Parts sourcing - Top-End Rebuild?

[kmisterk]

Have you tried Motobins or Motorworks in the UK? UKP33, USD40.

Btw, which model? It helps anyone reading to add it to the initial post.

I’ll check them out!

Also, I added the model. It’s an ‘80 r100t

[Rob Frankham]

*** Warning ***

Before you even think about buying big end bearings you need to dismantle the engine, measure the crankpins to see whether they have been reground, whether they need regrinding or whether the crank is scrap. There are no fewer than sixteen big end bearing sizes, four for standard crankpins. 4 for 1st regrind, 4 for second regrind and 4 for 3rd regrind. Measuring the pins will reveal one of the following situations...

1) The pins and the bearings are fine and the noise is from somewhere else, in which case you don't need new shells at all
2) The pins are fine but the shells are worn. New shells of the relevant size required.
3) The pins are undersize, barreled or ovaled but are suitable for regrinding. The 'grinder' will normally supply correctly sized shells.
4) The pins are worn beyond further use. Replacement crank required.

Rob

[gspd]

Can't say 1000%, but my experienced ears would bet ($100?) that the knock in the video IS a big end bearing knock, nothing else sounds like that. Even on shitty computer speakers. Only an autopsy will confirm that.

*** Warning ***
There are no fewer than sixteen big end bearing sizes, four for standard crankpins. 4 for 1st regrind, 4 for second regrind and 4 for 3rd regrind.

Please elaborate....Where did you get this info????
Stock type 247 crankpins are all the exact same size and there are only 4 sizes of shells available:
1 - stock
2 - .25 over
3 - .50 over
4 - .75 over
Obviously you need a total of four shells, two per connecting rod.
The depth of the damage to the crank, hence the amount of material that has to be removed, determines the oversize needed, not the amount of times it is reground.

The pins are worn beyond further use. Replacement crank required.

This is a definite possibility as very few shops (if any) on the planet can perfectly regrind these cranks.
A special machine is needed. They are too short to fit on all conventional (universal) crank grinding machines, and the critical radius-ing at the edge of the pin is almost impossible to duplicate correctly. There are also issues with proper heat treating of the reground part. A very experienced technician is also needed.
Reground cranks that didn't break prematurely are very, very rare, therefore I suspect kmisterk's crank has never been reground.

[Rob Frankham]

Can't say 1000%, but my experienced ears would bet ($100?) that the knock in the video IS a big end bearing knock, nothing else sounds like that. Even on shitty computer speakers. Only an autopsy will confirm that.

Please elaborate....Where did you get this info????
Stock type 247 crankpins are all the exact same size and there are only 4 sizes of shells available:
1 - stock
2 - .25 over
3 - .50 over
4 - .75 over
Obviously you need a total of four shells, two per connecting rod.

You're right, I was misreading the data for the main bearings... not that it makes a lot of difference, 4 or 16, you still need to know which to buy.

The depth of the damage to the crank, hence the amount of material that has to be removed, determines the oversize needed, not the amount of times it is reground

OK, I accept that point but it is normal to refer to them as a 1st stage to 3rd stage regrind, I just omitted the word 'stage'... again, I don't see that it makes much difference to the point, which is that you can't order big end shells before you know the state of the crankpins.

It is perfectly possible to regrind an airhead crank and have it give satisfactory service provided it is done by an expert engine rebuild service with the necessary equipment. I acknowledge, though, that many are not done well and can fail. A 'proper job' will be expensive and good second hand used cranks are probably a better bet...

Rob

[/quote]

[gspd]

You're right, I was misreading the data for the main bearings...

I always knew that deep down you had a human side

you still need to know which to buy.....which is that you can't order big end shells before you know the state of the crankpins.

All you can do is order the stock shells and cross your fingers that the crank is only slightly blued and will hand polish up satisfactorily. You do the polishing with compound and a shoelace and hope for the best. No amount of polishing will reduce the crankpin O.D. enough to fit even the smallest oversize shells available; that would require a crank regrind. You won't remove too much material from the hardened surface by polishing. As far as measuring things, forget it. Measuring any used crank is an exercise in futility. If you precisely utilize the best measuring tools, you'll soon realize that every used crankshaft in the world is out of round and out of spec to a certain degree. That doesn't mean it won't work 'virtually' forever. Precise measuring is only required during and after regrinding to correctly purchase and fit the new shells and bearings.

before and after.jpeg (25.64 KiB) Viewed 940 times

It is perfectly possible to regrind an airhead crank and have it give satisfactory service provided it is done by an expert engine rebuild service with the necessary equipment. I acknowledge, though, that many are not done well and can fail. A 'proper job' will be expensive and good second hand used cranks are probably a better bet...

Can you provide a link to one of these 'experts'? I can't find one, and god knows I've tried. This is an extremely specialized job and needs a very special machine and/or a very unique setup of a conventional machine. The short airhead crank won't fit any commercially available crank grinding machine currently in production. Theoretically an extension could be fabricated to extend the crank and make it fit but that would be very costly and somewhat imprecise. The oversize shell fiche listings don't really apply to airheads, they are remnants from the older BMW automotive parts fiche. Back then, (some) BMW cars used the same shells as our airheads. Pretty well anybody with a standard crank grinding machine can successfully regrind a BMW car crankshaft.

