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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Superfly on October 09, 2015, 10:23:16 AM
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Hi, all.
As part of the resto of my 1985 K100RS I'm planning this winter, I would like to do a complete teardown and rebuild of the engine for two reasons:
1) I have never done this, and would like to learn.
2) I bought the bike with 21K on the odometer, however the speedometer has been broken for at least 20 years according to the service records. I would like to get a look at the insides to assess the engine's condition and get it back to as close to zero-miles condition as I can.
With that said, what do I need to know?
Are there any rebuild kits for this engine...rings, gaskets, etc? Besides the rings and gaskets, what other parts should I replace with new ones while the engine is apart?
I've read that the output shaft for this model year is not as robust as later years due to it having only 1/2 of the rivets. Are the 12-rivet output shafts directly replaceable for the 6-rivet models?
What other stuff should I be doing while I have the thing torn down? I plan on sending the block to an engine shop to have the cylinders honed, if needed.
Like I said, I'm new at engine rebuilds and throw myself at the mercy of this forum for any tips, tricks, avoids you can recommend. If you were rebuilding a K100 engine, what would be the things tops on your list? What considerations would you make sure to answer before undertaking something along these lines?
Thanks, everybody!
Brian
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Look at the spark plugs. Test the compression. If it ain't broke, don't fix it.
Tom
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greetings superfly...
im thinking you oughtta choose another engine for your 1st rebuild...
butts if you must... doo your doo dilergence on that cylinder honing... the brick aints no chevy 350 4 bolt main... its gotts some stuff in there you may wanna leave alone...
looking forward to reading all about it... keep us informed with photos...
j o
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yeah what they said . cylinders are nickel plated at factory and were not intended to be honed. your going off the deep end . If you have time to kill try home brewing .at least you can drink your mistakes :hehehe
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I wouldn't be planning on honing those cylinder bores either. Can't recall where I saw it, but someone else did a teardown and posted photos that showed that after 51,000 miles the rings were just beginning to be broken in and the honing marks in the cylinders looked like they were done that morning.
Do a compression check before you do anything on your motor. I just did my K100 16V and the compression was good on all cylinders after 111,000 miles. Your bike has a high quality automobile engine in it and will last as long as any car's engine, if not longer since it is so lightly loaded.
If it was mine, I'd be checking the valve clearances, cam chain, tensioners, and replacing all the rubber bits around the throttle bodies along with vacuum, breather, and fuel lines. Do the clutch o-ring and drop the oil pan to see how sludged up it is. Change ALL the fluids, lube the splines, new brake pads and lines, clean all the electrical connections and spend the rest of your time on cosmetics.
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+1 what Johnny what said. For your first engine rebuild, do a small block Chevy or several. These engines are not typical.
Tom
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I'm with you on the rebuild part! However... parts are painfully pricey. Cylinders are exquisitely hard, shouldn't be honed or bored unless diamond is involved. I actually want to buy a used engine and just take it apart,doing a deep cleaning.
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Here's is a link to the first part of a seven-part series of one man's K75rebuild, Brian, so you'll get a partial understanding of what's involved. Seeking a "zero-miles" outcome will be a costly education. Rebuilding a thumper would be my choice for a training engine then selling it off to get more and better tools and gauges.
https://www.youtube.com/watch?v=pKrbjMhZCAE
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I did a tear-down and refurbish on my K75, putting a new (used) engine in to replace a tired one a couple summers ago.
Logic for the teardown is that I wanted new gaskets and was worried (correctly) about crap in the cylinders from storage. Also wanted to inspect to see that it is at factory spec.
With a K bike, you really aren't doing a rebuild when you do an overhaul, especially short of a ludicrous mileage. The cylinders are not able to be overhauled (absent serious cost/effort to re-coat with nikasil and custom pistons/rings), and rarely need it: the Nikasil is insanely hard. Pistons and rings wear, the bores do not. At 100k on my previous engine, with some serious abuse (questionable maintenance history, 4000 HARD miles with a stuck open thermostat, probably more before that.), you could feel a slight ridge at the top of the cylinder, and there was noticeable piston/cylinder clearance. However, you could still see some crosshatching.
