Pontiac Deck Height
 

What have you found the deck height to be on most blocks that don't appear to have been decked before? I've been seeing around 10.235ish. The reason is that different piston manufacturers such as DSS, Icon, Autotech, Ross, are using various deck height numbers in their calculations for compression height.

Just had my 455 decked earlier in the year. With DSS pistons, to achieve proper quench it was decked at 10.213.

4.21" stroke
6.625" rod
1.48" CH

10.21 for a piston that is in the hole .003 in the hole with a .041 gasket to achieve .044 quench distance

DSS part number 6120B-4190

My machinist did not indicate how much material they had to take off the deck, but I didn't gather it was a lot.

Do you have any stock assembled rods and pistons to see what you have now?
Most 389 and 400s I have worked on check in at .023” to .026”.
421s. 428s and 455s are more like .019, so between all the motors where not talking a big difference.
With a 4.150” bore and a .021” deck your looking at 4.65 CCs or like a 1/4 point change in compression.

I'm not talking about one specific block per se. I'm talking about the deck height in general on an uncut Pontiac block. Icon pistons assume 10.225", DSS uses 10.210", Autotech uses, I think, 10.230". It's all over the place. I'm trying to get a general consensus so I can convince DSS to use a different number for their off the shelf pistons. I like their pistons but 10.210" is too low. I don't want to deck the block that much each time if not needed.

Your initial calculation is probably pretty close nominally.

The pistons I took out of my engine where the old TRW L2359 which had a compression height of 1.497. When I did the top end on the previous short block, My pistons ranged from .005 to .007 in the hole, with the deck being taller towards the back of the engine vs the front. This led me to speculate that the engine had not yet been decked on it's first rebuild.

2.105 + 6.625 + 1.497 is a compression height of 10.227". If we nominally add another .006, that would put my likely original deck surface height at 10.233"

I do believe that many of these piston manufacturers like DSS, Icon, Ross etc. will anticipate that the engine will have a surface done on it and build their pistons accordingly. Your cheaper off the shelf stuff like the Speed Pro and KB hypers may assume a more budget friendly rebuild approach and may engineer the piston CH based off of that. Speculation on my part obviously.

For what it's worth, it cost me $250.00 to have my block decked with a BHJ fixture. That cost would likely be absorbed by having a piston with an altered CH built. It probably washes out either way. I also get that what you're trying to accomplish is having a company that supplies a quality forged piston that utilizes a modern ring pack, metallurgy and technology without the need to spend extra to deck the block according to its construction.

I believe the problem then becomes in situations where a block must be decked, that shelf piston may no longer be a good choice. You can run an engine with the piston in the hole easier than you can run it with the piston out of the hole. That would mean the manufacturer would need two skews and that adds cost.

I had my deck cut .025 to put put the trw/speedpro pistons at .000 deck height. The CH of the piston is advertised at 1.714. The calculations for an uncut block comes to 10.239.
My other block had 8 vr pistons with a 1.700 advertised ch were measured at .039 in the hole. Again the calculation puts the uncut deck height at 10.239.
I don't know if either block deck was previously skimmed to true them up. if so that would make the original deck height greater.

The .025 cut block is getting a makeover with a 4" stroke, stock length rods and off the shelf DSS pistons. The CH puts the deck height at .005 in the hole with the CR coming in a 9.9:1.

The good thing about custom pistons is you can specify what compression height you want for your block deck height. I measure the deck height at all 4 corners and specify a compression height of the piston for a minimal cut on the block deck to square them.

The 10.235” that was mentioned in the first post is what I plan on for uncut factory blocks, usually been a few thousands either side of that.

I almost always have any blocked decked to ensure that it's square and has a flat surface. That's pretty much a given. But I'm trying to avoid spending the extra money DSS charges for a custom compression height every time I buy pistons from them. The 10.210 assumed deck height they use will pretty much always put the pistons in the hole .020 or more on an uncut or slightly cleaned up block.

Looking at my records of my 1969 400 block, the deck height before machining was 10.234. Took off approximately.021 for Icon pistons. That still left .005 in the hole but the machinist was adamant he wanted to leave a little material in case I ever had to deck it again.

My 455 HO is 10.22.tom

For you machinists out there when the block is in the fixture to deck it is that height there after you cut it square? ?

Both of my ‘72 481988s are 10.225-10.230; my ‘59 389 block is 10.245-10.250. Seems like PMD let the spec drop over time, Tom.

We always measured the block deck heights BEFORE we cut them to insure equal deck heights. I’ve never seen an OE block that had equal block deck heights from the factory.

Far as I know--and maybe this is wrong--the PMD blueprint spec is 10.250 for all but the 265 and 301. I don't have a spec for those.

Pontiac changed the deck height a few times over the years. I've only seen a few 10.24 and most fall around 10.235 that was mentioned. I like to remove around .008" to .012" on average to square up the block and remove pitting - and even then most aftermarket shelf pistons are still too far down in the bore.

9.143"


:)

68 and up 400s and 455s are the engines I assume are 10.235ish. I also agree the older blocks tend to have taller decks. When picking out pistons or planning on decking the block I generally assume 389s and 421s are 10.24”, and the first gen 389 from 59 to 61 10.245”. 10.25” might be close on those old block though. I have not seen enough of them to make a good guess.

Dick Duclow when involved at Indian Adventures reported the factory blueprint he had on hand stated 10.2437



.

Would be interesting to see what kind of tolerance number was on that blueprint. The 10.2437” could be a single limit tolerance for an upper limit max number, which would explain why few measure that tall. Some blueprints work off the extreme limit’s for better fitment or interference. I recall reading the piston to deck height off blueprints for the 77 and later Pontiac 400s was -.0038, which I have never seen anything close to that, and it is off similar to that 10.2437” height. All the engines later 400s I have ran across were around .012” below the deck (taller pistons than pre 77 early 400s. The -.0038” was probably a single upper limit number like the 10.2437” was an upper limit number for the deck height.