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Thread: 1973 R75/5 Cylinder & Piston Measurement Questions

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    1973 R75/5 Cylinder & Piston Measurement Questions

    Folks,

    The facts:

    I've read about removing the ridge at the top of a cylinder when replacing the rings. What I have read states the ridge is due to less wear of the bore at the very top of the stroke leaving more metal creating a ridge. How much more metal is too much I haven't been able to find out.

    I've have also read some statements that ridge reamers can inflict damage on the bore if not well made and properly used. One piece of advice was "never use them", as a ridge that would harm a new set of rings indicates there is so much wear you need to re-bore the cylinder.

    Opinions:
    1. Ridge reaming advice?
    - Skip it.
    - Do it
    * Good ridge reamer tool recommendation

    I've also took some measurements of the cylinder bore and the pistons. I used a bore gauge with dial micrometer (0.0005 inch minimum graduation) and 3-4 inch digital micrometer (0.00005 inch minimum measurement). I measured at the top, middle and bottom of the cylinders and at 4 different directions across the bore at each height. I measured the pistons on the wide side (perpendicular to the wrist pin hole) about 1/2 inch from the bottom of the skirt.

    These initial measurements show the cylinder-piston clearance is okay, the ovality is okay, but the taper is just above the maximum of 0.0004 inches on the left cylinder (0.0005) and as much as 0.00025 above in one direction on the right cylinder (0.00075).

    Here is a link to my spreadsheet summarizing the initial measurements.
    http://www.flickr.com/photos/brook-reams/8670617474/

    Opinions:
    1. Would ridge reaming remove enough metal to bring the taper back to specifications?
    - the maximum taper is 4 ten-thousandths of an inch (0.0004), and that's small.

    Dumb Question #467
    Is the BMW specifications for cylinder-piston gap the difference between the diameter of the cylinder and the diameter of the piston, or the distance from one side of the piston to the cylinder, which would be the (Dia. of Cylinder - Dia. of Piston)/ 2.

    Advice
    Any one have recommendations for who I would send the cylinders to for honing, or boring out, preferable in the Metro-Denver, Colorado area?

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

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    Administrator 20774's Avatar
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    Certainly not an expert on this, but I just did my R100/7 with similar miles and found that my cylinders were out of spec. Interesting that you have little cross-hatching left but for the most part your cylinders are OK. I might have expected more wear than that.

    As for the ridge, it was my thinking that the ridge only comes into concern if you had been running a base gasket and now you were going to remove the gasket. That means that the rings will travel that might higher into the bore. But if that's not the situation, why worry about the ridge? Isn't the ridge some remnant material from combustion that doesn't get continuously scraped off.

    Anyway, in my situation, I opted to not deal with any of that and went with an aftermarket kit to replace the entire top end.

    You might consider a place called Bore Tech which can apply a carbide coating to the bores which are similar to the Nikasil for the '81-on models. This might add a little dimension back to the bore. In order to do this, they would need the pistons for the bores so they can be sure everything will fit properly. Probably need to check with them.
    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!

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    over sized rings

    I do not know about the 750 rings, but on a 900 the first over size is to big for standard bore. the ring will be egg shaped and will not seal correctly. If you feel you need to put in something oversized check out Hastings web site, they have just about any piston ring you couldwant, and the woman that helped me was good at her job.

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    Quote Originally Posted by RS36VH View Post
    I do not know about the 750 rings, but on a 900 the first over size is to big for standard bore. the ring will be egg shaped and will not seal correctly. If you feel you need to put in something oversized check out Hastings web site, they have just about any piston ring you couldwant, and the woman that helped me was good at her job.
    Ah, well sure. But, I'll be grinding the ends to get an exact fit, so no worries.
    ---------------------------------
    Brook Reams - Arvada, CO
    Various Two-wheeled Vices, All BMW
    2004 R1150-RS || 2002 F650-GS || 1975 R75/6 || 1973 R75/5

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    Quote Originally Posted by brook.reams View Post
    Ah, well sure. But, I'll be grinding the ends to get an exact fit, so no worries.
    Isn't he suggesting that if too extreme, the resulting shape of the ring will become such that it will never wear enough to seat perfectly around? I can see where taking a 5x over ring (just an example) and cut enough off to fit into a stock cylinder, by the times the ends came back together, the overall shape of the ring is no longer close to being round.
    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!

