M50 bore tolerances?

[e46s54dr]

To do any boring the block will have to be stripped.

If you just want to keep the 2.5 (which I think is a mistake btw. but this is not my car).

In this order.

Take block to machine shop
Ask them to inspect
You will probably need to bore up.
Bore up .025
Order oversized pistons with rings
Have the machine shop, measure, file, clock and install your bottom end.
Re-assemble your 2.5 long block
Smile


Option 2.
Do everything above but with a reassembly of a 2.8 crank and rods.
I would just find a 2.8 rotating assembly. You will need new pistons anyway so just get a 2.8 crank and 2.8 rods, order .025+ 2.8 pistons. You have a fresh 2.8.

If that was the scenario, hes much better off just buying a used s50/s52 engine because parts and machine work will add up quickly.

[jrdeamicis]

To do any boring the block will have to be stripped.

If you just want to keep the 2.5 (which I think is a mistake btw. but this is not my car).

In this order.

Take block to machine shop
Ask them to inspect
You will probably need to bore up.
Bore up .025
Order oversized pistons with rings
Have the machine shop, measure, file, clock and install your bottom end.
Re-assemble your 2.5 long block
Smile


Option 2.
Do everything above but with a reassembly of a 2.8 crank and rods.
I would just find a 2.8 rotating assembly. You will need new pistons anyway so just get a 2.8 crank and 2.8 rods, order .025+ 2.8 pistons. You have a fresh 2.8.

[e46s54dr]

Basically, if you want to run stock pistons, you should just hone your block and run standard rings. The rings will need to be filed, fitted, and clocked also.

The most important thing for you to do right now is to take your block and pistons to a machine shop and have them assess the fitment of the two. Hopefully youll be able to get away with just a hone job if you cylinders arent egg shaped.

[e46s54dr]

Play with fire, expect get burnt.

Yeah. I respect a man that is willing to do his own work, but when it comes to engine building, its a whole new ball game. The tolerances/specs must be on par. Unlike other jobs, there really is no leeway when it comes to engine assembly.

[ForcedFirebird]

Not a mis read, you are correct.

Play with fire, expect get burnt.

[GunMetalGrey]

Not a mis read, you are correct.

[ForcedFirebird]

So long story short, I will be best off going with a +0.25 ring, and then bore the block.

Are you talking about using oversized rings with stock pistons and over-bored block, or is that a mis-read?

[e46s54dr]

So long story short, I will be best off going with a +0.25 ring, and then bore the block.

If you plan on running the stock pistons, Id avoid boring the block completely. Widening the bore without matched pistons will produce piston slap and lower the longevity of your engine. Hopefully a good machine hone and youll be golden.

If youre totally unsure, just take it to your local machine shop of choice and theyll set you in the right direction.

[GunMetalGrey]

So long story short, I will be best off going with a +0.25 ring, and then bore the block.

[e46s54dr]

Either way your going to have to measure the exact bore size and go from there. Once you get that number you'll factor in the ring gaps and it'll tell you exactly what you'll need.

[328ijunkie]

From all the M50 blocks ive dealt with if you want a good fresh bore youll have to get a set of overbore pistons. With any mileage over 150k on a M50 block the walls are egg shaped and will need a good solid cut to get back to normal (yet the engines never seem to care that this happens???! lol)

[GunMetalGrey]

Alright, for money and times sake I think at this point I would like to try to keep the stock rotating assembly. Which leads me to my next question.
It shows on pelican parts that my options for piston rings are 83.98mm or +0.25 (84.23mm) Do you think it would be worth it to go get the block bored out a tad to accept the 84.23mm rings and know that it is a 99% perfect seal instead of playing the guess and hone game with my current cylinder walls, but still replace the rings with the 83.98mm set?

[Wanganstyle]

Would that result in a higher compression ratio? Or does the crank have a smaller stroke.

it would have compression like a M54b30 or s52b32; m54b30 is a small bore s52. Not very high compression, a S52b32 usa is a low performance engine with relatively low compression.

The 3.0 L crank has a longer stroke, that is why the same 84mm bore (or overbore) can equate a total of 3000cc instead of 2500cc.

If you desired 3200cc you could bore your m50 block to 86.4mm and use the s52 crank and rods in your block.

[NbAmTwE]

Would that result in a higher compression ratio? Or does the crank have a smaller stroke.

You can do either with that setup. They're custom pistons he's referring to.
This is similar to the Frankenstein M50 setup just with overbore and custom pistons. So you can choose pistons that will raise/lower compression depending on na/FI

[ForcedFirebird]

If you plan on running stock pistons with your block then the tolerance should be right around .0015 (+/-.0003") based on the stock bore size. Any more may result it piston slap.

Stock pistons are 83.98" (b25) and the bore is 84mm. Just a hair tighter than 1.5 thou...

.02mm=.0008"