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Thread: which size of rings do I need to buy?

  1. #1

    Thumbs up which size of rings do I need to buy?

    I was going to size my rings yesterday and then assemble the top end
    I finally have everything cleaned, honed, and ready or so I thought

    the bike is a 1971 R75/5
    pretty clean internally as it sat for 20+ years
    I've measured out the cylinders and everything has low wear on it and measured out with in specifications

    I bought standard "0" rings for it and yesterday when setting my gaps and prepping for assembly, I realized I would need different rings

    I believe on my model year the rings gaps are supposed to be:

    top .30 - .45mm
    seconds .30 - .45mm
    oil .20 - .35mm

    so my gaps with these brand new standard size rings are all out of spec.
    I'm in the approximate range of .55 - .60 (each of the rings measures slightly different)


    My question for the group is?

    I see that there is a first over and 2nd over ring set sold by the dealers
    how much (in mm) larger is a first size larger ring set... will it cover the approximate .20 - .30mm gap I need... to be within specification, or should I find/buy the 2nd over ring size?

    thanks a ton in advance

  2. #2
    Liaison 20774's Avatar
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    I'm interested to hear answers to this. I tried to do some circumference calculations to find an answer but I got confused.

    Also what confuses me is how the cylinder can be in spec but yet when original spec rings are installed, they measure too wide.
    Kurt -- Forum Liaison ---> 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
    for my /5 and /6 I went with Hastings piston rings in one over-size. The ends of the rings can be filed to get the proper gap. I called and spoke to the Hastings customer service folks to get the proper size rings.

    https://www.hastingspistonrings.com/

    Depending on the mileage and wear on the cylinders, it might be advisable to have them bored. One consideration is how the cylinder walls have "bowed". The basic piston engine turns linear (up and down) motion into rotational motion (via the connecting rods and crank shaft). When the piston is at mid-stroke the crank is at a right angle to the piston travel; when the piston is at either top dead center or bottom dead center, the crank is aligned with the piston travel. There is greater side wear on the cylinder walls when the piston is at mid-stroke.

    To measure the ring end gap and assess the cylinder wear place the rings near the bottom, mid-point, and top of the cylinder. If the wear is excessive, the cylinder can be bored and appropriate over size pistons and rings can be used.

    Or one could replace the old /5 cylinders, pistons, and rings with some newer nikasil-lined cylinders with new pistons and rings. I believe these are made by Siebenrock. I went this route with my /7 (a 1977 R100RS); the new cylinders, pistons, and rings came from Ted Porter and were obtained when I had him re-do the heads on the RS.

    https://www.siebenrock.com/en/produc...c/1100100.html

    https://www.beemershop.com/Merchant5...t_Code=1100085

  4. #4
    Liaison 20774's Avatar
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    I also bought a Siebenrock kit from Ted for my /7. But for the /5, a different kit is needed due to the smaller engine case opening:

    http://www.beemershop.com/Merchant5/...t_Code=1100072

    Might be a good idea to call Hastings...an older thread said they had good customer service. If you do call, let us know what they recommend for new rings.
    Kurt -- Forum Liaison ---> 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!

  5. #5
    B Reams brook.reams's Avatar
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    Quote Originally Posted by brant914 View Post
    SNIP ...
    so my gaps with these brand new standard size rings are all out of spec.
    I'm in the approximate range of .55 - .60 (each of the rings measures slightly different)

    ...SNIp

    thanks a ton in advance
    Hi Brant,

    Getting accurate cylinder measurements for taper and ovality (0.0004 in.) takes some care and a bore gauge with 0.0001 in. precision. Getting the cylinder bore measurement requires 0.0005 in. precision from the bore gauge. I show the tools I used and how I did the measurements and how to compute the ovality and taper in this post on my site.

    11 BMW 1973 R75/5 Measure Cylinders and Install Pistons & New Rings


    I have used one oversize rings and filed the ends to set the gap to the bottom end of the published range when cylinder wear is not enough to require boring them oversize. In that post above I show how I ground the ends of the rings to get the gap I wanted.

    Since the circumference of a circle is Pi*Diameter, or 3.1415*Diameter, every 0.01 mm (0.0004 in.) loss of diameter due to cylinder+ring wear creates a bit more than 0.03 mm (0.0012 in.) increase in the ring end gap. The published gap range allows 0.15 mm (0.006 in.) increase in the gap. That means the bore size can increase up to 0.05 mm (or about 0.002 in) before the gap is out of the published range.

