motors up at about .040" quench distance. In a dynamic situation, there is a lot more going on than at rest so I like to set mine up tighter than that, to insure that the mixture can't detonate since it's a lot hotter and more turbulent than in the textbook example. Also, with a tighter quench distance the piston passes through the critical detonation distance (.120"-.080") a little more quickly because the rod is at a larger angle than if the clearance is set at a wider minimum quench.
I don't have laboratory evidence of these theories, I've just sussed them out over the years and haven't had any bad results I can attribute to them.

[chargd72]

Thanks e.  I always appreciate the effecient explainations you give    But, I think you were assuming I was running zero deck pistons which would give me that .025 quench, right?  My pistons are a wee bit in the hole (don't know the exact amount) so I thought running a gasket with .025 crush would get me closer to the .040 optimum quench.  I quess I should get an exact measurement first.  I was mostly concerced with the reliability.  As long as I have proper valve clearance and a good quench, would it be easy to blow a .025 head gasket?  That's where I would throw in a .039 gasket if that were the case.  What do ya think?

[elacruze]

Actually, the thinner the head gasket the better-in theory.

Compression doesn't blow head gaskets, detonation does. If your block and head gasket surfaces are flat and the heads are properly torqued you shouldn't have any problems with those just because they're thin. I can't speak to the manufacturer or quality of those particular gaskets. I went with Cometic because I know they're first-rate. On a 318 with less than 11:1 static, I don't think you have anything to worry about. Be sure that there is no gasket material overhanging the cylinder or combustion chamber-normal combustion will eat the edges of unattended gasket and start a problem.

[chargd72]

 

Actually, the thinner the head gasket the better-in theory.

Compression doesn't blow head gaskets, detonation does. If your block and head gasket surfaces are flat and the heads are properly torqued you shouldn't have any problems with those just because they're thin. I can't speak to the manufacturer or quality of those particular gaskets. I went with Cometic because I know they're first-rate. On a 318 with less than 11:1 static, I don't think you have anything to worry about. Be sure that there is no gasket material overhanging the cylinder or combustion chamber-normal combustion will eat the edges of unattended gasket and start a problem.

[TylerCharger69]

A question to add to the topic.....Can over re-surfacing the head cause clearance issues??   Meaning  if the heads have been worked more than once.  And if so,  what is the most you can shave off of a set of heads safely??