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Bore Wear - Do I really need to overbore?

will740turbo

Member
Joined
Jun 15, 2006
Location
Portland, OR
Similar post is over on Brickboard.

94 b230ft squirter block with 175k miles. Crank, mains, and rod bearings all original and looking good after a basic polish (not oversize, still under 2 thousandths of gap on all of them). I bought it used and good with 145k miles on it.

The machine shop is saying that the bores are a bit egg-shaped at 3 to 5 thousandths of an inch. They are recommending overbore and replacement pistons up to 10 thouandths over. I had about 155-165psi compression from all the cylinders except number 3 (the redheaded stepchild), which was 60 due to a blown HG checked with a leakdown tester. I almost just did the head work and put it back together, but wanted to check the bottom end too.

Volvo Greenbook says the wear limit is over 4 thousandths *and with excessive oil consumption*. There was basically no oil consumption other than a little leakage from a HG leak from cylinder 3 to the oil galleys. This engine had 0 piston slap, even on 19F mornings running 15w-40 diesel oil - but maybe that thick oil masked the slap.

I'm ok with new mains and rod bearings, and was planning on it, but overbore+pistons adds $400 to my rebuild.

I have never rebuilt a multi-cylinder engine before, and want some advice on this type of thing. If I'm really going to regret not boring it - like the piston slap will be extreme and punching a hole in the block, then sure. I guess I can afford it. But I would rather spend $400 on something else, I think.


I know the TB opinion is usually "Add zipties and replace it when it breaks", but I would like this to last a while. I might be moving into an apartment, and would not be able to yank the engine back out easily. Meanwhile, I've got lots of time on my hands.

So what kind of egg shaped wear is enough to justify overboring? What am I risking if I don't?
 
Just barely visible. More polished than not.

117413221_10220111927742810_8401990966988489033_o.jpg
 
Have it bored, then and honed with a Ford 2.3l Lima torque plate.
No use in slapping it back together as is. New OS Mahle pistons From FCP are like $300 for a set.

5 thou of egg or taper means the rings are changing approximately 15thou in circumference every time it goes through one stroke. So you end up wearing out the grooves in the pistons.

You’ll also have a decent amount of pistons slap. The cast mahle pistons need like 2 thou or less of piston to wall clearance for quiet operation.
 
BORE IT. that much out of round NFG>. Don't cheap out, I have had to fix motors that people went cheap on,ended up costing 2X as much to fix it. I have been in the auto/restoration/racecar business for 50 years and have seen short cuts that failed, have a wall of shame of the failures in my shop.
 
Have it bored, then and honed with a Ford 2.3l Lima torque plate.
No use in slapping it back together as is. New OS Mahle pistons From FCP are like $300 for a set.

5 thou of egg or taper means the rings are changing approximately 15thou in circumference every time it goes through one stroke. So you end up wearing out the grooves in the pistons.

You?ll also have a decent amount of pistons slap. The cast mahle pistons need like 2 thou or less of piston to wall clearance for quiet operation.

BORE IT. that much out of round NFG>. Don't cheap out, I have had to fix motors that people went cheap on,ended up costing 2X as much to fix it. I have been in the auto/restoration/racecar business for 50 years and have seen short cuts that failed, have a wall of shame of the failures in my shop.

I can go ahead and have it bored, but one thing doesn't add up for me - this engine has *zero* piston slap. Never, not even in sub-freezing temps running 15w-40 or when feeding it seafoam/water. I figured it would sound like crap at that level of wear, but maybe not. Try as I might to get them to wiggle or clack in the bores, I couldn't get them to make any noise.

Aren't the OEM Mahle cast pistons? Could thick oil and squirters hide piston slap?
 
The electric fan turbo redblocks wear the bores more than the n/a or the older mechanical fan cars. Because the cooling system cycles a wider span of temps than the older constant cooling flow fan system. Those heat cycles cause more wear. Credit to reading that on pbase stealthfti.
 
You have to ask yourself why you tore the engine down in the first place. IOW, disassembled the lower end. Now that you have done that, you opened Pandora's Box. With 175K miles on a squirter block, I would have replaced the head gasket and sent it. With that much taper, I would not want to hone it and install new rings. Chances are they will never seal as well as the set that was in there with that much taper. Pull the trigger and put in the new pistons now. That is where you should be spending your $400 at this point in the game.

Another solution would be to hone the bores straight and have the pistons knurled to take up the clearance.
 
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Sorry for a bit of a threadjack, but for an N/A engine and the general scarcity of oversize pistons, is sleeving the block and reusing the original pistons a viable option?
 
I am going through this right now. Cylinders 1 and 4 had a very noticeable step at the top of bore, 2 & 3 weren't as bad. Turbo squirter block. Step was so large it would not overbore, so I am resleeving all 4 cylinders and going with the first size overbore Mahle pistons. We could not find standard bore pistons.

Sleeves are about $600. So if you can overbore with pistons for $400 that's better.
 
Usually a bore/hone is $50 per hole, so $200 for a 4cyl plus cleaning fees. Aftermarket pistons will be $650-850 depending on what you want. So that's $850-1050 for getting custom pistons installed.

Mahle OEM pistons are ~$350 for a set of 4, plus sleeving at $500-600. Basically the same overall price.
 
Usually a bore/hone is $50 per hole, so $200 for a 4cyl plus cleaning fees. Aftermarket pistons will be $650-850 depending on what you want. So that's $850-1050 for getting custom pistons installed.

Mahle OEM pistons are ~$350 for a set of 4, plus sleeving at $500-600. Basically the same overall price.

I'm seeing 1st OS Mahle pistons for ~$60 each for the FT pistons. I guess I'm going to bite the bullet and do it, I'm just kind of incredulous. I couldn't feel any ridge at all, but I'm no machinist.

:e-shrug:
 
Sorry for a bit of a threadjack, but for an N/A engine and the general scarcity of oversize pistons, is sleeving the block and reusing the original pistons a viable option?

https://www.skandix.de/en/spare-parts/engine/engine-repair/piston-assembly/piston/776/

This has the OE Volvo part numbers (prices are in euros), but I also saw some NA pistons here :

https://www.volvopartswebstore.com/...-kit-Genuine-Classic-Part/1107725/271332.html

Yoshifab has them, but custom-made, for about 700.
 
You have to ask yourself why you tore the engine down in the first place. IOW, disassembled the lower end. Now that you have done that, you opened Pandora's Box. With 175K miles on a squirter block, I would have replaced the head gasket and sent it. With that much taper, I would not want to hone it and install new rings. Chances are they will never seal as well as the set that was in there with that much taper. Pull the trigger and put in the new pistons now. That is where you should be spending your $400 at this point in the game.

Another solution would be to hone the bores straight and have the pistons knurled to take up the clearance.

All right, thanks. The main reason I wanted to tear down the bottom end was 1) for the experience and because it felt foolish to not even look and 2) to check the bearing clearances (all just under .0020, limit .0025). I took it to the machine shop to avoid buying micrometers and to get the measurements done for me (and to make sure I ordered the right bearings).

I didn't expect *any* issues with the bores, other than that they might benefit from a touch-up hone because the walls are sort of mirror-finished and have light crosshatching. But this is my first time, so I'm going to act like I don't know squat.

Now I have to talk myself into or out of an oil pump. Good thing I never measured the oil pressure :rofl:
 
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