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New Pistons For My Engine Rebuild - Some questions for you

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Hi Team,

 

I took my engine block, pistons/conrods, flywheel and crank to a well know Engineering shop. The owner is very knowledgeable and after inspecting my stuff he has concluded that the crank is straight and crack free therefore will only need to be polished. The bores could be honed (3 thou wear) but the pistons have suffered ring-groove wear. As a result he has advised me that it would be a good idea for me to bore and hone to the first oversize 0.5mm (20 thou). He has locally available pistons to match with new piston rings and so he has asked me to decide what I want to do. 

 

I don't see any issue with honing and boring and going the next size up. He mentioned that the new pistons he has (QSC brand) are a 10:1. I recall reading somewhere that the original MAHLE pistons were 9.5:1 compression. Is this suppose to be a concern, having a slightly higher compression (1972 BMW 2002tii with E12 Head)? To be honest my knowledge is limited in this area and so I am uncertain if using higher compression pistons will cause other issues on my tii? The crank is stock on my car. 

 

What makes a piston high compression? Is it because it has a taller height so it is compressing more within each cylinder? If this is the case then I am worried will the piston hit the cylinder head on its way up? I have had cylinder head rebuilt and it milled by a very small fraction and I am guessing the engine block will probably be milled to by a small fraction. Put these two together and I am thinking is there going to be a piston clearance issue.

 

These probably are silly questions..... I am man enough to admit I don't know a lot about engines haha....

 

Any sort of advice or suggestions appreciated

 

Regards

 

Raj

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Raj,

 

It may be a silly question to ask him but does he realise that the different head types use different pistons? The tii with the 121 head used 10:1 bathtub pistons while the tii with the e12 head used 9.5:1 grand piano shaped pistons. Of course, these are not stock pistons so they could well have 10:1 grand pianos but just concerned that I have seen QSC bathtub pistons advertised in a few places and he may not be aware of the foibles of the M10 engine. 

 

As for a potential clash clash between piston and valves, this is up to you to determine when you reassemble. I assume that you are using the stock cam which at least won’t introduce any additional valve lift but it would be wise to get some plasticine to place on the head of the piston, ‘dry assemble’ the head with no head gasket and the head bolts just nipped up. You can the roll the engine over by hand and the valves will leave an impression in the plasticine. You can then cut a section through this and measure the clearance. Report back what you find and issues can then be most likely corrected by a thicker head gasket. 

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Unlike a lot of engines, M10's vary their compression by different piston heights, not by milling the head.  So you buy pistons not only by compatibility with one of the three heads used on the engine (121, E-12, E21) but also by compression ratio.

 

Also keep in mind that if you run 10:1 pistons you most certainly will need premium gas--and you won't gain that much power over 9.5 CR pistons, which will do OK on mid-grade (unless you're driving in the mountains).  Other than the earliest tii's, US 2002s all had either 8.3 (68 & 69) or 8.0 CR pistons.  

 

Tii's don't take kindly to other-than-stock cams without a lot of fiddling with the fuel injection (which doesn't like to be fiddled with), so you'll probably want to keep the 264 cam that came with the engine.

 

mike

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Search QSC pistons.  Someone just recently had feedback.

 

t

 

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Just to throw fuel on this fire...

Wasn't there something about a 121 head working with "grand piano" pistons IF a small bit was removed from the head?

 

Reason I  bring this up is I have pulled apart my 72tii motor that was rebuilt 16 years ago. It is a 121 head, and has grand piano pistons. The "correct" barhtub pistons are supposedly more expensive/difficult to find. 

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Easiest solution for the e12 Head is the 0.5mm over which is second oversize, first isn’t available, get the 9.5:1 grand piano Mahles from Walloth, cheaper than BMW which didn’t have any in the US recently.  I bought these for a stock 75 2002 which would originally have had 8.3:1 shallow piano tops.  The above was recommended by Carl Nelson.

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(edited)

I rebuilt my engine this past year and used JE pistons and worked with Top End Performance. They were very helpful and have worked alot with the M10... another alternative to confuse you :) Here is the contact info.. 

 

Also, they are made to order for any bore and comp ratio you want/have. $695 for 4... on sale now, but that might be an always on sale price ...

 

Steve Nelson
President Top End Performance Inc.

JE Pistons and Total-Seal Rings
http://www.JE-Pistons.com
7452 Varna Ave N.Hollywood CA 91605 USA
818 764-1901 Phone
e-mail: topend@racetep.com

Edited by worzella
More info
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3 hours ago, worzella said:

I rebuilt my engine this past year and used JE pistons and worked with Top End Performance. They were very helpful and have worked alot with the M10... another alternative to confuse you :) Here is the contact info.. 

 

Also, they are made to order for any bore and comp ratio you want/have. $695 for 4... on sale now, but that might be an always on sale price ...

 

I did this exact thing a couple of months ago. Steve  of TEP was very helpful, in fact he called and spoke to my builder directly then drop shipped the correct pistons, rings, and bearings to the shop, after I paid for them first of course

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On 3/19/2018 at 4:12 PM, SteveJ said:

The "correct" barhtub pistons are supposedly more expensive/difficult to find. 

 

The 121ti bathtub pistons are now back in new production with Mahle.  I keep a couple of sets of 1mm oversize pistons with rings in stock for about $525 a set.  The quality on them is even better than the old pistons.  

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11 hours ago, oldguy said:

 

The 121ti bathtub pistons are now back in new production with Mahle.  I keep a couple of sets of 1mm oversize pistons with rings in stock for about $525 a set.  The quality on them is even better than the old pistons.  

 

That’s good to know. I’d go with the Mahle over pistons from TEP. 

 

Why: On the couple of engines I’ve torn-down with ‘TEP’ pistons, they used 2618 alloy forgings which - to keep the pistons from rattling - are installed ‘tight’ (for 2618).. And then they scuff/drag on the skirts. Install them too loose and they’ll be noisy, rings will wear sooner than some will like. Perhaps they’ve changed alloys lately. For folks above who used them, you might inquire and document what piston-to-wall clearance was used, and save that info for future reference. -KB

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IMHO 2618 should not be used in street car applications.  4032 works fine for street/competition.  2618 is a stronger better choice but only when we prefer strength over longevity and can hold tighter than normal operational temperatures.  I used to hot hone blocks receiving 2618 slugs.  Mahle has been producing their own custom blend of 4032 called M142 which pushes the competitive edge even further and brings 4032 expansion but malleability of 2618.  I mean if you wanted to get fancy, you could get them to make a set of race pistons but I think they run about $200 a hole with a min order of 8.  

 

https://patents.google.com/patent/US20080245335A1/en

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