Wringing hands over whether or not to replace a 100 engine. 223,000 miles but was running decent before bike tear down. PO had all scheduled maintenance done at BMW shop since 4,000 Miles. Compression cold was 190 175 175 190. Compression at operating temp was 120 75 75 115. I think the throttle plates snapped shut on the hot test so not sure about accuracy. Found an engine for $450 shipped with 58,000 miles.
With this many miles I'd pretty much always disassemble and mic pistons and bores and replace rings if that's all it needed and rebuild head. But: the gasket set alone is $230 plus whatever rings, head, water pump costs are and then there's the possibility of having to replace pistons, valves etc. those costs could easily run $300 best case to $500 + in worse case... The injectors are probably candidates fir replacement as well. And again I have the opportunity to install a "plug and play" engine with just a throttle body clean up and clutch. For $450.
Any thoughts?
Thanks
Ron S

Buy the reasonably priced engine and keep it in the garage until the high miler goes pop? What's prompting the work?

I would have expected that if you have such a drop in compression at operating temperature and it was the bores that you would be losing oil or having big problems with blow by into the crank cases? If not then it would seem you are looking at top end...valves, valve clearances, timing chain jumped a tooth but the cold readings seem to rule out a valve timing problem?

For what its worth I would recheck it with another test. Never believe a first time result and never simply accept a single reading but note it carefully as you have done.

As Snod Blatter said I would just get the other engine....but you might be waiting a long time to use it unless there is something specific doing this.

Snod,
The whole bike was a rat. Never cleaned and there was no paint left on the frame just rust... Seen cleaner farm implements. If it was leaking anything one could not have found the leak... Rolling chunk of dirt grease and rust. AX bearing shot... Figured at 223k it was due for a teardown.
Still pondering the 58k engine .....
92kk
Can't do another op temp test as the engine is sitting outside my shop having been thourougly douched and aluminum cleaned... Don't like pulling spark plugs out of a filthy rat engine much less covers...
Noticed an amount of oil and carbon in inlet ports and throttle bodies but no more than I'd expect in a 223k engine... So there may be a bit of blow by but not much... It ran when I began the tear down. Seemed a bit rich but 223k injectors are bound to flow excess. Valve timing not an issue.. The intake valve stems don't even have baked crud on them and the ports cleaned out easily with a little carb cleaner while tilted downward...
If it were not such a project to pull the engine I'd adjust the valves, throw o-rings under all covers, replace injectors and toss a new clutch in it...
How do you feel about taking cold compression readings?
I Really Appreciate all of your feedback guys!!!

Those readings don't sound right to me. I f there was any doubt then you should have done it again.
Having gone this far I would be inclined to put the low miles engine in and then you at least have a known quantity.

Thanks Rick. I'm a beliver in "known quantities".
I was going to do a second hot test but was sidetracked by a frantic call from a friend regarding his Ducati catching fire. Then proceeded to return home and promptly tear the bike down to where I could only spin it cold...
I'll probably order the 58k engine tonight.
In the meantime I now have a really clean 223,000 mile K100 engine sitting in my shop.
Maybe I can ebay it or Craigslist it to recoup a little $.

Yes I can sure understand how that could be a touch frantic. Had it happen to me once with a customers bike and that was near as frantic as walking into a spider web at night. A friend used to call that the aracna rap dance.

I like the "aracna rap dance!
Nothing like toasted motorcycle... The Duc was an electrical fire... He smoked his wiring harness.
I can't look at an Amal carb without thinking of my 'Trident hop-n-wave" dance I did in a remote location while extinguishing three flaming Amals with my $500 Hein Gerick jacket. Most of my bikes have baby fire extinguishers on them these days.

Paul Glaves had over 300,000 miles on his K without a rebuild.  Only reason it didn't go to 500,000 was an automobile that turned it into scrap.  As everyone on this forum knows, these motors are not delicate flowers.  If it has good compression and runs, leave it be, maintain it and ride it.   Replace it when it really needs it.

stilbo wrote:Wringing hands over whether or not to replace a 100 engine.  . . . I have the opportunity to install a "plug and play" engine with just a throttle body clean up and clutch. For $450.
Any thoughts?
Thanks
Ron S

Here was my "experience"

I got my first K bike dec 2012 with full intent of the rebuild (and the rebuild) I did on it in mind prior to purchase.

