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Thread: M70 piston rings

  1. #1
    Join Date
    Sep 2015
    Location
    UK
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    35
    My Cars
    1988 BMW 750i

    M70 piston rings

    Hi guys,

    I posted previously about head gaskets for my M70 engine rebuild. I've now stripped the engine down and measured the bores and they are in pretty good shape (other than one which has a small scratch). Anyway it seems silly not to replace the piston rings now that i've come this far but i have a problem getting the right sizes.

    My rings are 1.75mm, 1.75mm and 3mm which equals Goetze 08-702800-00. However i can't currently source these in the UK, everywhere is sold out. I'm even being told there's none available in Europe.

    Interestingly a lot of sellers are listing Goetze 08-704700-00 for the M70 but these are 1.5mm, 1.5mm and 3mm. I gather these are for the M70 from 92 onwards.

    The original BMW part for my year (89) is 11251718708 (1.75mm). This is upto 09/92. From 09/92 the part is 11251436986 which i think is the 1.5mm version. Here's the weird thing - the piston itself is part 11251747346 for all years which means the 1.5mm piston ring was fitted to the same piston as the 1.75mm rings - i think.

    Does this make sense?

    Thanks
    Simon

  2. #2
    Join Date
    Oct 2008
    Location
    Romania
    Posts
    110
    My Cars
    1994 BMW E32 730i M30
    Be careful about drawing conclusions strictly based on part numbers on realoem.com, or the like.
    A while ago I searched data for the pistons applicable for BMW M30/M90 engines, and one can get it really wrong based on part numbers only.

    My opinion is that you cannot use 1.5mm if the ring grooves in your pistons are for 1.75mm rings. Typically, a clearance of max. 0.12mm is acceptable, measured with a feeler gauge inserted between the ring and groove.
    Using 1.5mm rings yields a clearance of +0.25mm, which sounds very wrong.

    If you cannot find the correct rings for your pistons, then I believe you have the following major options:
    1. just re-use the pistons and rings that you already have. If there's no specific technical reason for replacing the rings, other than peace of mind, then I believe this is the best option. Ideally, the pistons have not been taken out of their bores. However, if the rings must be replaced, then the following two options may be considered;
    2. order a set of custom-made pistons, with brand-new rings - but that's most likely too expensive and not worth it;
    3. try to build a M70 / M73 hybrid engine. The following link may provide some inspiration:
    http://forums.bimmerforums.com/forum...ghlight=m70b54
    --
    1994 E32 730i M30 manual
    ex- 2006 E86 Z4M Coupe
    ex- 1986 E28 535i

  3. #3
    Join Date
    Oct 2008
    Location
    Romania
    Posts
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    My Cars
    1994 BMW E32 730i M30
    Here's other idea which may be worth investigating.
    I noticed that the M70B50 engine has the same 84mm bore as the M20B25 engine, which came with 1.5mm - 1.75mm - 3.0mm rings.
    Obviously, the top M20 ring would not fit M70. But the mid and bottom rings may fit. If your pistons are out, then it's worth testing this. You may also want to check if the ring material must be specific to Alusil bores, or not.
    It means that you would only change 2 out of 3 rings, and leave unchanged the most battered ring - but it's a reasonable compromise, if you rings must be replaced.
    --
    1994 E32 730i M30 manual
    ex- 2006 E86 Z4M Coupe
    ex- 1986 E28 535i

  4. #4
    Join Date
    Sep 2015
    Location
    UK
    Posts
    35
    My Cars
    1988 BMW 750i
    Thanks Robert for your detailed responses.

    I 100% agree that it seems wrong to fit 1.5mm rings in a groove designed for 1.75mm. There is a BMW main dealer about 5 mins away so i'll ask them to check their system when i pick up a sump gasket. Even if the parts do match it still seems wrong and i wouldn't do it unless i could get confirmation from someone technical at BMW who knows these engines.

    The reason for stripping the engine down is due to excessive smoke on deceleration and high oil consumption. I was in a way hoping to find something obviously wrong like a broken ring but they are all intact and not gummed up. I will check the end gaps though as reusing them may be the only option.

    I've also got a few valves covered in thick carbon and a mix of different valve stem oil seals so that could also be the cause of the issue.

    Thanks for the tip on using the other rings, i'll check it out.

    Cheers
    Simon

  5. #5
    Join Date
    Mar 2013
    Location
    Green Bay, WI
    Posts
    1,332
    My Cars
    1992 750il / 1991 560 SE
    Valve seals are a known wear point. The guides are not replaceable last I read. Either it can not be done/part not available. Something like that. When I refurbished my heads the guides were good at around 200k miles. Seals were shot and easy to replace. I would clean up and lap the valves - again, easy to do. Just make sure you always match the valves to original locations but I am guessing you know that bit.
    ______________________________
    1992 750iL

  6. #6
    Join Date
    Jan 2005
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    11/88 E32 750iL+98 E36M3
    "excessive smoke on deceleration and high oil consumption" >>>>
    I had that too on my 750 abt. 10 years ago. Start with the easiest repair, crankcase ventilation. I tested it first, made a catcher for the oil, see my website , there PVC M70 prototype http://twrite.org/shogunnew/upgrades...or_Shoguns_M70
    Changed this and problem was solved
    11611708475 : DK paper gaskets (get 2)
    11151718642 : VCV valve (one side)
    11151718713 : VCV valve (the other side)
    11151720131 : Non-return valves that connect to hose between MAF and DK (get 2)

    http://www.ow.no/index.php?option=co...d=23&Itemid=13

    if that does not help, next step: Valve stem seals, also do-able with the rope trick, see Johan's site, or the pics on one of my wrenching buddies site, where he even made an animation how that works with the rope, we did that in my garage, took us 2 days as we had plenty of time and also installed on my M70 new painted valve covers , actually do-able in 1 day http://e32b12.blogspot.com/ click in the index stemseals.

