[muthahucker]

Got my engine running after a rebuild, ran for 15 minutes while doing a break in, started making alot of noise in the bottom end then seized.. Tore it down and have found the Main crank bearings have detonated. These were brand new with all the other bearings, seals and gaskets.
Why would the main bearing give up so fast? Is it something that could have been missed while rebuilding or just a crappy bearing unfortunately? OEM Crank was also rebuilt before reassembly.

 

[312R1]

What kind of bearings did you use?

 

[muthahucker]

Not sure of the brand of the bearings, came from a reputable store and were specific for the TRX 250R.. but this doesnt mean they were good quality I guess.. Any ideas for a good quality brand I should get?

 

[312R1]

Get OEM 89 bearings..they have 8 balls.

 

[Oppossum]

agree'd! the ole cr500 8 ball is the only way to go.

you didnt by some remote chance tap your seals in too far did you? i've seen it happen before.

 

[C-Leigh Racing]

agree'd! the ole cr500 8 ball is the only way to go.

you didnt by some remote chance tap your seals in too far did you? i've seen it happen before.

Thats what I was wondering as well.
Did it mess up both of them or one messed up & other ok.

If the flywheel side seal leaking, it can cause the bearing to run dry & not get lube like it should. Realy dependant though how much the seal is leaking.
On the clutch side, if that seals leaking it should have sucked in some trans oil & maybe smoked a good deal more.
Thats why you see most everybody over filling their transmissions oil. By doing that on a 250R, the oil will be high enough that if the seal is leaking it will suck the oil in & smoke like the dickins.
That high oil level has save many pistons & crankshafts sence people started doing it.

Another thing you need to do after you put a bottom end together. Once you install the bearing in the case & then install the crank in the left side bearing, that crank & bearing will be bottomed out in the case & then when you put the engine all together & put the sleeve, those two gears & tighten the bolt up on the clutch side, it pulls the crank up into the clutch side bearing race & kinda puts the crank in a bind. Now after doing that, the crank is pulling on the bearings & causing side load on them as well as stressing the crank itself around the big end rod pin.
You can tell this is happening once you get it all together because the crank wont turn as free as it did before.
To releave the stress & since the crank or bearing cant move on the clutch side, put the flywheel nut on the crank & then take a dead blow or rubber mallot & frap the end of the crank kinda like driving it back into the bearing, freeing it up some.
You dont have to hit it hard & rotate the crank after you frap it each time till you feel it free up.

If it was in a side load bind that caused the bearings to mess up, I would be wondering about that crank as well, what if any damage did it do to that.
Neil

 

[muthahucker]

CRAP.. This is sounding like a bigger problem then I first thought it was. I didnt build the engine, I had a local shop rebuild it. He rebuilds alot of 2 stroke bike engines and also 2 stroke boat motor engines. I did notice the seal on the stator side weeping a small amount of oil. As far as I can tell so far it was just one bearing, unsure of which side at this time. Thanks for the info so far.

 

[Oppossum]

it will be really evident as to which side went....... there will be plenty slack on that side. could very well be the side that was weeping. a scorched seal will start to weep if it had been riding against the bearing. as you can see, there are plenty of variables that can cause failure. but atleast now you know what to look for in the future. keep us posted. i'm curious now.

 

[C-Leigh Racing]

If he builds a lot of 2 strokes, he probably knows how to install a bearing in a case or a crank into the bearing, but just ask him if he pressed the bearing in the case or pressed the crank into the bearing. Just ask him that in a way that dont lead to him thinking your questioning his abilitys or else he might give you a good sounding answer.

Best way to do either is to heat the case, cool the bearing & drop in. Then heat the bearing, cool the crank & drop in.
Realy, when your rebuilding a 250R bottom end & got to press the crank out of the cases, that flywheel side bearing shouldnt be used over again, because it is a bunch of side load being put on the bearing during the pressing out process & it damages that bearing.
Neil

 

[muthahucker]

Thanks for the info everyone, will followup with my builder to see whats gone on with the bearings for sure.

 

[370]

Get OEM 89 bearings..they have 8 balls.

you can buy the same bearing from koyo. for alot less than honda charges..

 

[muthahucker]

Think I have found part of the problem.. Checking the cylinder, it has been bored out alot and it looks like it has been bored way off center, over to the left side. I would imagine that this would put pressure on both sides of the engine out to the bearings. Looks like I need a new cylinder to work with.

 

[C-Leigh Racing]

If it is off center & binding the rod, it will wear the wrist pin & crank pin bearings first & probably burn them out before one of those main bearings get damaged.
Neil

 

[Oppossum]

and who bored the cylinder last?

 

[muthahucker]

Unknown who has done the work to this machine. I bought it, was told it was in good shape. Didnt know what I was looking for at the time between good and bad machines. Turned out to be a piece of crap with a nice paint job. Everything has been rebuilt or replaced right down to a new frame. Ugly little problems like this keep turning up.