The main obstacle is that BMW type 247 airhead cranks are a one piece forging. Don't confuse them with /2 and most British motorcycle cranks that can be pressed apart, fitted with new journals and pressed back together. This is also a very specialized job, but tons of shops do it.

[kmisterk]

before and after.jpeg

The pics you've provided here and before are both *super* helpful.

I believe that I may have been mistaking the term "Big-end bearing" with "Crankshaft bearing." Just to confirm I am not mistaking this further, am I right to assume that the "Big-end bearing" is synonymous with "Connector rod bearing shells" in this case?

Reground cranks that didn't break prematurely are very, very rare, therefore I suspect kmisterk's crank has never been reground.

Based on what I know of the bike's first owner (I'm effectively the second owner, as the actual second owner rode it for a month and put it in storage), who was adamant on proper services, updates, fixes, and always took it to an airhead-specific mechanic (Formerly BoxerBarn here in the Pacific Northwest), I believe this to be true. Aside from my own negligence and ignorance thus far, the bike has been running really well, and only when I finally realize what I'm doing wrong in operating the bike have I revealed how good of shape the bike is in.

In any case, the bike was towed over to my parents ranch today, and I hope to be able to tear into it here in the next couple weekends. It's a busy set of weekends, with a trip or two in the mix, but...I really want to get her back up and running and healthy asap.

[gspd]

I believe that I may have been mistaking the term "Big-end bearing" with "Crankshaft bearing."

All connecting rods have a 'small' end and a 'big' end;
Obviously the SMALL END is the hole the piston wrist pin goes through.
The BIG END is the crankshaft end; Your con rods can be split apart to renew the 2 piece bearing shell(s) inside. You'll need a 12 point allan socket, readily available at any auto parts place, to remove the bolts. Get 4 new OE bolts if you can. You should never re-use these bolts, but I've done so more than once in the past without issues. When it's Saturday night in the paddock and Sunday is race day, and you don't have spare bolts, you do what you have to do.
The CRANKSHAFT BEARINGS are the bearings that locate the crankshaft in the engine case. Your engine has one at the front and one at the rear of the crank, but I wouldn't worry about them for right now.

I didn't hear it in person or witness what happened before the knock started, but based on my previous experiences with these engines, and your description of what happened, the shells will be scrap in one big end (it's rarely both) and usually the crank journal will be salvageable. You'll need some valve grinding paste, some rubbing compound and a long thick shoelace for this.

Let's not put the cart before the horse. Let us know what you find after tearing it down.
Still no takers on my bet, eh?

[jackonz]

I am in New Zealand and have done a top end rebuild on my R80/7, all the parts I have sourced have come from Motobins and Motorworks in the UK, very good service and pricing.

My biggest concern for you is damage to the crankshaft and if it is damaged replacing just bearing shells is going to be wasted time and money, I would suggest you somehow get the crankshaft journals measured before you shell out hard earned cash.

[gspd]

I would suggest you somehow get the crankshaft journals measured before you shell out hard earned cash.

This is still all complete speculation, we still don't know the exact nature or extent of the damage.

When and if a con rod bearing failure is confirmed, go ahead, waste your time and measure your heart out, Official BMW journal specs are 47.975 to 47.991 mm. Go for it! and good luck!
You'll have to remove the crank and you'll need super accurate tools (and know how to calibrate and use them) to achieve anywhere near credible results. Inexperience in this arena could very well be your downfall.
Now please tell me, at exactly what point does one decide to declare the crank 'unworthy'?
47.974? 47.973? 47.972? 47.971 ? 47.970? 47.969? 47.968? 47.950? 47.930?
What if you measure 47.973 in one plane and 47.975 somewhere else?
What if there's a tiny smudge or scratch that won't polish out?
Hint : If you were standing in front of me in a Armani suit at the Porsche service counter with a Platinum Visa in hand it could very well be 47.974...

OK, now let's get real for a moment....
These journals have an EXTREMELY hard surface.
The shells are basically a (relatively) very soft cushion between the super hard crank journal and the super hard conrod. If the journal polishes out nicely to a smooth finish, with no visible scoring or deep marks when observed with the naked eye, it will run virtually forever, even if it's technically a bit 'out of spec'
In my book, 47.9xx is acceptable and experience tells me that if it looks perfect it's still well within that range. No measuring necessary. A million old airheads are running around out there waaay looser than that.
Anyhow, don't trust me, I've only built (at least) a few hundred air cooled VW and BMW engines. And most of them are still running.

Worst case scenario: (very unlikely)
The shell has completely worn through and disintegrated away to nothing,
the rod turned blue and hammered the journal and tried to weld itself to the journal,
the journal looks like black and blue jagged shrapnelled scrap metal.
If this happened, the only thing you need to measure is your credit card limit, to see if you can afford a new or used crank.

[Rob Frankham]

This is an extremely specialized job and needs a very special machine and/or a very unique setup of a conventional machine.

Thats pretty much what I said... ', not going to get involved in a pissing match about how many engines you've rebuilt etc... Suffice it to say that I have known a number of reground cranks go on to give perfectly good service for years. I've also heard of (never seen) one or two that have failed not long after regrind. While I haven't been in a position to forensically examine those that failed, the assumption (not my assumption) was always that the regrind had been badly done.

In any case, I go back to the point of my original post... it's a bad idea to source new bearing shells until you know the state of the engine... Someone who works in the industry can just put them back in stock till the next job comes along but an owner who buys parts they can't use is out of pocket.

Rob