1991 KTM 125mx 87 Motor Project

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4/29/2018 2:01 PM

Came across this 1992 KTM 125SX. First KTM for me. Looks like a cool bike. Dyno Port pipe, aluminum subframe. Brembo master cylinder. Bottom end was not assembled correctly im told... Its too tight. The bearings, seals and crank was rebuilt, but i was told that whoever did the connecting rod used shims? Anyway im gonna open it up, see whats going on in there. Does anybody know of any good ktm resources/info for Ktms of this generation? Cheers...☺

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4/29/2018 2:02 PM

Photo Photo Photo Photo
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4/29/2018 2:11 PM

I’m pretty familiar with them....looks like a bit of a Frankenstein project. The rear brake and motor look 87ish..but it’s definitely 91/92 plastic. Everything is available on eBay or from KTM.

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4/29/2018 2:12 PM

Mine runs good!Photo

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4/29/2018 2:16 PM

Hmmm... Now its getting interesting!

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4/29/2018 2:29 PM

I think that’s a older bike they put newer plastic on. And I had a DynoPort but it looked nothing like that....
Photo

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4/29/2018 2:34 PM

wpark89 wrote:

I think that’s a older bike they put newer plastic on. And I had a DynoPort but it looked nothing like that....
Photo

Gotcha. Thanks for the info. Does anybody know frame and engine numbers? Ill see what i got and post em...

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4/29/2018 2:50 PM
Edited Date/Time: 4/29/2018 2:52 PM

Send them to me and I’ll figure it out.

Just off looks I think it’s an 87 or 88.

87-90 are similar packages and 91-97 are similar (plastic changed in 93 but tank didn’t and everything is interchangeable.)

92 was the first year for the 2 part removable clutch cover and you don’t have that.

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4/29/2018 4:01 PM

Perhaps this will shed a little light? Photo Photo

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4/29/2018 5:15 PM

Looks like its a 1991 frame... M in the vin, upon some research, corresponded to 91.

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4/29/2018 5:24 PM

wpark89 wrote:

Send them to me and I’ll figure it out.

Just off looks I think it’s an 87 or 88.

87-90 are similar packages and 91-97 are similar (plastic changed in 93 but tank didn’t and everything is interchangeable.)

92 was the first year for the 2 part removable clutch cover and you don’t have that.

The engine is in fact a 1987. The 7 in the engine number gives it. Thanks for your input. I think i should rename this thread...

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4/29/2018 5:40 PM

Just bizarre they would put an 87 engine in the 91 frame and use the 87 swingarm/rear brake. Who knows...all kinds of weirds stuff out there. I ran into this on an 87 YZ 80 I restored and kept thinking Yamaha had all the parts wrong I kept buying....turned out the previous owner swapped the whole clutch/cover/water pump/primary gear/ for parts out of an 83.

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4/29/2018 5:55 PM

wpark89 wrote:

Just bizarre they would put an 87 engine in the 91 frame and use the 87 swingarm/rear brake. Who knows...all kinds of weirds stuff out there. I ran into this on an 87 YZ 80 I restored and kept thinking Yamaha had all the parts wrong I kept buying....turned out the previous owner swapped the whole clutch/cover/water pump/primary gear/ for parts out of an 83.

Yes its bizarre... Can only imagine why. With how fast the manufacturers were changing designs, i am very suprised they did the 87 engine swap. The frame doesnt look fabbed up for the swap. Must have been an easy thing to do i guess. Gonna be a learning curve all over again. Oh well, ive seen some Franks and this is actually decent. Wish me good luck please...☺

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4/29/2018 6:51 PM

I bought a complete 96 motor on eBay for 100 bucks. The motor changed very little from 87-97. Lots of mid 90s are being parted out, you could probably find the right swingarm cheap.

