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Old 05-15-2015, 08:17 AM   #1
JGT OP
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R50/5 new rings or rebore and new pistons?

My R50/5 is using about 1 quart of oil per 200 miles of riding. Quite high.

When riding at night, someone behind in a car can clearly see smoke -- most noticably when I am accelerating but also a bit when downshifting.

Compression is about 135 psi each cylinder. Leak down is about 30% each cylinder, and sound is coming from the oil dipstick. So seems to be rings are the culprit.

When I had the motor apart in the winter (it was not using oil then), measurements were done by a machine shop with good tools:
L piston 66.9468mm L cylinder 67.0687mm
R piston 66.9417mm R cylinder 67.0687mm

Both of these show piston to cylinder clearance at about the maximum allowable. I put the engine back together without new rings or a hone, hoping for the best, but obviously that is not what I got. Maybe cleaning the carbon off messed up some delicate balance of sealing. I am confident that rings were put in correctly, in correct orientation, and in the correct cylinders.

I don't want to bore out the cylinders to a larger displacement (600 or more) because I like the smaller engine.

Seems like my options are rings and a hone, or bore out to first oversize and get new pistons.

Given the maxed out clearance between piston and cylinder, would rings and a hone get me anything more than a very temporary fix? Should I just go for the full oversize rebore? Existing pistons appear to be in good shape when I had them out. Not a lot of scuffing.

Thanks for any opinions on this.
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Old 05-15-2015, 08:36 AM   #2
Solo Lobo
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If you are going to keep and ride this one, just do the oversize.

Rings and hone may improve things, but it would be a temporary fix

Brings to mind the saying "good money after bad"

I'd add that I did re-ring an R60 once and ended up buying pistons about 300 miles later.
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Solo Lobo screwed with this post 05-15-2015 at 09:29 AM
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Old 05-15-2015, 09:17 AM   #3
Stan_R80/7
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I'm in the try new rings corner. A new set of rings looks to cost ~ $100.00 (OEM, BMW). A hone is free from a tool loan. The time for this work is ~ 4 hours.

The cost for new pistons is ~ $520.00. A machine shop will need to be located which does not employ a dufus which will damage your cylinders. The time for this work runs into multiple days or weeks, depending on the machine work.

I am inclined to patch up the problem, ride the bike during the spring and summer and evaluate the situation. If oil consumption is still too high (and that values is subjective), then buy a new oversize piston set (which comes with rings) and seek out a competent machine shop. But, that's just me.

Stan_R80/7 screwed with this post 05-15-2015 at 09:39 AM Reason: added wrong
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Old 05-15-2015, 09:54 AM   #4
983Bob
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R50 oil consumption

If it were my bike, I'd use aftermarket rings if the cylinder wall isn't scuffed or grooved. Hastings has a size and application chart for most bores and your rings are 1.75, 2.00 and 4.00 mm in width. You can specify a coated cast iron or moly compression ring and three piece oil ring . They usually come in sets of four, but since the price is often not much more than one side from BMW, you are still ahead. Check your pistons to make sure that the ring lands aren't badly worn. I use these rings in my and my wife's 900s and oil usage is minimal. [.25L-5000Km on motors with about 400k Km each]

As an aside, I'd also check the valve guide clearance. With that kind of oil usage, the motor is probable high mileage and if the valves have been running loose for a long time, you run the risk of one breaking. [Especially exhaust]
This will really ruin your day.
The R50 is a nice bike. Good luck with it.
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Old 05-15-2015, 11:22 AM   #5
thetubespoke
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Hi Bob,

I don't have the best of opinions for whether to get rings or whole new pistons. Does the bike feel down on power? I'm not sure if your compression figures are close or not, but you might be able to try running some premix (1%, 2% at most) and see if it behaves any differently. I imagine that won't help with the oil consumption though.

I have an R50/5, myself. Why do you prefer the smaller motor? I can see some advantages:

Easier to start
Less strain on drive-line
Possibly longer life / even more overbuilt
Much harder to run into limits of chassis and brakes
Smoother?

I think fuel economy is supposedly the same or better on the R75, I think due to a taller gearing in the final drive.

Sincerely,
TheTubeSpoke
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Old 05-15-2015, 11:33 AM   #6
JGT OP
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I guess I prefer the smaller motor for its relative rarity, and for its reduced likelihood of getting me in over my head with acceleration and speed (my abilities are limited).

Can't say if it feels down on power, as I don't have a lot of experience with other BMWs. It is certainly sluggish off the starting line -- I have to rev it up, let out the clutch, then it gets going and relaxes a bit and then gas it up again. It doesn't feel like it's going to die or anything, but it certainly is no jackrabbit. But again, I am not looking for a fast bike, just one that is sturdy and does not burn oil like this. It will get up to 70mph or so on the highway but that is topping out and faster than I want to go anyway.

How much of a black art is boring the cylinder? Many years ago I had this done for an old car engine just by finding a reputable machine shop. That was before the internet and all the opinions (very good and useful) that give one doubts. There is a good shop here in Nashville that seems to do a lot of race car engines among other things, but they are not motorcycle or BMW specialists. They said they would use a torque plate and did not seem scared by me asking about a bore and hone. They did the measuring on the cylinders when I took them in earlier.
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Old 05-16-2015, 07:18 PM   #7
bmwrench
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Something Supershaft said (over and over) was that BMW rarely matched their own specs. If your cylinders are just at the maximum, I would seriously consider honing and new rings, assuming the ring grooves are in acceptable condition. Take a look at the thread about oil usage in a rebuilt engine.
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Old 05-16-2015, 07:42 PM   #8
Bill Harris
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I'd side with 'Wrench on this. New rings and a judicious hone may be all that is called for. The only thing that you might get from the max'd out piston clearance is a bit of piston slap on a cold engine, which simply adds charm to the beast.

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Old 05-17-2015, 08:42 AM   #9
JGT OP
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Ok, this is all useful information.

If I do with the new rings only, do I just get the standard (not oversize) rings? That is all that is generally available it seems.

This approach makes sense if it would buy me a couple of years, as I may be up for a more major engine rebuild (heads, bearings, etc.) by then but I just would rather not take it on right now. If I had to rebore at that future date, it would be ok.
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Old 05-17-2015, 01:53 PM   #10
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I never could figure out when supershaft was being facetious, and when he was saying something factual. His (continual) comments about BMW's tolerances being completely arbitrary and their published values inaccurate never made much sense to me. That statement implies BMW is a less competent company than FORD - which I don't think is true. That topic was one of several which was full of inconsistencies.

Having worked with many types of engines and their specifications and tolerances, I've yet to find one in which the published values were inaccurate. That's not to say the engine would not operate OK when out of tolerance, but that the BMW motorcycle spec's were not worth using is a very bizarre statement and raises the WTF flag.
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Old 05-17-2015, 03:53 PM   #11
Bill Harris
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My take (and I'm not trying to mind-meld with the SS) is that BMW sometimes has parts that are within spec but have "tolerance stacking", so they have to improvise. I've always looked at this as "old-world craftsmanship" and not cookie-cutter mass production. Like on my old lathe, if you need to take a fuzz off, you don't dial in 0.00003", you'll tap the cross-feed dial a couple of times and try it. Crazy, but it works...

YMMV. :)

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Old 05-18-2015, 06:42 AM   #12
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If I do go with just new rings for now, would the standard size work or would I need oversize rings? The bore is "standard" (B size) but it obviously worn somewhat.

It appears that complete sets of standard size rings are available, as are some of the oversize in individual rings.
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