The New And Improved Unofficial R65 Forum V2
Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: jep on April 07, 2008, 11:48:15 PM
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Hello all. I had help from you guys about a valve job on a '82 that was helpful. But this question is reguarding a '80 R65 that I am doing a valve job on. The heads are back from the machine shop (those guys that advertise in Airmail) ready to be put back on. The compression was 110 and 90. The compression came up a little bit with oil squirted into the cylinders. A friend miked (spelling?) the cylinders and determined that they have a .003 taper. The Haynes manuel as far as we could figure stated that the tolerance was .001. My feeling is that I'm gonna put new rings in and not bore the cylinder and replace the pistons. Any thoughts on this? Any suggestion on rings? I have heard that there is a gapless ring set available some where. Anyone head about this? Thanks. jep
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It will really be false economy just slapping rings onto the pistons with that much ovility. Going 1st OS is the best and most reasonable repair, you will basically have a new engine. The new top end job will make engine blowby even worse by nature of the fact the combustion pressures will be higher than before. You will risk blowing engine seals as well, then the costs rise!!
Also, not honing the bores before putting in new rings will mean they won't bed in properly, then just watch it eat oil.
So come on, bite the bullet and do the job properly, you'll be glad you did.
Bill..................;-)
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Bill, yes I would absolutely hone any cylinder that I replaced replaced rings on. My thoughts, or hopes, that I could get away with not having the expense of new pistons. Sometimes you can push the limits. There are the specs in the Haynes manuel and then there is "real life". But if this bike is going to blow oil with a new set of rings, then it is off to my friends at Moto Bins. Thanks. jep
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Just a dumb question: Why is it necessary to hone the cylinders when you put in new rings?
greetings from a grey and cool north with excellent driving conditions
trolle
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Honing puts a cross hatch surface on the bore face. This acts much like a file that rapidly helps bed the rings. If it didn't do this, the rings will not seal effectively, allowing blow-by of the combustion gasses past the rings, and also inefficient oil scavenging of the oil control ring which will lead to high oil consumption.
Honing a bore is pretty well a science as crosshatch angle and honing stone pressures during the procedure decide how effective the results are.
It helps in other things as well, and can be still visible on well looked after high mileage machines. A mirror finish on a bore is not desirable!!
Bill..................;-)
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Flame suit on:
Conventional wisdom has always suggested honing cylinders when replacing rings. This is an urban [gear head] myth! Heres the dope. Honing a cylinder that is within spec will only create extra piston clearance and ring gaps, and an engine that will need a rebore and oversize pistons a lot sooner! If you need [out of spec] to rebore the cylinders then honing is the last step in the oversizing process. You'll need oversize pistons, rings assembled DRY to complete the process. If you rebore & hone you can go with chrome rings. If you're within tolerance get a nice set of cast iron ones assembled dry. Hone only a freshly bored cylinder, never one in spec that you're just slapping new rings on.
rich
who may or may not have bored cylinders for a living at some point.
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So, if one or both cylinders measure as out of tolerance, but don't seem to meet the criteria for forcing re-boring to 1st oversize, what should one do ?
OK, I have re-read the posts again, and it is now sinking into my addled gray matter....
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I guess I could look this up but what amount of expenditure am I looking up for new oversize pistons? This job is getting to be like the layers of an onion. jep
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Here's a link that pertains to what Rich is talking about:
http://www.meridentriumphs.20m.com/rants/dnthone.htm
I have NOT done it this way; my only motor rebuild needed a rebore so this procedure was not an option. It does make sense to me, but that's not the same as it being true.
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Ed, you have outed me as a disciple of Pete Snidal. I've tried several assembly techniques and his advice werked on my Britt rebuilds. I just can't seem to find enough worn out BMWs to try it on!
Motobins has Pistons listed as: 650cc R65 (+0.25mm) 1ST O/S for £78.00 Which converts to $153.987 USD as of this evening. Each piston comes with rings, gudgeon pin and circlips Whoo Hoo!! Ring set alone is considerably less
650cc R65 1978-80 STANDARD £16.00 or $31.5975 USD also as of this evening.
(https://bmwr65.org/smf/proxy.php?request=http%3A%2F%2Fwww.motobins.com%2Flibrary%2F037.jpg&hash=2f71d18751c4a9c2997be55c9a947a3f9eae462b)
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Thanks, I have had good luck with Motobins in the past. This is a great forum! jep
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Although not convinced that re-rings shouldn't be honed, or that mirror bores are ideal (what can the oil cling to??), I'm open to more information (for one thing I'm fairly open minded, the other is I can't find anything else on the net about it!!).
Most of my experience is VW in the '70's, honing after re-rings or re-bores etc, and never experiencing any problems doing it, in fact being associated with some killer Beetles, as well as some very high mileage ones, so convention rules my experience/thinking. I'm just having my 90/6 jugs done at the moment. One side needed a re-sleeve (rust from standing for 18 years), the other just taking out to 1st OS (same as the sleeved one obviously). It will be honed, and I intend running it in hard with plenty of on/off throttle as per this advice http://www.mototuneusa.com/break_in_secrets.htm
Now his advice goes against conventional wisdom, but is starting to gain much favour and know several who have used the technique and now have really sweet engines. Maybe the same for your guy?!?!?! BTW, MotoMan recommends honing for re-rings and better break in!!
I might throw this to the wolves over at Boxerworks for comments later on, see if anyone else has heard about it.
Bill..................;-)
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After going to the site it rang a bell, so to speak. I think this was discussed at some length at boxerworks several years back...