91 K100RS broken valve spring-destroyed engine ?s - K-Bikes.com - Excellence in Motion
 
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post #1 of 6 (permalink) Old Jun 21st, 2010, 9:36 pm Thread Starter
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91 K100RS broken valve spring-destroyed engine ?s

Hey all,

I'm a new member and this is my first post. Forgive me if this subject has already been discussed -- got tired of looking through old threads.

My beloved K-bike broke a valve spring on the interstate which resulted in a destroyed engine. More specifically: the head is trashed (camshafts are fine). There is a hole in the #4 piston. The block is cracked near the crankshaft. The large gear at the end of the crankshaft is severely damaged. And, there are surly lots of other things I can't see. But, the rest of the bike is really nice, so I think I am going to replace the engine myself, and some parts interchanability info would be very useful.

My bike: '91 K100RS with 4 valve head and ABS.

Would a K1100 engine bolt right in without modifications? I would guess the fuel injection computer would need to be changed.

Would any 4 valve head from a K100 or K1 or K1100 work on a K100 engine? (This site says yes: http://www.beemerboneyard.com/11121461550.html)

Would a 4 valve K100 head bolt right onto a 2 valve K100 engine without modification?

Are there any differences in the crankshafts and blocks among all of the K100, K1, and K1100 engines?

Thanks for any help,

Brian

Rochester, MN
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post #2 of 6 (permalink) Old Jun 22nd, 2010, 6:26 am
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It's easy to get emotionally tied to an old bike you've had a while, but unless your hobby is classic bike restoration, you need to think that rebuilds with used parts carry a risk of hidden problems, unless you know their history. You will spend a lot just replacing gaskets, seals and consumables. If you already have a used motor or one not costing much I follow your logic.

With such severe damage I'd cost out the options carefully, including scrapping what's left of the bike for parts and buy another bike. Sounds brutal I know, but sheer economics may make that decision for you.



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post #3 of 6 (permalink) Old Jun 22nd, 2010, 11:23 am Thread Starter
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Quote:
Originally Posted by voxmagna
It's easy to get emotionally tied to an old bike you've had a while, but unless your hobby is classic bike restoration, you need to think that rebuilds with used parts carry a risk of hidden problems, unless you know their history. You will spend a lot just replacing gaskets, seals and consumables. If you already have a used motor or one not costing much I follow your logic.

With such severe damage I'd cost out the options carefully, including scrapping what's left of the bike for parts and buy another bike. Sounds brutal I know, but sheer economics may make that decision for you.

Actually, economics is what is driving my decision. Here is how I look at it. I bought the K100 for about $3200 last year from a motivated seller (he already had a newer machine). I adjusted the valves (one was slightly out of spec) and did some routine maintainance: coolant flush, brake fluid flush, driveline lube, new fork bushings and seals. etc. (did all work myself.) I invested in a new Givi top-box and mounting hardware and new Throttlemeiser bar ends. I also just put on a new back tire 200 mi before the engine let go. Aside from the blown engine, I like knowing that everything else on the bike is in really nice shape for its age. Now I have 2 options:

A
Sell the bike for parts for say $1000 and use that money towards a replacement bike. Even if I add another $2000 out of pocket, I would be hard pressed to find as good a bike as what I had.

B
Spend around $1000 (more or less, hopefully less) for a replacement engine plus whatever I value my time as. I am a competent home mechanic, and an engine swap is a pretty easy bolt in job--especially on this bike.

Option B is more attractive because it gets me back to a comparable replacement bike for the least amount of money. The trick, of course, is to find a good used replacement engine. Now remember, I stated with what I thought was a good used engine.

I'll admit that I am somewhat attached to this bike. But not that attached. This is my first BMW, and the thing that I like best about it is its build quality. Every fastener is beefy and of high quality so things come apart easily and go back together easily. I've had Japanese bike before, and it seems like if you took something apart, you were lucky to get it back together without stripping every fastener.

There are some things that irritate me about this bike. For example, when was the last time I had to lube any driveshaft or diveline parts on a car--NEVER.

Anyway, I have no idea why my exhaust valve spring broke. I was on the last leg of a 3000 mi trip when it happened, and admittedly, I ran the bike well above 6K RPM for many hours across MT and the Dakotas, and the bike IS 19 years old. But, jeez. Maybe BMW doesn't have the build quality I think it does.

In hindsight, I think I should have titled this thread "K1100 engine in a K100" or something similar. That would have been more descriptive of my main question. My guess is that BMW owners are less apt to doing such major engine mods as what I am proposing due to the high cost of parts and perhaps low occurrence of blown engines.

Anyway, thanks for the advice. So far, I have taken off the head, crankcase cover, and various other bits to inspect the damage. Yesterday, I loosely bolted everything back together to keep things organized, cleaned off 3,000 mi of road grime, and took some pictures. I'm all set to put the thing on Craigs List as a parts bike, but I still want to figure out if a replacement engine is feasible.

Brian
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post #4 of 6 (permalink) Old Jun 22nd, 2010, 5:38 pm
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post #5 of 6 (permalink) Old Jun 22nd, 2010, 8:04 pm Thread Starter
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Ha. Just a tad too Mad Maxish for me, but cool nonetheless. I would be happy with a K1100 motor.

Brian
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post #6 of 6 (permalink) Old Jun 24th, 2010, 11:38 am
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Brian,

Not sure about where you are - but here in the UK a good used engine will be around £300[ K1100 ]
With all the damage to your engine, be best to strip it and keep the good bits for spares. With a hole in the piston there'll be debris all round the motor.

Again depends on local prices, but I recently bought a crash damaged [ cosmetic] K1100RS for £300 ! [ $500??].

My next project involves putting an 8 valve head on a 16V bottom end and then using Jetronic Inj from the K100.

Reason? There's LOADS of 16V bottom ends going for next to nothing as the heads have been pirated for Mini [ A Series] twin cam conversions.

There's nothing black science about the swap - I'd go for it, I know what it's like to "keep an old friend" going !!
TJ

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