There is no single right answer but there are some general guide lines.

Excerpt from
http://www.speedomotive.com/Building%20Tips.htm

"Excessive cylinder pressure will encourage engine destroying detonation with no piston immune to its effects. The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.
If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!"

I've looked through the online KB catalog for 400 pistons. I couldn't find any. There were some for 351C and 351W with Cleveland heads but the 351M/400 use a larger pin diameter.
http://www.kb-silvolite.com/1toc.htm

I know the rods can be bushed for Cleveland pistons but there is still a compression height issue. 10.297 block -2.0 stroke -6.58 rod is 1.717" for a piston with 0 deck clearance. The KB Cleveland piston is 1.670" so with a .041" head gasket the piston to head clearance is .088". Milling the head will reduce the chamber size but it won't affect the P to H clearance. More later............

 

Ok, now i understand better the situation...
but there is no company offering a 400 piston with the correct height so the quench effect will work? does this means i need to get cu$tom Pi$ton$ ??? i hope there is a cheaper alternative? if not, then is it worth it any to put the aussie heads over my 351M block without any mods to my block? my 351M only has 42K miles on it, my bottom end is still good. So i can wait to convert it to 400 when it needs it more. if i drop the aussie heads on it instead of the stock ones, should i expect detonation? don't know what comp ratio i will get but i expect it to be around 9 something. Stock 351M has a 8.0:1 ratio. i don't even know what size is the dish, or relief, whatever you call it on my piston, so i cant check with a comp calculator. AARGHH! need help! i hope i have not bought those aussie heads for nothing!!!

HELP!!

Werk




GMC = Generally Misconstructed Crap

 

yeah, the best way to get quench on a 400 that i have seen is the KB pistons for the 351c - the ones with the 12cc "d-cup". but to clese the piston/head gap, you'd need to get the thinnest possible gasket and mill the block deck surface. most "block & head" machine shops aren't set up to do that kind of milling, and if you find one that is, then you'll have to get the intake (or the spacer plates) cut down to match.

the other way is custom pistons. KB doesn't do customs, and their competitors might ask a fairly high price to copy a KB top...

i had a couple other ideas - don't know how practical they'd be.
one is to offset grind the crank just a hair. i was planning to look around and see what the smallest "off the shelf" undersize rod bearings are. probably too much money for just an extra .01 or so though.
my other idea was that since you have to bush the small end of the rods anyway (for 351 pistons), bore it a little over, use a thick bushing, and offset-bore the hole for the pin a little higher.
...also known as a "rod-stretcher"!
the question on that technique would be cost (what else is new!) and how well the pressed-in bushing would resist the tendency to rotate to where it was giving you a shorter rod instead of a longer one.

 

Hmmmm...
how about custom rods instead of custom pistons?
maybe that will run cheaper?? if we get proper lenght rods,
then problem solved so we can use Badger flat tops.
Sounds like what i would do instead of customs pistons...

let me know if this could be fesible.


thanks,

Werk.



GMC = Generally Misconstructed Crap

 

In reply to the post which questioned gaining only 20hp from fitting ported aussie heads fitted with 4v valves, remember the original post was inquiring about just fitting the stock aussie heads, without any porting work etc.

 

The 302C Aussie heads are good heads, the best of the factory small port castings. They bolt on to any 335 series engine and 289-351W blocks (with a custom intake, pistons, and water passage relocation).
To get the maximum benefit, the quench area has to be maintained.

Bolting a set to a low mileage 351M engine for short term benefits is a good idea. I wouldn't install larger valves unless I could verify there would be no interference issues with the block or pistons.

Figure about 12cc for the 351M dish. Reducing the combustion chamber from 78 to 62cc would bring compression to about 9.3:1. A 351M has a much shorter deck clearance than a 400. The piston is only .017 below the block so the quench benefits can be maintained.

Off set grinding the crank is possible. For est. $180 I'd prefer to go to 4.17. A slight increase in stroke doesn't justify the cost. .030 over bearings would only allow about 4.015 stroke if the crank was standard. Bearing material is softer than the crank so strength becomes an issue and .030-up bearings are usually much more expensive.

Rod "stretching" is possible with off set bronze bushings. They would have to be tight so they don't rotate. Cost, anyone?

How about pressing a steel plug into the 400 rod and boring a .912
opening off set to about 6.62" length. It's hard to explain but the bushing can't spin this way. Using stock press in 351C pins will save about $150.

Or maybe easier to off set the Cleveland piston .912 to the 400 pin size of .975. Gain would be 1/2 the difference.
.975
-.912
-------
.063/2 or +.031 increase to the piston compression height.

 

Ok so if i install aussie heads on my 351M, i am looking at .017 deck clearance plus the gasket thickness, which my caliper suggest it is .040 which means i now have a quench clearance of .057 now my gasket thickness has been measured uncompressed, it is a brand new gasket. Don't know how much it will compress once i install it, someone relate about this please. Now, with .057 total clearance, do you think i will get a usable quench out of it? your article mentionned that things are getting worst at .060 so i am a little on the limit here, unless the gasket will squeeze when i install it so
assuming gasket is gonna squeeze .01, i will then get .017 + .03 = .047 total clearance, which is looking better. Entering those data on the compression calculator, i end up with a 10.00:1 comp ratio with usable quench so it won't ping. I am now looking at 2 full points comp ratio more over stock. With all the mods i will do to the aussie heads, i should expect MAJOR difference. I hope this is unfortunately a short term solution, because i will upgrade this 351M to 400 But WITH the aussie Heads and WITH appropriate quench!
quess i will have to order custom pistons or use one of the other ideas you suggested....


waiting for your reply.


Thank you for taking time to read this.

Werk



GMC = Generally Misconstructed Crap