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Decking a 496 Block

Old 03-31-2019, 01:29 AM
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Default Decking a 496 Block

Hello my mechanic took my 496 block (from mercruiser base model) for decking and boring to a machine shop. When I passed from the machine shop it seamed that they guy didnt know how much he decked the block and when i asked he said decked 020 mil and bored 030 inch.

I ordered Wiseco PTS524A3 pistons (Compression Heigh 1.340 inch) and CH6700APRB8-A rods (6.700 height) and after mechanic installed them while being at the machine shop (they wanted to check if the rods will be hitting the lower end if the cylinders) the found out negative clearance 010 at one side and 020 at the other side of the block.

I know that the standard head gasket included in the Mercruiser gasket set is 0.51 inch.

1. Will it be a problem that deck clearance is not equal at left and right cylinders
2. Will it be a problem that deck clearance is negative 010 and 020

3. What is the CH of the Mercruiser pistons (i know that rods 6.693 inc) but I cant find CH of pistons
4. What is the Deck Height originally before decking and what is the deck clearance with the original pistons and rods

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Old 03-31-2019, 04:18 AM
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I would take it back, ( I am very anal on motor building). .030 (-.010 to +.020) difference in deck height between bank 1 and bank 2 is HUGE. That will mess with quench terribly.
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Old 03-31-2019, 04:32 AM
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What about the intake manifold...it needs to be cut also! Not going to work out too well with the mismatch and the intake not cut.....lots of oh boy moments in your future!
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Old 03-31-2019, 05:20 AM
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Both sides need to be cut the same. The intake manifold need s to be cut to match. May need a different machine shop. This is very basic stuff
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Old 03-31-2019, 05:57 AM
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I’m prettty sure he meant negative clearance on both sides so only a difference of 0.020-0.010=0.010.
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Old 03-31-2019, 09:18 AM
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Originally Posted by streeter69 View Post
I would take it back, ( I am very anal on motor building). .030 (-.010 to +.020) difference in deck height between bank 1 and bank 2 is HUGE. That will mess with quench terribly.
010 with 020 is 010 difference. the 010 between bank 1 and 2 is big difference?
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Old 03-31-2019, 09:20 AM
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Originally Posted by kicker30 View Post
I’m prettty sure he meant negative clearance on both sides so only a difference of 0.020-0.010=0.010.
yes you are right
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Old 03-31-2019, 09:43 AM
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The block more than likely came this way from the factory. If you want a fully blue printed engine, the machining will be quite a bit more than decking the block to take out the warp in the deck.
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Old 03-31-2019, 09:51 AM
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And the pistons should be ordered to fit the machined block - I wouldn't use off the shelf pistons
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Old 03-31-2019, 09:59 AM
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Then if it is .010 not to bad.
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Old 03-31-2019, 10:25 AM
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Originally Posted by rdiesel01 View Post
And the pistons should be ordered to fit the machined block - I wouldn't use off the shelf pistons
Even with a set of custom pistons, he is going to see a .001" - .004" variance, possibly more, from hole to hole. Unless he is building a high compression motor set on kill, I don't see any issues with what he currently has.
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Old 04-01-2019, 01:46 AM
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Originally Posted by Ianl931 View Post
Even with a set of custom pistons, he is going to see a .001" - .004" variance, possibly more, from hole to hole. Unless he is building a high compression motor set on kill, I don't see any issues with what he currently has.
I am not building a high performance engine, just rebuilding my engine because i had be broken rod+piston and camshaft bend. I used all mercruiser parts except pitons+rings and rods because I read that they are the week point of the engine.
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