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Eggbert

Crank replacement

16 posts in this topic

I have my 2009 RMZ250 top end apart now and checked the crank. The crank and rod are pretty stout pieces, but the question of the lower rod bearing is the worry.

Up/down and side clearances of the rod all are within spec. Crank main bearings check out tight...no end or side-to-side play.

So far all recommendations (from forums) are to change the crank/rod assembly, but obviously that's a fair expense plus the effort of splitting the case, etc.

In my defense of being cheap, I still think a lot of advice is still based on 2-stroke engines. A 4-stroke should have better lubrication to the rod bearings if the oil is kept clean.

Of course if I don't replace it, I am running the risk of massive engine damage.

So......how may of you have had crank/rod/bearing problems, and if so, how many hours were on the engine (roughly)?

Hearing from people who have not had a problem for X amount of hours is also welcomed.

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How many hours are on it and how was the maint. on the bike? Cranks can last a long time if cared for and not abused. I have about 160 hours on my 07 KX250f motor with the stock bearings and crank.

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Somewhere between 200 and 300 hours. Of that, only about 3 hours total racing time (8 to 10 minute races). Maybe about 60 hours on a practise track where you can open it up. The rest mostly trail riding and short course practise track (corners and small jumps)

Oil and filter changed regularly although probably not quite as often as a bike that would be only raced.

Rest looks good. Valves are fine and don't even need to have the shims changed.

Cylinder was worn a bit, in spec but sort of shiny. We're replacing it, piston, ring, and small end bearing. Piston was fine, and even cylinder could have been re-used.

Everything looks good and measures within spec, but I have a friend with a 2-stroke HOnda 250 where we checked the big end rod bearing and it was within specs and not long after the bearing did self-destruct.

I'm thinking 4-strokes should be more durable due to the lubrication system, but I don't know, so that's why I'm asking.

I do admit to taking the risk. If I replace everything, than it may add up to so much $$$ that I'd be better off getting a new engine, or even a new bike. If it blows up, I'd be faced with that anyway. Maybe that's bad thinking......

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We have a 08 RMZ with 175 hours. The stock crank went out (rod bearing) with 18 hours then the 2nd (oem) crank seazed up with 80 hours. Now with a Wisco crank we're still going strong.....Something worng with the oem cranks!!!!

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Well now. That's interesting. Ours seems fine at over 200 hours, so maybe there was some duff OEMs for a while. On the other hand, your Wiseco seems to be holding up pretty well, and that's a good sign that a 4-stroke crank can last.

Didn't know Wiseco made cranks....thought it was Hotrod or something like that. I'll have to look into it.

Thank-you for your reply.

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Just had the stock big end bearing go out in an '08 with 60 hours. Top end looked fine, hadn't even replaced the rings yet.

-Kerry

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How long a lower rod bearing will last is always a crap shoot. I had a 05 rm 250 go 130hrs and a 06 rmz 450 go 70. Nothing is ever certain, but the best way to tell how worn a bottom rod bearing is would be to se how much "rock" it has, not up and down play or side clearance, but actual rocking across the length of the bearing.

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If you see any site of brass in the oil....replace the crank. Last year I did an oil change and noticed just a few brass particals. I pulled the engine and replaced the crank....glad I did, you could see the bottom bearing was starting to let go......If your bike has 200 hours on it.....I would repalce the crank...IMO your taking a big chance.......

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How long a lower rod bearing will last is always a crap shoot. I had a 05 rm 250 go 130hrs and a 06 rmz 450 go 70. Nothing is ever certain, but the best way to tell how worn a bottom rod bearing is would be to se how much "rock" it has, not up and down play or side clearance, but actual rocking across the length of the bearing.

Yes, I would think too much rocking indicates wear in the bearing cage. I see my manual has a spec for that, but havn't tested it yet.

I was surprised as usually there is no spec for rocking.....just up/down and side-to-side clearance.

I guess I better measure it.

