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squirrelscoundrel 01-07-2010 11:37 AM

a bunch of airhead questions for all you experts. Thanks in advance.
So I have my bike in pieces and am wondering what to do next. The engine is out, I have a gasket kit on the ready but am wondering about other jobs that I should while it is apart. History: 70,000 miles 1974 r75/6. The bike sat for 5 years. previous owner said it was well mantained... who knows

How do I know if the timing chain needs to be replaced? Can I know this without taking the whole engine apart?

How do I know if the cam shaft bearing needs to be replaced? Can I know this without taking the whole engine apart?

The cylinders look really shiny and are free of any visible imperfections (this is my first time ever seeing a cylinder, but I am amazed by how clean they are) So should I do anything to the rings or the pistons? i.e. hone and replace?

I have attached a pic of one of the cylinders...if it helps.<a href="" target="_blank"><img src="" border="0" alt="Photobucket"></a>

Thanks a lot for the help. This is the link to my thread. Thanks again!


ChromeSux 01-07-2010 12:29 PM

Anton or some of the other guys can give you the exacts but if i went thru all the trouble and took it down as far as you have i would go ahead and do the entire motor too, i know there are many BMW out there with allot more miles on the motor bottom end but if you are going to keep it why not do it right.

crazydrummerdude 01-07-2010 12:34 PM

When I saw my first cylinders, I was surprised how shiney they were, too.

With a more mature eye, I can now see carbon ridges and imperfections, etc. We'd need a better pic to assess yours.

If you're on a budget, like I've always been, I would not worry about the bearings you mentioned just yet. I'd ride it and keep an eye/ear out, for sure, though. If you have infinite time/money, or infinitely more than me, sure, tear it completely down while you're in there. Then, you at least know what you've got when you've got it all back together.

What I did to both my resurrected BMW's was hone the cylinders and put in new rings.

batoutoflahonda 01-07-2010 12:49 PM

Timing chain

more info from Snowbum:

DUNDERHEAD 01-07-2010 12:58 PM

Did you try to start the bike prior to tearing it down ?

Wirespokes 01-07-2010 01:12 PM

That's the best reason to start it up before tearing into it like this - a running assessment will point out things like smoking, rough running, low compression, knocking, tapping, scraping or other out-of-place sounds.

The timing chain may be ready or it may not, but it's easy enough to get at with the engine mounted. But this is about the mileage they tend to go, so could be a good time to replace it. And they're cheap, so why not?

I wouldn't worry about any of the bearings, if it wasn't run low on oil they should be fine.

As for the pistons and rings - there's another item that's just so easy to get at that I wouldn't worry. If it's working, don't fix it! I don't see any scraps or broken stuff, so just clean it up, replace the push-rod seals, stick the pistons back in and go. The head gaskets are re-useable, so the whole thing could be torn back apart next week if need be if for some reason you needed to get back in there, without needing to replace gaskets.

Most of these parts last so long, that even if it's got only 20K life left, that's three or four years for most riders. Why get impatient? Spend the money on a good set of tires, or nice rear shocks, or for some better riding gear.

squirrelscoundrel 01-07-2010 02:43 PM

Thanks for the info. I think I'll at lease replace the chain and based on the articles, possibly the sprockets , depending on how they are worn. Still thinking about the rings....they are right there and easy to get at should the bike start burning oil. I just don't want to have to take the whole thing apart again to get to the chain at some later date.

I ran the bike for about a half mile before disassembly and it was the first airhead I had ever ridden, the first I had ever even seen up close and as far as knowing what to look for and listen for, I really had no idea.

Thanks again

carpetburn 01-07-2010 03:31 PM

when i bought my r75/6 i got it running when i got it home[that was my motivation] and then pulled the barrels to have a look, i did a valve grind,hone and new rings and also replaced the bigend shells so i knew where i stood, i did the timing chain at a later date- i should have done that at the time of the other work as the bike idled and ran far better[with new chain]. All this helps you to know your bike inside out, this was my first airhead.

mykill 01-07-2010 06:40 PM

Good bike goes basket
I wouldn't worry about the chain yet. Pull the oil pan and see how much goo is in there and read it like a gypsy reads tea leaves. If the sludge is minimal, I would leave as much as is as possible. Since the jugs are off and pistons out I would measure the ring gaps, if in spec, a quick hone and back together. Big end shells would probably be a good "While I'm in there" but you will need to purchase new bolts and a 12pt male wrench to do this. If there is no in and out play then they are probably good, there is always a bit of deceptive side-to-side play.
If you cannot help yourself check out the heads. With heads off, inspect the mating surfaces of the head-to-barrel for head gasket failure, I'd personally renew, but the existing may be just fine.
Look at the exhaust valves for recession in the head. If the valves are still "proud" in the head leave them alone. If it makes you feel better, remove the valves and inspect seats and edges. You may want to lap them in to clean up a bit.
Don't get too deep too fast. Be patient and diligent. The torque wrench is your friend and the BMW engineers knew what they were doing, follow directions.
After it is back together and running, time it with a strobe light. A bouncy timing mark at idle is a sign of a loose timing chain or a sloppy advance mechanism. The chain will most likely be noisy if it is an issue.

