CRANKSHAFT BEARINGS

[sooty157]

Hi all,
Right as some of you may know i recently posting a thread regarding jetting my mk.2 125lc and hey that's sorted now but ho, now it runs sweet i've noticed a tapping sound from the cylinder area at idle so i took the barrel off today, you know just for a laugh! Barrel, piston and little ends are fine as they should be as it had a 2mm oversize rebore and piston before i bought it but there is a little play at the big ends accompanied by a slight knocking which i'm sure is the cause, just wondering before i take the lump out will it be a case of replacing bearings and seals or is the con-rod likely to be damaged too. thanks.

[neil]

you bye a conrod kit which is bigend conrod and little end. you will need to replace mainseals as a matter of course. i would do the mains as well. even if they seem ok. also you sure its not piston slap? cheack skirt to bore clearence. should be 2 thou new. mine was bored to big at 5 thou. it slaps quite bad. i noticed also that the mitaka pistons you get are for the dt,s. the windows in the skirt are smaller than rd ones. dont no if they done this to increase surface contact with the bore, to make it quieter. i opened mine up to match an old rd one, it did contibute to the noise. so this is worth taking into account. i think they all tend to rattle abit when a few miles have been covered.

[billy]

2-strokes are quite rattly, you'll hear the rattle clearly if you're running a stock pipe as those makes the exhaust puffs quieter. But yeah, it could also be something that is getting worn. Not necessarily the crank though, it could be the bearings for the counterweight, but these bearings rarely fails as far as I know (which is actually pretty strange as there must be enormous vertical forces going through them).

You can feel if the conrod (bigend) has any vertical play. Just get the barrel off, then hold the crank steady in one position and carefully lift/push down the conrod and see if you can feel any play.
Sometimes they are a bit rattly sideways though so the bigend can feel sloppier then it actually is. However, excessive sideway play (even thoguh the conrod has no vertical play) is also a sign that the conrod/bearing is getting old
(OR if you have a 2-cyl engine it can mean that the crank has split in the press fittings. This doesn't happen to 125's though as far as I know).

The main bearings are harder to check, you pretty much need to get them out of the case to check them. I wouldn't advice anyone to run with used bearings though unless the engine is easy to tear down. Crank bearings can feel nice and smooth and have no vertical play at all, but that doesn't necessarily mean they have a lot of lifetime left.
Crank bearings in general holds up nice until they start to sing on their last verse. WHEN they start to go, they go FAST! Thing is, when they manage to wear so they get even the tiniest ammount of play, they will be knocking up/down, particularly on idle. This will be bashing the bearing races and also the balls inside it, which causes even more play, which causes even more bashing, which causes even more play and more bashing... I think you get the idea now lol.

I did the mistake of assembling my 350LC engine with used bearings that felt perfectly fine.
They lasted for like, I dunno, 10 miles, and then the engine suddently started to get the horrible rumbling noise/vibe which is typical of crank bearing failure. Oh well, you learn from your mistakes I guess.

[sooty157]

well the plot thickens, took the engine out today thinking if it's gonna be right best be having a proper look. once i had the crank in my hands i realised the thrust washer between the big end and the crank webbing had done one on the clutch side allowing the con-rod to connect with the webbing at the top of the stroke leaving a light mark on both surfaces. i'm guessing the crank will have to be sent off and rebuilt now as some expense so if anyones got a good bottom end going at the right price...........

[neil]

your better off getting yours done, you dont no how long another bottom end will last. Theres conrod kits on the site for thirty rubs. cant think it would cost that much for your local bike engineers to fit.

[sooty157]

yeah i know and that's what i'll do in time as the bikes got matching no.s but hey i'm already buried in it so i'd be happy to chuck a motor in it to get back on the road. trouble for me is i've got rangerovers desperate for my attention at work and that's where my bike is so i've only got lunch times, i could get her running again with a motor in an hour.

[neil]

yeah i no what ya mean, ive got to much crap too.

[sooty157]

i've managed to find a 10w engine at sensible money the guy reckons it's a dtlc engine i thought 10w's were mk.1 rdlc's but either way that will go straight it my mk.2 wont it?

[Norbo]

Yes it will mate all 125lc bottom end will swop over and so will the first Dt125;s

[neil]

gear ratios are different. 10v full power 12w restricted.

[sooty157]

thanks for that guys thought as much just needed reassurance heads a mess at the min. with all my plans - by the way norbo piston kit arrived today for tother motor thanks a lot for getting it to me so quickly will defo come and see you when bike is feeling better!

[sooty157]

that's sort of what i did gary, i used the crank out of the dt engine to rebuild mine as it was in very good order. bikes done 150 miles now since rebuild and all is well sounds really nice with small can micron on!