    Based on your measurements, your cylinder bore would have increased by 0.05-0.1 mm (0.002-0.004 in) in diameter from the original bore, as that much increase in diameter would add 0.15-0.30 mm (0.006 - 0.012 in.) to the stock ring gap and your gaps are on the order of about 0.15-0.20 mm greater than published range (0.30-0.45 mm). Since a one-over bore increases the cylinder diameter by +0.5 mm, your wear does not seem to be enough to warrant boring out the cylinders and buying new pistons and rings. That assumes your bore measurements and ring gap measurements are accurate.

    When you measure the ring end gap, it's important that the ring be square in the bore. Using a piston to push the ring about 1/3 of the way into the bore from the bottom will insure the ring is square before taking the gap measurement.

    Now, one other interesting bit of information from Bob Fleischer, aka, Snowbum, on this topic can be found here:

    http://bmwmotorcycletech.info/cylinders.htm

    "... It is perfectly OK to use next oversize rings, adjusting the end gaps, when you have a bike with a good bore. If you do this, we shall assume the bore has low taper, but the rings are perhaps on the edge of high specifications or bit over. You can purchase an oversize set of rings and adjust the end gaps to specifications, but when doing this, use the smallest measured part of the bore ...usually near bottom, where the diameter of the bore in the lowest area that the rings normally contact, is the smallest ...adjust the new oversize ring gaps for that diameter. I don't usually bother replacing rings if the end gaps are just a bit larger than maximum specifications."

    Unfortunately Bob doesn't define ".. a bit larger than maximum" in this text.

    I hope this is helpful.

    Best.
    Brook Reams.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  6. #6
    Quote Originally Posted by brook.reams View Post
    Getting accurate cylinder measurements for taper and ovality (0.0004 in.) takes some care and a bore gauge with 0.0001 in. precision. Getting the cylinder bore measurement requires 0.0005 in. precision from the bore gauge. I show the tools I used and how I did the measurements and how to compute the ovality and taper in this post on my site.

    11 BMW 1973 R75/5 Measure Cylinders and Install Pistons & New Rings ....
    Very nice write-up. One question about honing the cylinders - did you consider using a ball-hone rather than the hone shown? I've understood that the ball hone offers some advantages, but never used one. It seems that you had good success with your hone.

    https://www.brushresearch.com/product-line.php?line=1

  7. #7
    You guys are Great!
    this is perfect info
    and Brooks, thank you so much for your time and help
    I use all of your articles... including this one that had led me down this top end work.

    If your ever in Longmont... needing a bite to eat, beer, or coffee... you would be most welcomed

    do my neighbor has a full mill and was nice enough to clamp down my cylinders and take very accurate measurement for me!!!
    I feel pretty comfortable with my cylinders and taper also was very low on one bank with basically none on the 2nd bank

    I have honed them, cleaned them, and feel pretty comfortable with them
    (if anyone needs to borrow the correct diameter of ball hone, I have one that is like new!)

    the one area I should have been more careful in would be what you pointed out Brook..... having the rings level in the cylinders when measured
    My piston is still connected to my rod... I used a can of something else from the garage to level the rings instead of a piston.
    I should probably double check that, and being out of level could cause the measurement to be inaccurate

    but I will seek out 2x over rings.... and I have the grinder !

    thanks everyone!

    brant

  8. #8
    B Reams brook.reams's Avatar
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    Quote Originally Posted by robsryder View Post
    Very nice write-up. One question about honing the cylinders - did you consider using a ball-hone rather than the hone shown? I've understood that the ball hone offers some advantages, but never used one. It seems that you had good success with your hone.

    https://www.brushresearch.com/product-line.php?line=1
    Rob,

    From what Tom Cutter has said, the best tool for honing a cross-hatch to seal the rings are the ball hones sold by Brush Research. They also make a ball hone that can be used with Nikasil cylinders to restore the cross-hatch and not damage them. That said, I have not purchased their hones, yet, but they are on my "new tools to buy" list.

    While on the subject of "hones", there are multiple kinds of them and they are designed for different jobs. There are rigid ones that you use to remove scratches without boring out the cylinders, and those designed to create a cross-hatch for sealing the rings.