The ~35k replacement engine was all at factory spec: I checked it all. You could barely tell it had been run, once the oil and such was cleaned off. Cylinders were crosshatched still. All you need to do is scrub them with green scotchbrite to get rid of some glazing.
The one part of the engine that does occasionally need (and is able to be) rebuilt is the head. The exhaust valves recede, and eventually you use up the available adjustment range. The seats are installed with a range of acceptable depths. If they are on the tighter side from the factory, there will be less life expected. There's a guy with (last I read) over 800k on a K75. He needed a replacement (used) head at 320k, and at 500K had a rebuild. At 500k, he had the head built for the full clearance to be availiable.
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How does it run currently? Are there leaks or other causes for concern? I'm with the other posters who advise leaving well-enough alone. Test your compression, oil pressure, etc. Synch the throttle bodies, adjust the valves... adjust everything you can and clean/replace worn parts along the way.
I just bought a 1990 K75s with 180,000 miles on it (I bought it for parts for my 1992 K75s). The thing ran great and still had all the factory fuel line clamps and airbox clamps on it. The transmission was going bad, and the previous owner had the heads done a couple times - he was also concerned that the heads might not endure another rebuild. Anyway, it's all in parts now (except the engine) and I am planning to take the engine apart later, mostly for the hell of it, but also to see if I can sell any of the parts. I did find oil in the water - but did not notice any water in the oil. Perhaps I should post some post-mortem photos on a 180k mile engine. That might be interesting to a few people.
I talked to a retired BMW mechanic recently. He said in his whole career he only saw the inside of one of these K-brick engines once - and that was in training. He NEVER had to rebuild one.
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Hi,
Thanx for all the good info.
I just bought a 1985 K100 with unknown mileage. It runs but smokes. I was thinking about doing a complete engine and tranny rebuild to build a cafe racer type. But since reading this thread, I'm having second thoughts about the complete engine rebuild. The foot shift lever is sloppy compared to my '95 k75 also so I'm gonna' have to do something about it. I haven't rebuilt an engine in over 40 years so I could use any help or suggestions.
thanx,
ed
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Hi,
Thanx for all the good info.
I just bought a 1985 K100 with unknown mileage. It runs but smokes. I was thinking about doing a complete engine and tranny rebuild to build a cafe racer type. But since reading this thread, I'm having second thoughts about the complete engine rebuild. The foot shift lever is sloppy compared to my '95 k75 also so I'm gonna' have to do something about it. I haven't rebuilt an engine in over 40 years so I could use any help or suggestions.
thanx,
ed
Do a compression check. If it looks good(and it probably will) You might want to replace the valve stem oil seals. I just did mine, but haven't reassembled yet so I can't say for sure if that was the problem. My K100 was burning about a quart of oil every 600 miles.
The shifter slop is the result of the grub screw on the shifter shaft loosening. It's maddening to know that they didn't use Loctite on that screw after dealing with all the other places where they did. Tightening it requires that you remove and split the cases of the transmission. It's not that bad of a job. My RT took me about 7 hours start to finish and that included a spline lube, swingarm bearing lube and all fasteners cleaned, anti-seized, and torqued. Worst part was the reassembly of the transmission which was finicky but not that bad if you take your time. You do want to address that problem. Besides not being able to shift gears, you don't want that screw loose in the transmission.
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OK thanx The Mighty Gryphon. I'll definitely check into those things.
Ed
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Compression test showed 120 - 80 - 90 - 90
So I bought a '85 K100 engine and transmission with 55k miles from Fallen Cycles in Daytona Beach.
They had it in eBay with video of it running and shifting thru all gears and no smoking. Paid $250, drove 440 miles round trip and threw it my trunk. Got a receipt that says " guaranteed to run and drive"
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Squirt a bit of oil into the cylinders and redo your compression test. If the compression goes up, the fault is with the rings, if it stays the same its with the valves.
Regards Martin