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My beloved 1985 RM125 has some concerning crank play:foul:. I'm wondering how much play is acceptable? If there is too much play what options do I have? Nobody makes a bottom end kit for my bike anymore :( 

Any help is appreciated, thanks:goofy:

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A little side to side is OK but any up and down is not good. If you can buy a new connecting rod you can have someone press it in.

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Hot Rods makes kits for tons of old bikes.  

here is an Ebay listing for yours.   http://www.ebay.com/itm/SUZUKI-RM125-1984-1985-1986-HOT-RODS-CONNECTING-ROD-KIT-/391726705129

You can measure big end wear with a dial gauge up at the small end and see how far the rod "tilts".   If you are a meticulous mechanic you can rebuild the motor yourself(have someone do the crank rebuild). You will need a few special tools like flywheel puller, torque wrench and preferably a case splitter tool. If you haven't rebuilt a two-stroke before either find a good shop who knows their two strokes or ask a friend if you are worried about mucking things up.

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I asked basically that same question about my Suzuki a while back.  The real answer is how much clearance is built in, by design, in a crank bearing.  Then you'll know what's "normal".   Main bearings are almost always a C-3 clearance.  Look up that spec. in a name brand bearing manufacturer's data.  You will find there IS a small built-in "slop", if you want to call it that designed it.  I seem to remember one of their web site's mentioned this is for a situation - like 2t crankshafts - where the inner race may run hotter than the outer race.  On my Suzuki, the radial (up and down) slop was a little less than .002 ". It's hard to check because you've got the whole weight of the crank fighting you along with main seals to hold it firm. I haven't gotten back to this Suzuki yet but I plan on putting a new main seal (this is the OE and 35 years old) and run it and I think it will be OK.

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Now this is some technical stuff! I'm not very mechanically inclined so this confuses me a bit. Is 1/8 inch of side to side play too much?

It looks pretty extreme but I can't find anything that states how much slop was present in stock form. 

I'd like to learn about this stuff because I have a tendency to buy POS vehicles and I always end up needing help fixing them.

thanks for all the help guys, I just wish I had more experience in this sort of thing, as I'm only 16. It'd also help if the bike wasn't an hour away at a friends house :banghead:

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Yes, I think 1/8 " side to side is too much.  The whole bottom end (main bearings, crank, even primary gear) should be inspected.  Yes, that means some major disassembly.  Got a manual?  What I commented on was vertical play but if it's really 1/8" side-to-side, that's not good.  If I were you, I'd post this question in the Suzuki 2 stroke sub-forum too.  Maybe there's something unique about how the crank is indexed or held in one position when it's designed/assembled. You may get an answer there like "put this in the vintage area" but it's worth a try.  Maybe a manual would explain the crank layout too.

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The big end of the connecting rod will move left to right on the big end pin of the crank. This is to allow for 'tolerance stacking' of all the other parts at final assembly. (I.E. cylinder bore slightly off set, cases, crank tolerances, etc...)

Put the crank at top dead center (with piston removed) hold the small end of the connecting rod straight up and try to twist it left to right, then try up and down you shouldn't feel any play at all.

This guy is the expert has done several cranks for me, fast service too. He does more than just KTMs.. I have sent him cranks I thought were questionable that he sent back to me saying no worries go use it.

https://www.cookseycrank.com/

 

Edited by BigDaddy22

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OP, that 1/8" movement you were referring to, was that crank movement or just pushing the rod side to side?

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On 3/25/2017 at 1:16 PM, motoxvet said:

OP, that 1/8" movement you were referring to, was that crank movement or just pushing the rod side to side?

It's crank movement 

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