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Why L28 Piston Deck Height?


joey.youssef

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This post is to find out if anyone knows why the L28 motors have negative deck height when running flat tops (piston sticks out of block).  From reading different people's post I see that the deck height measurements are roughly 0.017"-0.025".  I never understood why when looking at the measurements from "How to Rebuild Your Nissan & Datsun OHC Engine"

 

L28 specs:

Block Deck Height = 207.85 mm

Connecting Rod = 130.2 mm

Piston Pin Height = 38.1 mm

Crankshaft Stroke = 79.0 mm

 

This should give a positive deck height of  207.85 - (130.2+38.1+(79/2)) = 0.05 mm (.002")

 

I was just wondering if anyone knows why in real life we get a negative deck of ~0.5 mm (.020")?

 

Which of the above numbers is not accurate to real life? Am I missing something.

Edited by joey.youssef
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This post is to find out if anyone knows why the L28 motors have negative deck height when running flat tops (piston sticks out of block).  From reading different people's post I see that the deck height measurements are roughly 0.017"-0.025".  I never understood why when looking at the measurements from "How to Rebuild Your Nissan & Datsun OHC Engine"

 

L28 specs:

Block Deck Height = 207.85 mm

Connecting Rod = 130.2 mm

Piston Pin Height = 38.1 mm

Crankshaft Stroke = 79.0 mm

 

This should give a positive deck height of  207.85 - (130.2+38.1+(79/2)) = 0.05 mm (.002")

 

I was just wondering if anyone knows why in real life we get a negative deck of ~0.5 mm (.020")?

 

Which of the above numbers is not accurate to real life? Am I missing something.

Well, for one thing the 207.85 figure was when it left the factory.  Any subsequent machining operations (e.g., truing the deck or align boring) that may have taken place in the intervening 40 years will decrease that number.  

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Well, for one thing the 207.85 figure was when it left the factory.  Any subsequent machining operations (e.g., truing the deck or align boring) that may have taken place in the intervening 40 years will decrease that number.  

Right. I think jy.s30z is talking about using factory pieces you still get a deck height of 0.5 mm (.020"). 

 

OP have you verified all those measurements yourself? 

 

Also subbed for answer. 

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Right. I think jy.s30z is talking about using factory pieces you still get a deck height of 0.5 mm (.020"). 

 

OP have you verified all those measurements yourself? 

 

Also subbed for answer. 

 

No I haven't....  I just received my parts back from the machine shop and waiting on a few more items before I start assembly and taking measurements. Iv'e just been doing calculations on what to expect and something not making sense.

 

Just wondering how/why the books are so off from real life.  0.020" is a lot to deck a block and everyone seems to be in this area and I don't think everyone has a decked block.

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Right. I think jy.s30z is talking about using factory pieces you still get a deck height of 0.5 mm (.020"). 

 

40 plus years later there's pretty much no such thing as "factory pieces" - maybe it's been machined and maybe it hasn't.  Odds are very good that it has been.  You simply have to measure the deck height of _your_ block and go from there.

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Yes. I agree with NZ.  Some of the dimensions in those " Hot Rod " books are not  the same as the FSM and Machinist spec tables. There seems to be a fair amount of conflicting information, particularly involving Piston Pin heights. Machinist tables on the other hand  are constantly updated and checked for accuracy. Measure twice.. cut once.

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Interesting question. I recently installed an F54 block with a P90 head in my 260z. The engine had been rebuilt by the previous owner some years ago but had been stored. I wanted higher compression so I replaced the P90 with a refurbished N42 head. While the head was off I noted that the Flat-top P79 pistons were sitting above the deck but I didn't measure the height. (Looked to be only just above. I'm guessing 20thou). Felpro gasket installed. Once assembled the cylinder compression with some oil added to the dry cylinders was between 220-225 psi across the cylinders. Higher than I expected. Lucky in Australia we have 98 Ron fuel.

Edited by Enzo250gto
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