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    Quote Originally Posted by 20774 View Post
    Isn't he suggesting that if too extreme, the resulting shape of the ring will become such that it will never wear enough to seat perfectly around? I can see where taking a 5x over ring (just an example) and cut enough off to fit into a stock cylinder, by the times the ends came back together, the overall shape of the ring is no longer close to being round.
    Well, sure, I suppose, if you cut 25% out of a circle and then tried to close the remaining 75% into a circle, you wouldn't be happy with the result. However, in this case, a cylinder bored one over is 0.5 mm larger in diameter than the original 81.96 mm +/- bore. So, the oversize ring has 0.5mm greater diameter than the original 81.96mm +/- circle of a standard bore. That's about 0.6 % increase in total ring diameter and circumference. As I grind some off the end of the ring to set the correct gap, each 0.1 mm I remove reduces the diameter by 1/3.14159 times as much or 0.032mm change in diameter. That means there is approximately 1/3rd the change in diameter for a unit change in length.

    Note some piston manufacturers supply "zero gap" rings so you have to grind them to the correct gap, and those gaps are on the order of 0.02 to 0.03 inches (0.51 to 0.76 mm) for large bore pistons. This is more material removed than I will need to remove.

    I'm not worried.
    ---------------------------------
    Brook Reams - Arvada, CO
    Various Two-wheeled Vices, All BMW
    2004 R1150-RS || 2002 F650-GS || 1975 R75/6 || 1973 R75/5

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    Quote Originally Posted by brook.reams View Post
    I'm not worried.
    Well, I guess that's all the matters!
    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!

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    Quote Originally Posted by 20774 View Post
    Well, I guess that's all the matters!
    Yeah, and I've said those words before; usually just before I learned something very important about life
    ---------------------------------
    Brook Reams - Arvada, CO
    Various Two-wheeled Vices, All BMW
    2004 R1150-RS || 2002 F650-GS || 1975 R75/6 || 1973 R75/5

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    Not an expert either, but the notion of a ridge and using a ridge reamer is more important than just the base gasket. If you did a microscopic look at the ridge and JUST BELOW it, you would see that the ridge doesn't form a perfect 90 deg. corner at its bottom. When new rings are installed, the very top ring will have a sharp 90 deg. corner on it. The rule of thumb was that the ridge needed to be removed so that the top edge of the ring didn't "crash" into the bottom rounded corner of the ridge. The ring could break.

    As for the quality of the ridge reamer? Don't know.

    In my past, the piston to cylinder clearance was total clearance. For example: if the piston measured 4.0000 (at the 90 deg skirt) and the cylinder measured 4.004 you would have 4 thousands clearance - .002 per side.

    Also rule of thumb (but not to be followed if you have mfg. specs) is that you allow about .001 clearance per 1" of piston diameter. This just gives you a NOMINAL number to go by.

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    Quote Originally Posted by JIMMYLEE View Post
    In my past, the piston to cylinder clearance was total clearance. For example: if the piston measured 4.0000 (at the 90 deg skirt) and the cylinder measured 4.004 you would have 4 thousands clearance - .002 per side.
    Thanks for the input. I also confirmed that the published clearance is the total clearance (diameter of cylinder - diameter of piston) and NOT the clearance on one side of the annulus created by the piston inside the cylinder.
    ---------------------------------
    Brook Reams - Arvada, CO
    Various Two-wheeled Vices, All BMW
    2004 R1150-RS || 2002 F650-GS || 1975 R75/6 || 1973 R75/5

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