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4/30/2018 3:04 AM

wpark89 wrote:

Just bizarre they would put an 87 engine in the 91 frame and use the 87 swingarm/rear brake. Who knows...all kinds of weirds stuff out there. I ran into this on an 87 YZ 80 I restored and kept thinking Yamaha had all the parts wrong I kept buying....turned out the previous owner swapped the whole clutch/cover/water pump/primary gear/ for parts out of an 83.

Yes its bizarre... Can only imagine why. With how fast the manufacturers were changing designs, i am very suprised they did the 87 engine swap. The frame doesnt look fabbed up for the swap. Must have been an easy thing to do i guess. Gonna be a learning curve all over again. Oh well, ive seen some Franks and this is actually decent. Wish me good luck please...☺

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5/1/2018 7:40 AM

Sorry for duplicate post...

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5/1/2018 7:42 AM

Does anybody have crankshaft / connecting rod specs for the 87 engine, i.e. overall length, runout etc...? Thanks for all the help!

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5/1/2018 7:44 AM

I want to open it up, get just the crank fit in the cases and go from there. I was told the previous owner just assembled the whole thing and found out something was not right, after it was put back together...ugh!

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5/2/2018 12:08 PM
Edited Date/Time: 5/2/2018 12:09 PM

kruspy378 wrote:

Does anybody have crankshaft / connecting rod specs for the 87 engine, i.e. overall length, runout etc...? Thanks for all the help!

specs for the rod back to 89'. Is it possible they pressed a 91' Rod into a 87' crank?

Pro-X connecting rod dimensions

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5/2/2018 12:46 PM

kruspy378 wrote:

Does anybody have crankshaft / connecting rod specs for the 87 engine, i.e. overall length, runout etc...? Thanks for all the help!

mxrose3 wrote:

specs for the rod back to 89'. Is it possible they pressed a 91' Rod into a 87' crank?

Pro-X connecting rod dimensions

Thats what i am thinking they did. I was told it was the pro-x rod. On the receipt for the rebuild it was written down as a 1991KTM. Previous owner must have torn it down and deleivered the crank only in guessing. Its all starting to make sense... The 91 rod is thicker at the big end etc...?
If this is the problem what rod / rods can i use or can the 91 prox rod be shaved down on each side? Thanks!

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5/2/2018 12:54 PM

Btw, thanks for the prox link.

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5/2/2018 1:38 PM

kruspy378 wrote:

Does anybody have crankshaft / connecting rod specs for the 87 engine, i.e. overall length, runout etc...? Thanks for all the help!

mxrose3 wrote:

specs for the rod back to 89'. Is it possible they pressed a 91' Rod into a 87' crank?

Pro-X connecting rod dimensions

kruspy378 wrote:

Thats what i am thinking they did. I was told it was the pro-x rod. On the receipt for the rebuild it was written down as a 1991KTM. Previous owner must have torn it down and deleivered the crank only in guessing. Its all starting to make sense... The 91 rod is thicker at the big end etc...?
If this is the problem what rod / rods can i use or can the 91 prox rod be shaved down on each side? Thanks!

I'm not familiar with early KTM motors, but hopefully someone will chime in who is.

Otherwise, I would start tearing it down and taking some measurements with a caliper. I would measure the case opening, then the total crank width, big end rod width, and go from there. Heck, maybe it is the right rod, and it just seized up on the guy that you bought it from.

No problem for the link - Pro-x has the thrust washer measurements on there site too.

I did a quick search for '87 KTM 125 connecting rod' and wasn't coming up with much either. You could also try www.ktmtalk.com - they have a 2 stroke section that you can ask questions like this also. There are probably some people on that site that know a little about the older KTM's too.

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5/2/2018 1:42 PM
Edited Date/Time: 5/2/2018 1:46 PM

I have a 91 service manual but it doesn’t give any of those specs. The best source for info is the couple vintage KTM forums on Facebook.
I’m using that Prox rod in mine. I used the Prox piston too for awhile and then found an oem ktm piston on eBay cheap.

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5/2/2018 1:45 PM

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5/2/2018 1:56 PM

Thanks everybody!

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