Need some advice & opinions please as I've got a bunch of parts for & from a customer. Need to build his 360.

He got a 360 block with a rotaing assy with it which contains KB373 'dome' pistons. Because of that dome (which sticks above the deck) and the fact he wants alloy heads we were thinking about 340 edelbrock heads which has a slightly bigger chamber and usa a thicker head gasket.

How many clearance would you suggest as a minimum and ideal between the dome and the machined part of the cylinder head chamber?

Guess there would have been a better choice for the pistons but they are already in his posision, balanced and all.

Any input?
Thanks!

In general, the use of the "340" Edelbrock heads with the squish area milled out is a bad idea. You want the squish area of the head and the squish area of the piston to come to within 1mm of each other. Milling out the squish area on the cylinder head to allow for a piston that rises above the deck in the squish area is useful only if your client has a brand new set of 340 high compression pistons. Buying those heads will forever limit you as to the combustion chamber working well.

The KB pistons have a solid dome, IIRC, and one could mill it flat, even a bit lower to reduce compression. That would be my preferred plan. Or deshroud the valves a bit and gain a few ccs in the chamber.

10.5:1 with aluminum heads and good squish should work on pump premium, even at 50 meters below sea level.

R.

With the parts the customer supplied (kb373 pistons, .060 cometics) and maybe the 340 we have on the shelf the distance between the milled out area in the chamber of the head and the dome on the piston would be around .030" In total this will give around 10.3 static CR.

Would the .030 be enough?

I'm not familar with those pistons.

A few years ago, 2007 I screwed together a 340 with KB 243 pistons and they were .029" above the deck. I had already years earlier had bought the Edlebrock heads with the open chambers for the early hi compression 340s.

It has 10.4-1 compression.

It runs really well now for approx 18K miles on 91 octane fuel. It makes more power than any 340 I've driven and has held up well as I beat the car hard out at Bonneville 3 times and Pikes peak twice.

Good luck with your build.

I'd want .040" but I'd mockup & CAREFULLY measure & see what you actually have. didn't read all the posts so if these are open chambered heads is the recessed area of uniform depth/slant all the way across?

As I understand it, you haven't purchased the heads yet. You also probably don't have a real number for deck height and connecting rod length. If you do, that makes it easier.

Another thing to consider is block deck orientation. Mopar decks are notorious for being off spec and that includes both height and rotation. It is not unusual for the deck to be tilted front-to-back or side-to-side.

Some of this can be discovered by mocking up the block with a single piston/rod combo and measuring clearance from top of piston to the deck at the four outside cylinders.

Do you have the machinery to correct the deck surfaces?

Either way, the minimum clearance or distance is the one to use in calculations.

Now on to pistons and heads. I would strongly suggest that after you get the deck height problem solved, you calculate how much needs to be taken off the top of the piston to get it to zero deck. This won't be more than a few ccs of aluminum, and if done consistently will reduce the amount of each piston weight by just a few grams. Then read a couple explanations of "overbalancing" and realize that the tiny amount of weight you removed from the piston tops will NOT affect the engine balance or performance.

Then buy the regular Edelbrock closed chamber heads. Set the squish with the gasket. 1mm (0.040") is a very good number, some run as close as 0.032. Ideally you would run out of clearance slightly above redline. Bottom line is you don't want the pistons running into the head, but anything short of that is OK.

www.theoldone.com has some interesting reading on squish. Here's a quote from Larry Widmer (Endyn) regarding quench clearances,

"Every engine I've been associated with made the most power had deck clearances so tight that the piston quench pads just kissed the corresponding areas on the head at peak RPM levels...."


Doing a little more reading it appears your guessed 0.030' would be in the OK area as long as piston-to-bore clearances were smaller, limiting piston rock. But I still think milling the tops of the pistons to get true zero deck is the way to go.

R.