109 Block Stock Deck Height

[turbodave231]

I recently decked a 109 Block on an RMC V-20 CNC mill. I ended up at a 9.506 deck height (with a nice mirror-like finish). I milled off nearly .010 to get everything square and cleaned up.

The decks were flat but NOT square. This is not uncommon from what I've found. Looking at the engine, I'm not sure the block has ever been cut before, but the deck height doesn't agree with what I've heard about the stock 3.8 deck height.

My Questions:

What deck heights have you found on your 109 blocks?
Do the earlier blocks (1984-1985) have a taller deck height? ( I know the piston are different between 84-85 and 86-87)

Dave

 

[TurboBob]

The deck height was taller due to the thinner head gasket if I remember correctly.

B

 

[charlief1]

I recently decked a 109 Block on an RMC V-20 CNC mill. I ended up at a 9.506 deck height (with a nice mirror-like finish). I milled off nearly .010 to get everything square and cleaned up.

The decks were flat but NOT square. This is not uncommon from what I've found. Looking at the engine, I'm not sure the block has ever been cut before, but the deck height doesn't agree with what I've heard about the stock 3.8 deck height.

My Questions:

What deck heights have you found on your 109 blocks?
Do the earlier blocks (1984-1985) have a taller deck height? ( I know the piston are different between 84-85 and 86-87)

Dave

9.535 is stock for a 109 block.

 

[Lee Thompson]

recently

Measured a virgin 109---9.526--that is very accurate. Not saying all are the same but that one was definately 9.526. And yes, when they went to the composite head gasket in 86 the deck height was dropped to allow for the thicker gaskets. An 85 gasket is supposed to be .018 thick compressed--86-87--.060 compressed. I once had a service bullitin on this. Altho it did not come out to be 1-1, it is close.

 

[turbodave231]

Lee,

Thanks for your measurements. That is what I suspected.

That makes sense. This engine appearred to be untouched. Still had std factory bearings, pistons and all internal parts. It measured 9.518 to 9.516 before I trued up the decks.

Dave

 

[turbodave231]

I have done some more research on the Deck Height of the 86-87 blocks and here is what I found:

The BMS Power Source manual shows a print on pg 50 that lists the deck height of the 3.8 block at 9.560". This book was published in 1985 so it was prior to the release of the 86-87 engines. I am also assuming that the 3.8 S1 and S2 blocks share the same deck height as production blocks.

I found GM bulletin 86-6-12 which states that 1986 3.8 Goodwrench engines P/N 25528795 had revised thicker composite head gaskets, TTY bolts and a .038" lower deck height........so putting this all together, we get a factory deck height of 9.522 for the 86-87 109 block.

This is consistent with the measurements that Lee and I have taken on virgin 109's.

Dave

 

[charlief1]

Thanks Dave. It looks like they're much shorter than was claimed. Good info.