Wirespokes 01-07-2010 06:56 PM

Good advice on pulling the pan.

But I see no reason to hone the cylinders if the same rings go back in. They're already seated, and just because the piston was removed doesn't mean they won't seat back in as before.

The 750 is a pretty easy-going engine so I woudn't get too concerned about big-end bearings - not unless you find a bunch of sludge in the pan. These bikes run at really high oil pressure and the bottom ends are practically bullet proof as long as they've got clean oil. They'll last hundreds and hundreds of thousands of miles. Proof? Last I looked, crankshafts new were about $500, probably more like a grand now. It's hard to give them away - nobody needs em.

The problem with measuring the end gap is removing the ring from the piston and then re-installing. Those things are brittle and it's possible to break at any time. If the bike ran decent - idled ok, didn't smoke, I'd leave them. It's pretty normal for the cast-iron cylinder bikes to get somewhere between 1K miles to 2K miles to a quart. Just to give you an idea.

Jasper ST4 01-08-2010 06:20 PM


Originally Posted by squirrelscoundrel
Thanks for the info. I think I'll at lease replace the chain and based on the articles, possibly the sprockets , depending on how they are worn.

I doubt very seriously that you would need sprockets, and it's no small job. Mine has 170,000 and they're fine. I'm glad I didn't have access to a lot of this stuff when mine was growing up, I would have been paranoid half the time.

Carrizojim 01-09-2010 08:06 AM

Just checked your other thread, my bike didn't look a whole lot different form yours. I paid $800. Took me about 3 days to get it running well.

I've got a 1977 R100/7 that was sitting outside someones house here in Cali. It wasn't running when I bought it. The outside of the bike was weathered pretty good.
It had leaks around the base of the cylinders, leaky pushrod seals and the sump. I'm not sure if it has 27,000 or 127,000 miles. The barrels looked great when I pulled them. I did nothing, got some new pushrod seals. I got Viton seals instead of the factory rubber because they don't deteriorate as fast and put them back on. Got the seals from Bench Mark Works, another great source for parts. Most of their rubber parts are better quality than the factory stuff.

No smoke or problems, plugs look great. Runs awesome.

I do need to do my cam chain, because it's pretty noisy. I wouldn't do it if it was quiet. When you re-assemble it, and you have access to a timing light. Check the timing mark and see how steady it is. if it's blurred you need to replace it. Got my parts from Ted Porter's Beemershop. Everything cost me about $90. I bought chain, tensioner, spring and gaskets. I'm waiting to get it apart to see if i need a sprocket ($150.00).

Just so you know, my biggest issues were with the carbs. I would see if it will run decently before tearing them down. I would pull the float bowls and make sure the jets are clean. I did a complete overhaul on mine including new floats and slides. It was rediculously expensive, but I wanted that problem to go away permanently.

The guys posted up links to Snombum's website. Excellent source of info. Follow it, the guy is an Airhead guru... Spend some time reading through it, It pays off. Keep us posted!

One Less Harley 01-09-2010 08:34 AM

If you are replacing the timing chain,DO REPLACE THE TIMING GEARS, you always replace the gears and chain as a set, not doing it is a false economy. You will want to do the timing chain tensioner.

As others have said check the ring gap and if they are w/in spec just put them back in. Decarbonize the heads and pistons. I read somewhere that you'll want to leave the carbon that is just above the top ring on the sides of the piston (read in the Clymer manual). Something about burning oil if you remove??? Look for scratches on the piston, if you have more than a few then you will probably need to have the cylinders mic'ed.

When you drop the oil pan, remove and clean the strainer and make sure the pickup is tight.

Go ahead and replace the rod bearings whether they need it or not. If they look beat up then you've got a problem. My rod bearing from a 100,000 mile G/S looked great, but I still replaced them.

Here's a post you might find interesting.

bpeckm 01-09-2010 08:34 AM

The only thing that I can add to the advice above is that we all tend to be too compulsive about finding potential problems... as mentioned, these bikes run for a long long time, and are so easy to get back into, that I would reassemble, maybe do the cam chain, ride it to get to know it better, and you will most likely be surprised at how little it really needs....

I read a book a long time ago that made the point, over and over, that he was always worried about something breaking.... and that, in the end, the worry was always about the wrong things anyway...! Something else broke!


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