Hello all,

My 1992 k100rs engine with 91k miles is pretty much dead. There is 0 psi compression on the 1st cylinder. The others are: cylinder 2 = 140 psi, cylinder 3 = 140 psi, cylinder 4 = 130 psi. In order to fix that 1st cylinder, regardless of what it is, I'm looking at around $1,500 - $2,500 USD, whether I do it myself or take it to a shop.

So, I found a 1991 K100RS 16v engine with only 29k miles. It has 200 psi on each cylinder, and is up for sale for $500. This seems to be my best option at the moment.

The question is: Can I replace my 1992 engine with that 1991 engine? Aka, are they interchangeable?

Thank you!

In a word Yes.
200psi in each cyl I would be checking my compression gauge 165psi is all you could expect.
At $500 it would be the best bet, I did that with my K100 when I reserected it from a near death experience.

Agreed, regarding the expected psi. That is, unless, the engine was modified with different pistons to have a compression ratio of ~13.6:1, which would make me concerned as to whether it would run on 91 octane (US) gasoline.

K75 pistons perhaps.....98 octane should be ok with that....

The only issue with that is: the highest octane gas you can get now-a-days, from a regular gas station, is 91.

Now, I know that ratings in other countries are different, and that, for example, Germany's 98 octane ~= US 91 octane. Is this possibly what you are referring to?

You wont get K75 pistons into a 16V engine

Just out of curiosity.... How come your looking at $1500 dollars for the cost of repairing your engine? Do you know what the cause of the the 0 psi in the first cylinder is? I would have thought the worst case scenario is either a bent valve or a broken ring... getting that one cylinder back to 140 psi would give you a working engine again your engine would be able to idle smoothly...

true enough twincarb ...even perhaps clearances on the shims ...but in any case a valve job by the sound of it

Because regardless of what it is, the valve cover needs to come off, the head needs to come off, and then, depending on what it is (most likely a burnt valve), but a new valve, clean everything out, take the head and valve to get fit to one another, get it back, put it all back together. And that's only if its a burnt valve. If its a compression ring, then the barrels have to come off, rings replaced, etc etc. If I were to take it to a shop to get this done, there is tons of labor involved. If I did it myself, just the parts and machine work would cost quite a bit.

I do see where you're coming from, and I agree, but it's just more cost and time effective for me to replace the whole thing with a motor in seemingly much better shape.

I would take it one step at a time first off to work out what the problem is. I used the following process to figure mine out...
1. Compression test (engine cold not a problem) inject some oil into cylinder 1 only about a teaspoon using a tube on a syringe is a good way.
2. If the compression go's up it shows that the oil is sealing the piston and most likely the rings. If it shows no change then its a closer look at the head.

now it depends on what the oil in the cylinder test shows.

If it is a valve taking the head off at this stage is still no cost, at the moment all you need to do is replace the gasket. once you have your head on the bench put the spark plugs back in (and with the cam-shafts removed) you can pour a small amount of petrol into the top and if there is a problem petrol will leak down the valve. You would then be looking at getting a new or second hand valve and all you need to do is lap in it. The tool for this is very cheap it's just a wooden stick with a sucker on the end and you just grind it in.

If it is a ring again it's head off but also the crankcase cover remove the 2 bolts on the piston bottom end and out it will slide. The nicosil coating on our cylinder's is incredibly strong and resistant to major scratching to prepare the cylinder there is are many cheap honing tools on the market that will allow the new rings to bed in. it's now the cost of rings, 2 new bolts for the bottom end and also a head gasket....

I know I have over simplified the tasks above but they are both achievable with the advice that's available on here and the manuals that are available for our bikes. The head can be removed from the engine with the engine still on the bike! It's a tad awkward but is achievable... Had you been on this side of the pond I would have offered to assist

There is no real machining that is possible for our engines. The heads can't be skimmed (not to the best of my knowledge) and the cylinders can't be rebored and relined (unless you have more money than the cost of several new bikes!)

One important thing is don't throw the old engine out they don't make them any more and one day repairing it will be the most economical thing for someone to do.