R51/3 pistons "mystery"

[schrader7032]

Michael -

I can't help with the markings...but I'd measure the diameter of the piston just to convince myself that is the actual size. If it is, then that's 5th over?? I can't imagine there would be enough meat in the liner to stand that many oversizes. Measure the bore as well. Is it possible that the cylinders are from another machine?

I've looked through the standard bore sizes and a couple of bikes have a bore close to that - the R66 of 69.8mm and the R61 (sidevalve) of 70mm both produced 1938-1941. But anything after the war was either 68mm or 72mm.

That's definitely a mystery...

Kurt in S.A.

[EuroIron]

http://www.realclassic.co.uk/techfiles/ ... 31600.html

follow this link and perhaps afterwards your questions may change a bit?

I'll suggest I've read nothing in your post that leads me to determine your cylinders do in fact need boring as this requires at least some basic cylinder measuring with precision tools unless there is visually detectable galling or wear resulting from a myriad of possible failures

it is quite possible you could even have some amount of measureable wear yet slight enough that a good honing to fit even slightly larger pistons would have you good to go....... ie..... 70mm or so

I'll also suggest those cylinders have enough meat to bore out 60 thou or 1.5 mm past standard and also that pistons larger than the nearly 70mm pistons you have are quite likely still available

of course, by the factory recommendations, or anyone who strictly adheres to factory recommended overhaul practices, it is quite likely your engine would be instantly deemed in need of new cylinders and pistons

for example, I am reasonably certain if I were wrenching your engine and was doing so at the BMW dealership I worked at some time ago, your engine would get new cylinders and pistons if the specs in the book said 69.95 was beyond factory limits....... and pretty much that is how it would have to be dealt with under BMW's flag

my personal opinion is that I'd not be the least bit concerned punching one of these out as far as 2.5mm or 100 over unless it was to be used for sidecar work or other stump pulling type duties which did not allow for higher speed airflows over the cooling fins

even then it would probably work just fine and last a long time

can you post good macro pix of what you have scattered on the bench?

do you have an appropriately sized dial bore guage and a set of feeler guages? You'll also need to measure your pistons as well. Again, all moot if you have damage in the bores or even a fingernail catching ridge at either end of the bore........ but whomever would be boring your cylinders would need to have the pistons to be fitted and good measuring equipment as well.

I think when you read the fundamental writings about the cylinder bore work, you'll likely be able to relax a bit and realize these parts are merely metal cylinders with alloy pistons which happen to fit on a BMW motorcycle.

nothing mystical or even to be so revered that any number of possible non-oem parts would not serve you very well.

The only advice I can offer is if the condition of your crankshaft is questionable, or there is visible physical or feel-able evidence that would make the condition of the crankpin, rods, or their needles.... seem dubious

then discretion would dictate a crank overhaul but there are people who can feel, listen, and look being able to say this should be good to go or otherwise

most needle bearing bottom ends do have specs to dictate the upper wear limits for these items and a simple procedure for measuring

I haven't cracked the manual to see what these numbers are for your engine but I am reasonably sure they exist, sounds like you are good in at least one measureable plane

[Darryl.Richman]

First off, the original bore size was 68mm. Each oversize bore is 0.5mm, so you're at the 4th oversize right now (eg, 68.5, 69.0, 69.5, 70.0). (These bore sizes are approximate, they should take into account the size of the actual pistons you've obtained and the piston manufacturer's recommended clearance.)

I believe that, at the time the bike was new, BMW only recommended going to 3rd oversize. However, Mark Huggett supplies 4th oversize pistons for these bikes.

I don't have the experience to tell you what will work here. I would call Craig "Vech" Vechorick and ask him (Google Bench Mark Works or find him in our links over on the main web site.) He will give you the straight answer.

My own R51/3 is on 3rd over pistons, and I do worry about the day that I need to replace them. I'll be interested to hear what you find out.