    Only in case all that does not help, I would go for the piston rings, Patrick has made a DIY write up http://www.bimmerboard.com/forums/posts/440624/
    Shogun tricks and tips for the E32 series are HERE!

  7. #7
    Join Date
    Sep 2015
    Location
    UK
    Posts
    35
    My Cars
    1988 BMW 750i
    Thanks guys, very helpful.

    The valves i'll be cleaning and will go back where they came from with new seals.I'll measure the play with a dial gauge to ensure they are within tolerances (0.5mm according to the Bentley manual).

    I tested the PCV system by disconnecting it and plugging the inlet manifold. Still got a load of smoke so don't think that's the cause. I had already replaced the valves previously and there was no oil in the inlet manifold on removal.

    I'm glad i removed the heads though as i found a small fragment of metal sitting on top of piston no. 7 but there's no sign of where it came from - and i've now lost it so can't post a pic.

    Anyway plan is now to measure the existing rings and see what the gaps are like and refit them if ok. Then i'll clean the heads and install new seals. Just paid £6 for each conrod bolt so not cheap.

    The engine is out of the car on a stand so all this work is easy to do.

    And thanks for the links Shogun, i'll have a look.

    Regards
    Simon

  8. #8
    Join Date
    Mar 2013
    Location
    Green Bay, WI
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    1992 750il / 1991 560 SE
    Keep us updated on the rings. A good cleaning will do wonders, but I wonder if they are worm at all. The silicon lining should have kept them in good shape - versus a steel sleeve lining.
    ______________________________
    1992 750iL

  9. #9
    Join Date
    Sep 2015
    Location
    UK
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    1988 BMW 750i
    Just thought i'd give an update.

    The only piston rings i could locate were in Latvia at a cost of £650. I would also need to hone the bores if fitting new rings and as there is no one near me that does that i've decided to keep the existing rings. For info the bore wear ranged from .015mm to .085mm. The ring end gaps on the top 2 rings varied between 0.6mm and 0.7mm. I did have good compression (175-180) on all cylinders apart from 2 that were 160. Interestingly those 2 cylinders bore measurements are the same as the others so i'm assuming the compression was being lost through the valves.

    Interestingly the oil slots on the oil control rings were all clogged with carbon so a good clean has sorted those. I also have excessive wear in the exhaust guides so they are being replaced along with refacing all the valves and seats.

    I'm hoping the excess play in the guides, bad valve stem seals and clogged oil control rings is the cause of the smoking. I'll post up the results when it's all back together in a few weeks.

  10. #10
    Join Date
    Mar 2013
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    Green Bay, WI
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    1992 750il / 1991 560 SE
    Shogun can answer for certain on this I think, but the m70 can not be bored anyway. There cylinders are silicone lined as I mentioned above. If you bore them, that is removed and those piston rings will make a quick meal of the aluminum engine. The only way to do it would be to bore for inserts.
    ______________________________
    1992 750iL

  11. #11
    Join Date
    Sep 2015
    Location
    UK
    Posts
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    My Cars
    1988 BMW 750i
    You can hone the bores if it's done with the proper tools and process - Dragon850 on here did it at home and it seems to have worked well. There are i think about 8 places in the UK that can do it but to be honest with the costs involved i'd be better off getting a secondhand engine.

    Anyway, pistons are all back in now and tomorrow i should have the bottom end back together. Then just needs the heads back and hope to have it up and running by Xmas.

    The car is only doing about 2000 miles per year so i'm not looking for perfection.

  12. #12
    Join Date
    Jan 2005
    Location
    Japan
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    11/88 E32 750iL+98 E36M3
    you could make a modified oil catcher system for the VCV for testing as I did that years ago when I had an oil consumption of 1 liter /1 kmile. In the end it was solved by new VCV valves, here what I posted today on the E31 forum, as there is someone with the same problem on his 850 https://www.bimmerforums.com/forum/s...3#post30174653
    As the tests with my simple made oil catcher showed the VCV is the problem, I changed this, and no more oil consumption:
    I installed then:
    11 15 1 720 392 vacuum hose left 1
    11 15 1 720 393 vacuum hose right 1
    11 15 1 720 131 non return valve 2
    11 15 1 718 642 2 stage valve left 1
    11 15 1 718 713 2 stage valve right 1
    11 61 1 708 475 DK paper gaskets 2
    Shogun tricks and tips for the E32 series are HERE!

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