I swiftly dismantled the whole thing to the frame and did all the cosmetics, leaving the engine "refresh" to someone else (why I am still asking myself !). The parts I wanted in was rings, recond head, water pump and timing chain and tensioner / guides etc. For the price of the parts alone (including front and rear crank gaskets etc) I could have got a whole nother engine. I wish now that I had decided that instead of pursuing the recond. As it happened the engine was recond for a grand price of $1800, 1000 of which was LABOR. Pissed me off but I (HAD) a frienship with the mechanic and I did not want to destroy that relationship because he did all my major car work as well. ( I run a small fleet of cars and am generally over doing my own car mechanicing)

Anywayz, to cut a long story shorter the recond engine only ran for 3000ks before the No 1 big end bearing shat itself, due either to incorrect re - install of the rods or incorrect tightening of the bearing caps ( be VERY careful if you go down the road of reconditioning 0- - - - a word to the wise, see HAYNES manual about tightening 80 degrees over the recommended torque settings)

Well  . . I found another whole bike RS done 48000ks with a REALLY sweet engine and repowered with that and I am SO glad I did. I swapped over all my cases, my new clutch, my recond starter motor and alternator and bunged it in to my bike and hey presto . . . good as a near new one, running REAL nice now

Having typed all that, I say . . . get the new engine.

Be Made,  I'd guess that the cylinders were honed or cleaned up with more than a scothcbrite pad thereby removing the Nikisil coating on the cylinder walls.  If they weren't re coated,  that will be the cause of the failure. 

Ron S,  What you haven't told us is how does it run now,  Before you blow a bunch of cash, consider doing or having done a proper compression check including a leak down test. Also pull the head, check the valves or have the head re done. if the motor runs good.  If your worried about some smoking on start up, most/some K100's smoke on start up. It's normal and a known design flaw.   If your "wringing your hands" over buying the motor, buy it.

k75RT Keith wrote:Be Made,  I'd guess that the cylinders were honed or cleaned up with more than a scothcbrite pad thereby removing the Nikisil coating on the cylinder walls.  If they weren't re coated,  that will be the cause of the failure.

Sorry mate, i fail to see how that relates to my big end dying

Be Made,  I apologize for my obvious miss. I did read that the big end went out and then it escaped me when I wrote my response.

heh . . . no problemo . . . .nun of us is purfect :-))

stilbo wrote:In the meantime I now have a really clean 223,000 mile K100 engine sitting in my shop.
Maybe I can ebay it or Craigslist it to recoup a little $.

What you have is an invaluable spare parts department. Like the starter motor, alternator, cases, etc etc. If it were me I would take the crank, and valve cases, and maybe the timing chain cover too, off the dud motor and slowly polish them up over time while using the bike once its going again. When they are all finished spend an hour swapping them over. Bet the bike would would look REAL nice

Be made wrote:

stilbo wrote:In the meantime I now have a really clean 223,000 mile K100 engine sitting in my shop.
Maybe I can ebay it or Craigslist it to recoup a little $.

What you have is an invaluable spare parts department. Like the starter motor, alternator, cases, etc etc. If it were me I would take the crank, and valve cases, and maybe the timing chain cover too, off the dud motor and slowly polish them up over time while using the bike once its going again. When they are all finished spend an hour swapping them over. Bet the bike would would look REAL nice

+1 The salvageable parts in that motor are worth a couple times what you can get for it.  I really like the idea of pimping the covers and doing a swap.

Point-Seven-five wrote: I really like the idea of pimping the covers and doing a swap.

Yep.

I actually gave mine to the shop to do while I was getting on with the rest of the customizing.

I am getting old and lazy to do a lot of stuff myself. I am starting to especially resent really tedious jobs like sanding and polishing so for the $500 the shop charged to do the polishing and the $300 the paint shop charged to do the clear coating with diamondshine special paint it was (for me) the greatest thing I could have done to give the bike that special look.

I never have to polish those cases again and they always come up mint when I wash the bike.

I now have a set of spare cases as I repowered my bike with another engine so, if any mishaps occur by way of a fall I can be back on the road pretty much straight away