[Micha]

Thanks a lot guys, you're great. You always reply and try to help. I really appreciate it. I e-mailed Mr. Vechorik and do hope he will reply. I need to rebore because there is much play between the piston and the cylinders wall. The rings are new with no tolerance, probably a desperate try of previous owner to make the machine live a bit longer. There is "trauma" evidence on the pistons body and the cylinder walls look exactly like my worst day in my life.
So now I wait hoping Mr. Vechorik will answer.
All the best to you all. Micha.

[EuroIron]

realistically, short of having the cylinders in one's hands

there really isn't much anyone can recommend other than to safely say in light of what you've posted about them, you need cylinders or relining

most all in the BMWrestoration industry would not agree that running them 2.5mm over is either safe or prudent so

I'll offer the basis for my opinion an even lightly qualify it prior

Certified Master Tech and also BMW(and others) factory certified

I own and operate a reasonably complete machine shop and even though I am quite capable of boring your cylinders, I am not seeking any such work

So, having run many similarly sized engines bored even more resulting in even less cylinder wall thickness than would result from poking yours 100 over....

and I am talking about engines with less optimum physical designs than the BMW boxer that were much more highly stressed due to running very high compression ratios, and they put out no less than 2 times the horsepower at the rear wheel as that boxer is going to put at the crank

ie........ the most radical was a HPI 12:1 CR 940 big bore norton with maximum area cams having duration in excess of 300 degrees... which requires poking a 750 Commando out to 940cc and I'll tell you, there is precious little room between the resulting bores for head gasket material and the walls are thin even after lining, less than 85 thou thickness

but that engine was very durable just as poking 650 Triumphs 140 over to 736....

now neither cared for summer parade type use but a 650 Triumph bored 100 over will tug a side car or even heavier trike rig just fine

and last a long time

so having inspected 500 and 600 cc cylinders

I can see no problem whatsoever poking out this large and running a mere 10:1 CR or less, especially with reasonably mild to stockish cams, and the net result being less than 30 horses at the crank

besides, they have far superior cooling too boot

[rennsport]

Sp 0.05, then there's a sign or so stamped, something that looks like a flag on a stick (not the KS factory sign, I know that one) and then 69,95 and another sign stamped, something like a capital N (?) and then 5.

The Sp 0.05 is the assembly clearance and the sign should be a direction arrow for the assembly direction. The N5 is for the date of manufacture. The info on the piston head is given for largest skirt dimension and ideal piston clearance. The nominal dimension of the cylinder bore is derived from the two dimensions.

69.95 mm = Piston
0.04 mm = Sp
69.99 mm

[VBMWMO]

I took the cylinders and pistons off the engine this morning, and unfortunately understood that there's no other way then going to a rebore and fitting new pistons. The old pistons are KS items and it is written on them:
Sp 0.05, then there's a sign or so stamped, something that looks like a flag on a stick (not the KS factory sign, I know that one) and then 69,95 and another sign stamped, something like a capital N (?) and then 5.
I will be thankful if you experts can shed some light on these pistons and help me out with some questions:
1. If there are, as far as I know, 3 possible oversizes (68.0, 68.5 and 69.0) for these motors – how come is this piston stamped 69,95? Are there more sizes?
2. If there are, what size of new pistons should I order now? Isn't it going to be now a new liner and back to STD?
3. There is no play on the connecting rods as I try the push – pull test; the rods are a tight fit on the crank inside. Must I even though replace the bearings inside? Is there a danger of bearing collapsing, as they can not compete with the forces produced by the new pistons and rings? Opening the engine and overhauling the crankshaft is not really what I plan to do next…
4. Can some one please tell me the KS part number for these pistons? They do not appear at the KS Pierburg site.

Many thanks in advance for any info,
Micha.

[ctydenhat]

I need to replace a piston ring set of BMW R50 STD (68mm), I see on ebay have a set which is use for R51/2/3 but it have same dimension 68mm (Link ebay).
Everyone help me that they can use for R50 or not?
Waiting help for my R50
Regards