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Mercruiser 496 base model - Pistons and Rods

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Mercruiser 496 base model - Pistons and Rods


Old 12-23-2018, 11:23 AM
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Join Date: Jul 2008
Location: Delray Beach, FL
My Boat: Baja 342 Raylar HO 600's
Posts: 2,995

kout - I dont know where you are the US or Europe but it isnt unreasonable to find these blocks or engines that have been pulled out on Craigslist. I am in Florida so there are typically both boat engine 496/8.1 and truck 496/8.1 on craigslist. At one point during my rebuild, I had salvaged 3 off craigslist in my shop. Maybe get one off eBay hand have it shipped? Or you can even buy a short block from Raylar for $6000 and have it shipped.
Keith Atlanta is offline  
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Old 12-23-2018, 11:44 AM
Join Date: Jan 2006
Location: Peoria, IL
My Boat: Center council
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Originally Posted by koutroullos View Post
4.250 plus max 0.040 is 4.290 so how they go to 4.340. i mean to use 4.340 i need to overize the 4.250 by 0.090 which is more than the 0.040 which tbe max. is is wise to change the 8 injectors too? what chnages needed to go 6.700 for the rod?
when decking you delete equal distance from the pistons or rod length is affected too and how?
4340 is the material they are made of, not the bore size he was referring to....
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Old 12-25-2018, 04:26 AM
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So correct me if i am wrong.
1.Sleeving 1 of the 8 cylinders and over sizing it all 8 of them to 4250+020 will not be an issue even if this block has no sleeves> New Pistons and rings will be 4.270 and I can use the 4340 series of pistons. He already bored the block to plus 020 and if I ask him to do it 030 I assume he will ask double money for doing 2 times the job.
2. All parts (block, head, manifolds and elbows will be pressure tested).
3. Additionally Manifolds with turbolator and elbow must be pressure tested as one complete part all together.
4. After decking the pistons will not be shaved but head gasket that will be used will be thicker by the size of the removed surface from the block decking (if 0.5mm removed then gasket must be 0.5mm thicker)
5. The existing rods are 6.693 and it will not be a problem to use any 6.700 rod without any other change??? What about the 2 hole sizes, screws of the rods (should they be specific size or type)?
6. I must take all 8 injectors to a professional to clean them and measure their flow and if anyone has issues I must replace it with new (no need to replace all 8 of them???)
7. Since the rods, pistons will not be standard I must rebalance the crankshaft to work smoothly with the new pistons and rods.
8. The camshaft that has a damage must be replaced with a new one exactly as the original
9. I must replace all bearings of the camshaft and cranckshaft
10. I must replace oil pump
11. I must replace exhaust flappers
12. Replace crankshaft position sensor who broke while turner tried to deck the block

Can you recommend me any model for piston 4.270 4340 series? Should I care about anything else like the pin type or something?
Can you recommend me any model of 6.700 rod that will be ok to use with the new pistons and existing crankshaft?
Is there any way to check the work done by the turner? is there any tool that I can use and see if they did correct work before installing thousands of dollars of new parts back to where they worked?

Thanks and merry Xmas!

Last edited by koutroullos; 12-25-2018 at 04:34 AM.
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Old 03-26-2019, 01:00 PM
Join Date: Mar 2019
Posts: 1
Default More explanation

When the boys on this forum are talking about 4340 rods, that is the material that the connecting rods they are recommending are made of. This material is much superior to the OEM rod material. The pistons are not 4340 style/make or anything.

People often put in 6.700" length rods (4340 material) with .990" (std) bore small ends so that they can use more easily obtained forged Big Block Chevy (BBC) pistons. Most BBC pistons use .990 wrist pins whereas the oem Merc pins were 1.004" I believe. The Enginetech pistons you are referring to are the 1.004 pin size and correct compression height (CH) 1.34 for the oem stock rods. I suppose you could get 6.7 rods re-bushed to 1.004, but why would you = Enginetech pistons are hypereutic material and nowhere near as strong as the forged pistons.

If you go 6.7 rods and forged pistons they are not likely off the shelf and will be somewhat custom = more money, but you are upgrading the motor at that point. People use the forged pistons because they are stronger and more tolerant to a " less than perfect" tuning situation, not a fix for ignoring the tuning\fuel injection issues though.

Turner = Machinist to us in N.A.

Are you sure the cylinders are finish honed to size? and what are they bored for, piston material wise? 4.270 is a +20 over stock size for sure but the finished size is dependant on the material the pistons are made of: forged 4032, 2618 or Hyper all have different finished sizes for the +20 oversize. The "turner" needs the pistons in hand before they hone it to final size or it is a guess.

I hope this is more clear.

Last edited by Weedy64; 03-26-2019 at 01:28 PM.
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