    Best.
    Brook.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  9. #9
    B Reams brook.reams's Avatar
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    Quote Originally Posted by brant914 View Post
    You guys are Great!

    [snip ...]
    If your ever in Longmont... needing a bite to eat, beer, or coffee... you would be most welcomed

    I have honed them, cleaned them, and feel pretty comfortable with them
    (if anyone needs to borrow the correct diameter of ball hone, I have one that is like new!)

    [snip ...]

    but I will seek out 2x over rings.... and I have the grinder !

    thanks everyone!

    brant
    Brant,

    Well, yes I get up there pretty often as I hang out with Radar, who lives there, to kibitz on his projects. Also, if you aren't a member, please join our Colorado Airheads Beemer Club news and notices site:

    --> https://www.downtomeet.com/ColoradoAirheadsBeemerClub

    You mention 2x rings, but I'd choose the 1x over, or 0.5 mm over rings. If you go too big, you run the risk of the ring not staying round in the bore.

    Best.
    Brook.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  10. #10
    Quote Originally Posted by brook.reams View Post
    Brant,

    Well, yes I get up there pretty often as I hang out with Radar, who lives there, to kibitz on his projects. Also, if you aren't a member, please join our Colorado Airheads Beemer Club news and notices site:

    --> https://www.downtomeet.com/ColoradoAirheadsBeemerClub

    You mention 2x rings, but I'd choose the 1x over, or 0.5 mm over rings. If you go too big, you run the risk of the ring not staying round in the bore.

    Best.
    Brook.

    I will join the Colorado club today!
    I don't have a running bike and honestly... it could be a year out
    but I still would love to meet more local folks and attend a gathering or coffee or?


    I will place an order today and go with the one oversized then? I was worried that may not be large enough and still put me close to the wear tolerances
    however I just looked a bob's and Hucky's... it seems the 2 over rings are NLA. So that makes my decision a bit easier.

  11. #11
    just an update for those searching in the future.
    I did call Hastings yesterday
    the rep that helped me said they sold no BMW air head rings and have never sold them
    I explained that real people have used them in the past

    They explained that perhaps the past customer knew all of the measurements and details/design to buy rings they already make, but have never made rings for an airhead specifically.

    I probably just was unlucky with whom I spoke with
    but I ordered factory oversized after that call and skipped Hastings

    too bad really, I've used hastings rings on a 914 race car about 20 years ago with good results.
    brant

  12. #12
    Liaison 20774's Avatar
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    ...and their website doesn't list BMW motorcycles either...

    https://www.hastingspistonrings.com/ring-finder

    In these three posts from the past, it seems that the owner had to work specifically with someone at Hastings to get rings that met the specs they wanted:

    https://forums.bmwmoa.org/showthread...l=1#post622630
    https://forums.bmwmoa.org/showthread...l=1#post744337
    https://forums.bmwmoa.org/showthread...l=1#post216360
    Kurt -- Forum Liaison ---> 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!

  13. #13
    Quote Originally Posted by 20774 View Post
    ...and their website doesn't list BMW motorcycles either...

    https://www.hastingspistonrings.com/ring-finder

    In these three posts from the past, it seems that the owner had to work specifically with someone at Hastings to get rings that met the specs they wanted:

    https://forums.bmwmoa.org/showthread...l=1#post622630
    https://forums.bmwmoa.org/showthread...l=1#post744337
    https://forums.bmwmoa.org/showthread...l=1#post216360
    you have a great Memory for finding info!
    Very Nice! and what a great skillset in an admin

    I think you have confirmed that I spoke with the wrong person yesterday
    oh well... I ordered from Bob's... so will go with the stock brand

  14. #14
    Liaison 20774's Avatar
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    Can't go wrong with OEM! My philosophy is to go with OEM with I deem something to be critical or really internal to the bike. Aftermarket is OK for more external things that can be changed easily if it doesn't work out.

    As for finding things, I'm nearly into the CRS syndrome! I have been using google's search feature. I type "site:forums.bmwmoa.org" into the search box and then list the things I want to find. I added "hastings" after the site search string. It's best to have an idea of a specific search term..."pistons" will return too much info, but "Nural" will be much more specific.
    Kurt -- Forum Liaison ---> 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!

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