So far the longevity doesn't sound good. Now imagine if someone else had started this thread. I'd come along and say "I've got between 200 and 300 hours on mine and it's fine".

I do appreciate all the replies. Would be nice if someone came along and said I've got 1000 hours on mine and it's fine.....LOL.

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If you see any site of brass in the oil....replace the crank. Last year I did an oil change and noticed just a few brass particals. I pulled the engine and replaced the crank....glad I did, you could see the bottom bearing was starting to let go......

Good tip. I take it the brass was from the washers on either side of the connecting rod. Havn't drained the oil to look, but last oil change it appeared fine.

Part of the problem is that it's at the Dealer's, so I need to go back and double-check some stuff. The mechanic appears to be competant, but no doubt he's more used to trail bike engines that seem to last forever.

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Hope you don't think I'm hi-jacking the thread, Eggbert.:busted:

I think we just stumbled over a critical problem with Suzuki 's recommended maintenance. How often do people verify their oil feed pump side strainer is clean? I could drain the oil and not see the brass, but I pull that pencil nub of a strainer on the stator side and it was clogged... Book says 12 hours or 6 races... maybe they should say check every oil change, and when it's clogged with brass, time for a bottom end?:p

For the record, the stock rod on our '08 did not exhibit excessive side to side or top to bottom motion prior to pulling from the engine. The big end just didn't move, period, unless forced - like when I was trying to get it to TDC. I can roll the case crank bearings. I have rods from other bikes that demonstrate the play of a bad bottom end - this one just flat out froze to the crank! Weird.B)

Point is, you never know when the failure is going happen, but talking with others - and reading these posts - makes me thing the OEM crank is a POS.:smirk:

-Kerry

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No, not all all. Your contribution is quite relevant. I do remove the strainer every second oil change, but never found anything on it. There's a second strainer, but I think you have to split the case to get to it.

I don't know if the cranks are really that bad as ours has lasted well. But it may be a quality control issue where some aren't as good as others due to inconsistent manufacturing.

Feezing to the crank usually means oil starvation in a solid babbit bearing, but it is weird for a needle bearing. Perhaps the needle cage became worn and cracked or broken and the needles jammed up.

It is starting to sound like I'm taking a risk not changing the crank.

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... the best way to tell how worn a bottom rod bearing is would be to se how much "rock" it has, not up and down play or side clearance, but actual rocking across the length of the bearing.

Turns out the book is confusing. Below is a pic of all the checks. The last check looks to be for rocking of the connecting rod. So....it appears that the deflection limit is total rocking movement, although one might think it would be movement measured from vertical, so half total movement.

Anyone care to comment?

ConRodSpecs.jpg

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I believe that is from the neutral/center position, but 3mm seems like a lot for half of the side to side deflection... I'd think 3mm might be a bit much for total overall.

As an aside, our trail bike's side clearance is way smaller. I understand that specs are not going to be indentical, but you'd think that a race engine would have tighter tolerances.

-Kerry

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I did a rebuild at 40hrs on my 08 last year. Its only ever been on motocross tracks. The main reason for the rebuild was a gearbox issue. But I did the crank pin, rod kit, bearings and piston kit all at the same time, and the timing chain too. I used OEM stuff. Nothing was out of spec, the play in the rod was close though. I just don't see the point in risking a failure, particularly when I had it apart anyway. I change its oil after every 4hrs, and I always check that strainer, and I always pass a probe magnet inside the sump to see if there's anything lying around. When I started seeing flakes of metal from the gears I knew it was rebuild time.

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Just an update. Engine had somewhere beetween 200 and 300 hours, but all we did was replace piston, rings and cylinder. The cylinder could have been re-used, but we opted for a new one and will keep the old one as a spare.

So far at least 10 hours since rebuild and it's working flawlessly. I guess the risk of the crank or crank bearings or rod big end bearing suddenly going south is still there, but so far, so good.

And interestingly enough, all the valves were in spec. No need to adjust the lash.

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