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Motor rebuild questions

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Hello all,

picked up a 2007 rmz 250 for a good price and the crank bearing is bad. I’m comfortable tearing into these motors just was wondering if there’s anything I should watch out for or do while I’m replacing the crank. Also I’m having a hell of a time pulling the kick side case off and I don’t wanna smack it to hard it seems to be hanging up in the kick shaft. Any suggestions to get it off easy. Also any advice cracking the crank case would be appreciated. Does the cylinder look like it can be just honed or do I need to do more I can still see the cross hatches in the scoring.

thanks 

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Get the case flat and lightly tap the kickstarter side until it breaks free. Never use a screwdriver to pry the cases. To split the case a crankcase splitter is easiest but can be done with a rubber mallet. The cylinder you can take to your local machine shop for inspection but if it was mine I would lightly hone and run it.

 

Edit!

You need to remove shift lever and disconnect the drum.

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Posted (edited)

The cylinder's plating is toast, it needs to be either replated or replaced.

 

Tearing this far deep into the engine and to avoid having to repeat it sooner than anticipated,

at a very minimum you should fix the cylinder, replace the piston, ring, cam chain and tensioner

and since there was a bearing failure, carefully inspection and cleaning of the entire oiling system (pump and passageways)

That said, you'd still be uncertain of the cylinder head and valvetrain's condition.

 

Also inspect the air filter for proper sealing at it's base and any rips in the foam element.

Edited by mlatour

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Posted (edited)
44 minutes ago, mlatour said:

The cylinder's plating is toast, it needs to be either replated or replaced.

 

Tearing this far deep into the engine and to avoid having to repeat it sooner than anticipated,

at a very minimum you should fix the cylinder, replace the piston, ring, cam chain and tensioner

and since there was a bearing failure, carefully inspection and cleaning of the oiling system (pump and passageways)

That said you still be uncertain of the cylinder head and valvetrain's condition.

 

Also inspect the air filter for proper sealing at it's base and tears in the foam element.

well I was trying to be positive lol but I agree on the cylinder I was trying to keep this budget friendly nut not looking that way I can get a full rebuild kit with crank, piston, cylinder, and gaskets for $650 seem like a decent deal? I'm used to the 2smoke world this is the deepest I have been in to a 4stroke. 

What would be your recommendation on checking out the valvetrain? The guy I got it from said he rode it a couple if times and felt good full power easy start with no notable issues. He had it die on him on the trail seemed to lock up on him. He tried to start it and said he couldnt get it to start.  I'm trying to find the cause of the failure. he took it to a shop and they told him the crank bearing went out which is what caused the problem they said contamination in the bearing but this bike is spotless inside and out seemed to have good maintenance done by the owner before him so I'm just not sure. could bad valves or valves out of adjustment cause the piston to slap then the bearing fail? all my 2 stroke just had cylinder issues never anything in the bottom end. everything seems fine minus the crank bearing which feels okish (some side to side play and a little bit of resistance on the balls) turns over by hand fine with the piston off and feels tight in the case. 

 

 

Edited by KyleYZ450_07

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Posted (edited)
8 hours ago, KyleYZ450_07 said:

could bad valves or valves out of adjustment cause the piston to slap then the bearing fail?

No not related,

the wear you see in the cylinder is likely from pushing a piston too many hours, clearances open up allowing the piston to 'rock' in the bore.

Though scratches may also suggest dirt/debris ingestion past the air filter (not good for the intake valves!)

 

The cylinder's plating is very thin and only about as deep as the cross hatching itself,

meaning by the time the cross hatch is worn, so is the plating.

 

For the cylinder head, hold it upright and spray some parts cleaner into the ports, observe if any liquid leaks past a valve.

also check the cam journals for wear (due to an oil / bearing related failure of the crank)

If everything checks out, re-install it and check the valve lash.

 

8 hours ago, KyleYZ450_07 said:

I was trying to keep this budget friendly

'Budget friendly' and failed 4-stroke can't be included in the same sentence...

With most 'project' bikes like this, unfortunately the cost to rebuild them reliably often equals or outweighs the value of whole the bike itself,

considering the age of the bike and everything else around the engine that also likely requires attention. (carb, chassis bearings, suspension etc.)

 

As another member once suggested, 10+ year old 250F can be considered 'disposable' bikes when the suffer a major failure

but since you mention the rest of the bike appears to be in otherwise very good condition, perhaps then it's worth investing a bit into it.

 

Edited by mlatour
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Posted (edited)
37 minutes ago, mlatour said:

No not related,

the wear you see in the cylinder is likely from pushing a piston too many hours, clearances open up allowing the piston to 'rock' in the bore.

Thought scratches may also suggest dirt/debris ingestion past the air filter (not good for the intake valves!)

 

The cylinder's plating is very thin and only about as thick as the cross hatch itself,

meaning by the time the cross hatch is worn, so is the plating.

 

For the cylinder head, hold it upright and spray some parts cleaner into the ports, observe if any liquid leaks past a valve.

also check the cam journals for wear (due to an oil / bearing related failure of the crank)

If everything checks out, re-install it and check the valve lash.

 

'Budget friendly' and failed 4-stroke can't be included in the same sentence...

With most 'project' bikes like this, unfortunately the cost to rebuild them reliably often equals or outweighs the value of whole the bike itself,

considering the age of the bike and everything else around the engine that also likely requires attention. (chassis bearings, suspension etc.)

 

As another member once suggested, 10+ year old 250F can be considered 'disposable' bikes when the suffer a major failure

but since you mention the rest of the bike appears to be in otherwise very good condition, perhaps then it's worth investing a bit into it.

 

is the scoring usually only on the intake and exhaust sides? in my snowmobiles the wear has been more uniform in the cylinder. 

could the rocking in the bore cause the crank bearing failure?

Would you consider this a major failure? with what I am into the bike now, I'm good with going the reliable route just wanted to take the best course of action to get this done since its a back up or loner bike.

Edited by KyleYZ450_07

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Posted (edited)

In some engine designs, skirt wear from rocking is more apparent on the intake side (was on my CRF250X)

if your snowmobile is a 2-stroke, the piston skirt is longer and resist rocking more than the ultra short 250F piston.

not related to crank bearing but as mentioned more likely from too many hours.

Ingested debris can be uniform around the bore, the intake valves/seats also take a beating if air filtration wasn't adequate.

 

As for 'major failure' well not really as it hasn't grenaded and taken out other pricey items like engine cases etc.

but, as with most high performance 4-stroke MX engines, to avoid sooner than anticipated failures and have decent reliability,

at this point you are reaching a costly 'might as well do it while I'm in there' situation where either you spend now

or risk having to tear into it sooner rather than later.  'partial' rebuilds never lasts very long.

 

IMO spending $$$ on fixing the bottom end and re-using that cylinder as-is would make no sense,

so would bolting a cylinder head that leaks past the valves. Ending up chasing issues and wrenching on it more than actually riding it.

 

If it wasn't done within the last few years, figure this will also be on your to-do list :

Not as simple as a 2-stroke carb but not all that complicated if you are mechanically inclined.

 

If the rest of the 12 year old bike is worth 700$+ in engine only repairs is solely up to you.

Edited by mlatour

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