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Thread: 1976 BMW R75/6 Piston Condition and Identification

  1. #1
    Registered User b25bsaboy's Avatar
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    1976 BMW R75/6 Piston Condition and Identification

    Good afternoon,

    Trust all is well with everyone and the crazy weather we are having is not causing to much of an issue.

    Took the pistons that came out of my 76 R75/6 yesterday and have started to clean them. I was wondering is it advisable to reuse pistons, as I have done this in the past with no issues. As well the markings on the top of the piston doesn't tell me anything other than the direction they must be installed. Can anyone shed some light on what these markings mean. Would like to know if they are standard size yet or have they at some point in the engines life been bored and fitted with over sized pistons?

    P1020548.jpg

    What they looked like when I removed from the con rod.

    P1020582.jpg

    The side view after cleaning and a light polishing!

    P1020581.jpg

    These are the marking I am not familiar with.
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 1976 BMW R75/6, 2009 R1200RT

  2. #2
    Administrator 20774's Avatar
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    I think the only number you need to be concerned with is the 81.975. Since the stock bore is 82mm, looks like you have stock pistons. Whether you can reuse everything depends on making bore measurements up and down the bore to show what the actual size is and if it's still more or less cylindrical. If the bore has grown a bit and/or become more oval, then you would have to think about going first over. But if the dimensions were still pretty close to stock and the ring lands were still in good shape on the pistons, they could be reused.

    Look on the inside of the piston. You should be able to see more lettering, some of which will tell you the date the piston was made. There will also be a mold number in there...not really of any value. See my post of the inside of my R100/7 pistons:

    http://www.bmwmoa.org/forum/showthre...l=1#post852965
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    B Reams brook.reams's Avatar
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    Measuring Pistons and Bore

    Quote Originally Posted by b25bsaboy View Post
    Good afternoon,

    [snip]

    I was wondering is it advisable to reuse pistons, as I have done this in the past with no issues. As well the markings on the top of the piston doesn't tell me anything other than the direction they must be installed. Can anyone shed some light on what these markings mean. Would like to know if they are standard size yet or have they at some point in the engines life been bored and fitted with over sized pistons?

    P1020548.jpg

    What they looked like when I removed from the con rod.

    P1020582.jpg

    The side view after cleaning and a light polishing!

    P1020581.jpg

    These are the marking I am not familiar with.
    Here is a write-up on how to measure pistons and cylinders should that prove of interest.

    11 BMW R75/5 Measure Cylinders and Install Pistons & New Rings

    Those pistons look fine to me. The ring grooves are clean with no signs of detonation having damage them. I am reusing pistons with 97,500 miles on them and these look better than mine :-)

    Rings, well, measure the end gap per the write-up. In general, I replace them if I have reason to pull the top end and the engine has more than 50,000 on it. You can install standard rings, or you can go with one over size (+0.5 mm) and then grind one end down to get the gap in specifications. That is what the write-up covers.

    Hope this helps.

    Best.
    Brook Reams.
    ---------------------------------
    Brook Reams - Arvada, CO
    Various Two-wheeled Vices, All BMW
    2004 R1150-RS || 2002 F650-GS || 1975 R75/6 || 1973 R75/5

  4. #4
    Registered User kwb210's Avatar
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    What's in store for the bike?

    What are your plans for the bike? Club rider, 100 miles from home? Or is it getting the heavy once over for extended trips? Also, how many miles are on the pistons? If they are just a little bit sloppy and you are just going to run it locally, use 'em. If you have a bit to much wear in the cylinders you can purchase 1st oversize ring set and simply file the ring gap to an acceptable range. The stock rings may have to big of a gap. It's not perfect but can work quite well if attention is paid to the details of assembly. Basically the fitment of the rings. Careful with the rings (!) taking them in and out of the cylinder bore and being the least bit careless and you might break a ring...I have two extra rings, I broke the oil ring while installing. Argh!

    What are you going to do to the heads/valves?
    Actually, what are your overall plans for the bike? Don't feel like the lone ranger if you think it is getting out of hand!
    Kurt
    1977 R100/7 1971.1972.1972.1973 R75/5
    1974 R90/6 multiple boxes
    Airhead Revival
    "Objects in the mirror appear to be losing" unk

  5. #5
    Registered User b25bsaboy's Avatar
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    Quote Originally Posted by kwb210 View Post
    What are your plans for the bike? Club rider, 100 miles from home? Or is it getting the heavy once over for extended trips? Also, how many miles are on the pistons? If they are just a little bit sloppy and you are just going to run it locally, use 'em. If you have a bit to much wear in the cylinders you can purchase 1st oversize ring set and simply file the ring gap to an acceptable range. The stock rings may have to big of a gap. It's not perfect but can work quite well if attention is paid to the details of assembly. Basically the fitment of the rings. Careful with the rings (!) taking them in and out of the cylinder bore and being the least bit careless and you might break a ring...I have two extra rings, I broke the oil ring while installing. Argh!

    What are you going to do to the heads/valves?
    Actually, what are your overall plans for the bike? Don't feel like the lone ranger if you think it is getting out of hand!
    Kurt
    Evening Kurt,

    Sorry for the late respond. What I'm I going to do with the 76 R 75/6? My thought was when I bought her is keep it and use it to attend bike functions in and around Calgary.

    I sold off my collection of Brit Iron and am down to three machines. My original 68 BSA Starfire, 2009 BMW R1200RT and the 1976 R75/6 that I am currently working on.

    I stripped the Vetter Fairing off the machine, as it looked out of place. I have stripped the complete frame down, powder coated the frame, added a second front brake caliper to bottom of the from fork and now have a rolling chassis. Next step is to get into the engine which I have started with the rear main seal being replaced and check the clutch thickness. Cleaned the engine cases and have the barrel's off and they are good to go as the ID is even from bottom to top. The rings need to be replaced as they are about .053 which is a wee bit much. No worried about fitting the rings, as I have an old English tool made just for that process.

    So I am am in the hunt for new rings and I will get the heads redone as the valve guides are to sloppy in my opinion. Biggest obstacle at this point is finding someone to walnut shell blast the aluminum covers, barrel's and head/covers. My next task.
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 1976 BMW R75/6, 2009 R1200RT

  6. #6
    Nutfarm
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    Quote Originally Posted by b25bsaboy View Post
    Evening Kurt,

    Sorry for the late respond. What I'm I going to do with the 76 R 75/6? My thought was when I bought her is keep it and use it to attend bike functions in and around Calgary.

    I sold off my collection of Brit Iron and am down to three machines. My original 68 BSA Starfire, 2009 BMW R1200RT and the 1976 R75/6 that I am currently working on.

    I stripped the Vetter Fairing off the machine, as it looked out of place. I have stripped the complete frame down, powder coated the frame, added a second front brake caliper to bottom of the from fork and now have a rolling chassis. Next step is to get into the engine which I have started with the rear main seal being replaced and check the clutch thickness. Cleaned the engine cases and have the barrel's off and they are good to go as the ID is even from bottom to top. The rings need to be replaced as they are about .053 which is a wee bit much. No worried about fitting the rings, as I have an old English tool made just for that process.

    So I am am in the hunt for new rings and I will get the heads redone as the valve guides are to sloppy in my opinion. Biggest obstacle at this point is finding someone to walnut shell blast the aluminum covers, barrel's and head/covers. My next task.
    I would suggest you get some modern rings for it, Total seal makes gap less rings and 3 piece oil rings. They are better rings and cost less than OE rings.

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