I acquired a rebuilt motor awhile back and was just getting around to taking a look inside it. It was assembled back in 90 or 91 according to the paperwork I've pulled the head as I plan on using the stainless steel O/S valves from engnbldr and shaving the head for more compression. I measured how far down in the hole the pistons were and it appears all are about .1875 or really close to that. Sound about right? Think it is necessary to deck the block? BTW it has a windage tray and double roller timing set installed - SCORE!

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65 Dart station wagon slant 6 - now under construction
47 Dodge Custom 4 Door sedan
87 D100 Short Bed slant 6

Retired USAF 1966-1986
Retired US Postal Service 2004-2014

It depends on what your build is going for.

That's about as deep as I've measured on a standard bore, with most being in the 0.140 area. There was a lot of variability in manufacturing.

With over-bore pistons, I've seen 0.060" over pistons that ran that deep.

Once you have all measurements, compute the compression. You will want to determine if your going to go with forced induction. Once you cut it, the only way to put it back would be going with a copper gasket to raise the head.

Mike has a Mopar performance book that goes into quite a bit of detail about the slant six to 1967. We were reading last night where it recommended limiting decking to 0.090" to maintain enough of the threaded part of the deck for strength at the head bolts.

Long rod builds tend toward zero deck, but not always. Adjustments can be made with specific piston compression heights. I did it with a Metric piston meant for a Toyota rebuild.

With oversize valves, check to make sure the combustion chambers are located correctly over the bores. The best way to do this is remove the reciprocating assembly, mount the head to the block and turn it over. Verify that the combustion chamber is lined up with the bores. With the valves you are looking at, it's not as critical as with bigger choices. If the bore is oversized 0.060" or greater, this becomes less critical. It should still be checked.

Points to ponder.

CJ

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Part of Tyrde-Browne Racing

You can actually cut to about .110 on the deck before you are in need of going to the ARP head studs, after that point you have some threads showing so you get to limit the amount the bolts will be able to do their job. On the early blocks they tend to have a thicker deck and the pistons are down in the bores a bit farther than the 70's blocks.

-D.Idiot

That's not what the book inferred. The later blocks may tolerate more. The book was printed in '67, and suggested the deck not be cut more than 0.090".

Argue if you want. I don't have a dog in the fight. Cut my deck 0.125" on a BH and have had no issues.

CJ

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Part of Tyrde-Browne Racing

But that was '67 and things have marched on and people have dared to go there...I have cut .120 on a 1963 block with no detriment (but have to use studs- it is currently back in the Hpak Duster), and last year's motor was the forged crank motor out of the BV so it was a stock untouched 1967 engine (decked it about the same).

No arguement, just want to make sure that the peanut gallery knows that they aren't limited by it.


-D.Idiot

With studs, sure. Be careful going further if your using bolts.

I use ARP with the deep cut. Wouldn't want to do it with bolts.

CJ

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Part of Tyrde-Browne Racing

I've had a couple that were .170-180 down the hole. If cutting the head gets the CR you want (& the deck is smooth/flat/square enough) then leave it alone. Why tear down the short block & pay the extra machining costs if you don't need to?

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63 Valiant Wagon
225 - 4 bbl

Most of the 20-30 Slants I have gotten into measuring were close to 0.180" down. We can discuss compression/buildup at Mason-Dixon in 2 weeks!

Lou

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Home of Slant6-powered fun machines since 1988

The '67 block I'm working with now measured aprox .166" down the hole at #1 and #6 and appeared untouched (original pistons, bearings, etc).

I'm glad this this subject came up again, it convinced me to stop being such a piker. I'm going with .125" off the block to get a good chunk of the squeeze without eating into the chambers too much later. I figure I can steal from my carb budget to get the head studs now and hopefully not have any problems. Later I will try my hand at juggling the gasket thickness along with the cut on the head(s) to get the SCR where I want it and the studs should help if I decide to go higher with SCR later.

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1966 Coronet Deluxe

The problem here is unless you are making a long rod, there is no squeeze until about .035-.060...with .125 off the deck you're in the hole .041, and the shelf in the chamber is still at least .100 up from the mating surface. Once you take the .100 off the head to get some quench then you will be pretty close to 11-12:1 SCR....

FYI.





-D.Idiot

That's what I was planning for actually, I figure the chamber is still a mile away and I'm too scared to take more off the block. This should allow me to run in the 10:1 neighborhood now (with the right head cut) and more later without too many concerns right?

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1966 Coronet Deluxe

I had a 1.78int and 1.47ex and .540" lift, I cut the block .140", the head was cut .090". I had 11.4/1 comp and .065" valve to piston clearance. which was tight but I never had any issues.