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Limit to overbore on an M10

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I'm in the process of pulling parts together for a stroker motor. Thus far I have the stcok crank and pulley from an S14 and am now sourcing a suitable block. I've been flip-flopping on whether to bore out an M10 or just get an S14 block. I thought I had the latter sourced through a fellow '02 enthusiast but as it turns out it is actually an M10 bored out to S14 stock diameter of 93.4mm. This was one of a couple at this spec done by Bill Arnold, a well respected BMW engine builder and mechanic and he claims he had this very motor in his car, along with a turbo, and never had any problems. He sees no problem at all boring an M10 motor to this size. My concern though is that this goes against the majority of what I've heard which is not to bore an M10 beyond 92mm. Is there anybody out there who has first hand experience with these kinds of bore diameters that can share some of your knowledge with me? Thanks in advance.

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2002 M10 standard bore = 89.015 mm (3.150")

................ 2nd rebore"C" = 89.520 mm

E30 S14 oversize piston (+.40) = 93.755 mm

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According to Markus Glarner a well known BMW Tuner. The M10 can be safely bored up to 90mm for race use, some guys in Europe use 90.5 VW forged pistons with good results. In the US a a few guys sell 91 and 92mm slugs for M10 and have been used on a few street '02's with no problems.

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Kerman is a fan of the 92mm piston. He feels that going higher it is uncertain that the cylinder is thick enough in all the blocks out there. In other words, it looks like you can safely and structurally soundly go to 92mm. In many blocks you may be able to go higher but without sonic thickness testing of the cylinder walls you won't know if they are thick enough to have stiffness at 7K with your foot to the floor. Not to mention about the distortion from the head bolts.

I have 92mm pistons with an S14 crank that will give (I think) 2230cc.

John

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Is there anybody out there who has first hand experience with these kinds of bore diameters that can share some of your knowledge with me? Thanks in advance.

Here's an answer from a different point of view. Don't bet the bank on this BUT........

Ok here goes: So a race motor revs really high right? Street motors don't and in general are more tame and have less stresses. It is because of this that I would offer the argument (that I have created in my head) that you can in theory actually bore MORE in a street motor than in a race motor. Since sidewall flex (leading to ovalized bores & cylinder bore failure) is the biggest issue race cars will be more prone to this because they spend a lot of time up over 5,000rpm. On a street motor when you have much less RPMs and in theory less stress (compression, rpms, rotating mass, sidewall force & flex, etc.) you could get away with a thinner sidewall because of less force.

just my .02 cents. BTW tom I will be boring mine to 92 or 92.5 (maybe) but personally I think that 93+ is sketchy - BUT the worst you have to lose is a motor worth a few grand :-)

Remember that there is only 100mm between centers so you can do the math from there.

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Also, I have heard that the M-10 318i block is the one to use but I'm using a 72 tii block since i had it laying around.

John

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While the factory has the second overbore at +.5mm, I know that they also had +1mm overbore on special order. The +1mm piston sets were also made by Mahle and cost $$$$. The +1mm oversize is not listed in workshop manuals.

Slavs

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While the factory has the second overbore at +.5mm, I know that they also had +1mm overbore on special order. The +1mm piston sets were also made by Mahle and cost $$$$. The +1mm oversize is not listed in workshop manuals.

Slavs

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I would agree with the 92mm limit, but if you have a question, get it sonic checked. I had a competent shop check mine and said 92mm was fine, but all blocks vary a bit. I don't remember the specifics, but I think it's thinnest spot is on the passenger's side of cylinder 3, where there is quite a bit of force to begin with. The only negative effect I have seen is that it runs hotter, but that may be due to the other work I did at the same time. Old hot rodders faced the same problem, something about less metal to dissipate heat, but I assume that with the stroker motor, you already have plans for an aluminum radiator, or at least an oil cooler. As for race motors, they don't care how long the engine lasts, if it blows up, they can have another one in short time.

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As for race motors, they don't care how long the engine lasts, if it blows up, they can have another one in short time.

Yeah. It's only money. And machining. Typically, getting a race motor built

takes 3 months...

Actually, there's no really good class to bore an M10 out for.

We're limited to stock +1mm, and a few extra cc's is not

worth getting torn down for. The 2002's not that competitive,

and not that tightly restricted (like spec Miata, for example)

t

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Vizard writes quite a lot in his books on A series about overboring. He stresses that leaving a thin 'liner' creates bore flex leading to ring blow by. He documents no apparent problems at low revs, increased torque etc, but at high rpms demonstrates the larger engine producing no more torque than the smaller one (due the additional blow by reducing the cylinder pressure). Hence concluding it was pointless (for the extra expense) to increase the engine inertia for no more power for a race car. He also concludes that turbo blocks and thin walls were a positive no-no. The extra cylinder pressure exagerating the problem.

That said, no idea how far the m10 will go. Horses for course. If you build the engine up for low revs, go for it. But then the engine is blessed with some lovely internals that simply scream out to be revved.... why not submit to natures demands?

Cheers,

Glen.

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I just put my two liter back in the car. I'm in the process of "refreshing" my stroker after 45 hours of use (a little over 2 years of racing). That engine is bored to 92mm.

As to the comment, "As for race motors, they don't care how long the engine lasts, if it blows up, they can have another one in short time." Can I send you bill? Just kidding....

Bob

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One of the most respected long time BMW specialist over here in sweden has sold engines bored to 92.5mm using forged volvo B20 racing pistons. I know that you can go up to 93mm with Volvo B21 cast pistons and simply get rid of the pressed in bearing in the connecting rod taking it up to 24mm from 22mm. However that would require you to drill a small lube hole from below into the wrist pin hole on each side of the piston.

But I have heard that one guy claimed that two blocks have been cracked going to 93mm. Clearly 92.5mm seems ok but should might also be considered to be the safe limit without experimenting further.

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