engine and drivetrain Maximum overbore for the M10?

[CR_MOE]

Greetings from Costa Rica!

Sorry if my mechanical techinal terms sound a bit odd but well english is my 2nd language.

I have a 74 2002 Touring with its original engine block on which I'm doing a full engine rebuild and when we took apart the engine we found that although the crankshaft was in good shape; the cylinder walls were in pretty bad shape. Currently the engine has 89.08 mm flat tops and an E12 head.

Before we took it apart the compression was around 130-120 psi and some cylinders (like number 4) had up to 60% pressure loss on "air pressure leak test". The car is used as a daily driver but I do race it a couple of times a year for fun on track day events that the BMW club organizes here.

The machine shop tells me that they will have to bore out the cyliners past the 0.50mm mark so my question is how much can you safely overbore the 2002 M10 engine?

I've looked around and I can only find 025mm and 0.50mm OEM pistons but I have seen 1mm and 1.20 mm oversized pistons at 2002AD and MaxSil respectively so assume it must be possible; the problem is that I'm not building a race engine so if the cylinder walls are too thin I might run into overheating problems or possible even cracking a cylinder wall.

The other option is putting in sleeves and going back to 89.00 mm but I doubt that the quality of the sleeves will better than the original material that the engine is made out of.

Any feedback is appreciated, I know most people here do not think well of MaxSil but at moment I just need to see if overboaring that much is a viable solution.

Thanks

[skipsfcr]

1mm oversize is no big deal (certainly WON'T cause overheating)...And as you said, pistons in that size are readily available. Several enthusiasts on this forum have blocks bored to 92+mm. BTW we've used several sets of "Max-Sil" pistons with fine results.

[CR_MOE]

That's great news, I was really worried about the overboring.

If I go up to 1.20mm will I need a custom head gasket or will I be able to use the standard gasket? I already bought a Victor Reinz upper gasket set so it would be great if I can use it, specially since everything I buy online has to pay additional overseas shipping charges and sometimes a customs fee.

So with 1.20 mm oversize pistons de engine would have (gueestimated) how many CCs? 2050, 2100?

Correct me if I'm mistaken but the M10 block is basically the same as the one used on the S14 engine right? That's why overboaring to 92 mm is possible I guess.

[Grover]

I can't answer your question about the headgasket with a 1mm overbore. However, in terms of how big you can get, I've heard of numerous people going in the 91-92mm range. I also know of at least two people who have gone to the stock S14 bore which I think is 93.4mm (somebody correct me if I'm wrong) without issue. One even did this with a turbo set-up that he drove very hard with zero issues. My understanding is that some of the later M10 blocks were made with slightly better process control by BMW and may be stronger. Good luck, whatever you do.

[Harv]

My pistons are 92.mm J&E With a custom comp ratio.

John

[CR_MOE]

Thanks John,

Would you perhaps know if the standard head gasket would work with 1mm oversize pistons?

What head gasket did you use on yours?

[TobyB]

Yes, the head gasket is 90mm stock.

And will work fine!

t

[Harv]

What head gasket did you use on yours?

I'm using the S14 head gasket.

John

[Grover]

Harv, is your motor put together already? One of the things I discovered when putting the S14 gasket onto my stroker motor is that the gasket doesn't line up perfectly with the lower timing chain cover. Basically, once the motor was warm, a small amount of oil would leak out and I ended up having to remove the upper timing chain cover and make a small "patch" out of some gasket material to block the leak, it has been fine ever since. If yours is already up and running with no issues then ignore this, but otherwise it might be something you want to look into. I can probably describe it better if need be.

[dapbmw]

92-93mm is okay i guess as long as you are using the later m10 block from and e30 318i. i would use a custom gasket like what cometic makes in copper and you can even have higher compresion ratios like somewhere in the 11-12.5 :1 range

[CR_MOE]

Well I will most likely be using the 90.2mm pistons on the original 2002 M10 block with the stock head gasket. I'm hoping that those 0.2 mm larger than the gasket won't be a problem.

If it is, I guess I'll have to get an S14 head gasket.

I'll post pictures later on the projects section.

Thanks again

[Mike Self]

for the past four or five years when I've been in Nicaragua I've seen an Agave roundie (looks like US version, 71 or later) sitting in a yard in Rivas, right on the Pan American highway. It's on the east side of the highway as you're coming into downtown Rivas from the south, just before the baseball stadium.

Body appears to be good, but it's been sitting for at least five years. Don't know where in CR you live, but Rivas is only 37 km from the CR border.

Might be worth checking out...

cheers

mike

[Harv]

Yea, I have heard about this problem. Eric Kerman said to just do a good job of sealing it up with a silicone sealer. I have an arsenal of aviation related sealents.

On another note. I know a guy that has close ties to Cometic. He said it wouldn't be too hard to make an M10 head gasket with S14 bore size. I'm sure it's more complicated than he eluded to.

Hmm.......

John

[CR_MOE]

for the past four or five years when I've been in Nicaragua I've seen an Agave roundie (looks like US version, 71 or later) sitting in a yard in Rivas, right on the Pan American highway. It's on the east side of the highway as you're coming into downtown Rivas from the south, just before the baseball stadium.

Body appears to be good, but it's been sitting for at least five years. Don't know where in CR you live, but Rivas is only 37 km from the CR border.

Might be worth checking out...

cheers

mike

Well that sounds like a 6 or 7 hour drive from the capital were I live but are you suggesting it for parts car or as project car?

I recently bought a 74 touring parts car for about $275 but it was in so bad bad shape that we just grabbed the stuff that worked and cut the car into pieces and sold it for scrap.

The engine was in worse shape than mine, some previous owner damaged the head and kept running the car until it stopped working. When we took it apart we found a nice "cream cheese" surprise inside all the hoses and radiator.

I'm probably going to get bashed for having chopped up a touring but there's probably more Tourings here than Coupes.

Allow me to explain; back in the early 70's there was no BMW dealership here in Costa Rica so FACO which is now the Honda delearship brought the 02s in to the country. As good latins they found loophole in the imports law that allowed the Touring chassis to be imported as a "delivery vehicle" if they replaced the rear and back windows for metal covers.

By being a "work vehicle" it would barely pay any import taxes and therefore could be sold at a lower price or at the same price but with increased profits. Of course they added the glass windows back before selling the car.

So if anyone is looking for an 02 Touring there's some down here; I saw a lime-yellow (not GOLF, but some other hideous color) Touring being sold for about $3000. The interior looks good but I haven't had the chance to check the VIN and the owner says its a 76 (they stopped making Tourings in April 74) so I assume the guy doesn't know what he has.

I'll see if I can get some pictures of it.