top end rebuild time??

T

tnutcher

Member
Jan 10, 2008
55
0
I've been trying to get my bike jetted correctly - 91 rm250. It had a aftermarket carb so it took some fiddling to get the float height set and crap and got it running and running pretty crisp finally. Well i was doing a WFO plug chop last i rode it and pushed it to my shop to check the plug. The plug was a lighter gray brown but i couln't see a ring developed yet down in the threaded section so figured I had to ride it longer than the 10 minutes i did to get a valid reading and that I'd try again later. Well tonight put the plug in and went to fire it up and it wouldn't build compression on the kick lever like i left the plug out. checked it and i couldn't build more than 65psi - before it was 140psi-ish. What the heck happened? Any guesses? i'm figuring top end but don't know what might have blown. . . rings or piston or rod. . . the clutch wasn't perfect it lurched when put into gear all the way in. . . and havn't torn into it yet I'd like to have an idea of what to look for.
Thanks for any help, advise, or ideas on what the heck happened to my bike.
TNutcher
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
You could cheat and have a peek in the exhaust, see if the ring seized. Bottom line, you have to yank the top off and check. Reading them plugs is going to be accomplished 1 of 2 ways. Buy a QUALITY plug viewer or cut the threads off. No grey area.
 
T

tnutcher

Member
Jan 10, 2008
55
0
Well it kicks over freely and all but yeah I'll tear into it. I'm a little aprehensive about doing so though if machine work will need to be redone. On a typical top end, what fails and is there any machining to be done?

And on the plug test how much riding is necessary to get a good valid reading off a fresh plug? Thank you
TNutcher
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
As long as nothing came apart, a LIGHT hone, check the new piston skirt to bore clearance, skirt to exhaust bridge clearance, ring gap, top and bottom rod clearance should be about it, and the reeds! In the future, ideally, after the set time on hours in that bikes manual, tear it down and check it. You will have a better idea how YOU are wearing YOUR engine. Running it till something seizes is always way more costly than a set of gaskets. And it pushes your bore out of round, and puts way more stress on the crank. New plug, warm the engine, take it out through all the gears and clear it out, turn around and perform the chop, wfo for few seconds and chop it.
 
T

tnutcher

Member
Jan 10, 2008
55
0
tore it apart and the exhaust side of both the piston and cylinder are scored pretty deep. Piston has a crack in exhaust side skirt that goes up 2/3 way. Intake side looks good. The rings are stuck in the piston and light metal dust from the scraping on the piston top near exhaust side. The rod-piston bearing rolls well but has left right play as does the free con. rod on the crank. . . Not sure how much side to side play is too much there but it does feel like a lot.

WTF happened? Was it me that put the bike over the barrel or hard to say? I understand whats messed up and why the bike won't hold compression but not sure what caused it and why it seems to be just exhaust side. . .
Thanks for your help I appreciate the knowledge
TNutcher
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
First questions: 1, how many hours were on it? 2, Does it have an exhaust bridge(thin metal strip, top to bottom of the exhaust port, right in the middle)? 3, How many hours are on the bottom end. There is a measurement for how much the top of the rod rocks, up and down, and in between the crank and thrustwashers. Check these also. Sure hope you have a shop manual.
 
T

tnutcher

Member
Jan 10, 2008
55
0
Not sure the amount of hours on the top or bottom end. This is the first time I've had the motor apart since I bought it in Feb 2008. The PO said the top had been done recently but. . . and not sure about the bottom.

Yes it does have the exhaust bridge - why do you ask?

TNutcher
 
J

Jbgabriel

Member
Feb 23, 2006
31
0
Were there holes drilled in the piston where the exhaust bridge would be? If not you found your problem. Those holes are crucial for the lubrication and cooling of the bridge. Without the holes you experience the problems you have now.
 
R

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
When you were running at wide open throttle, how long did you hold there? A few seconds you should be okay, but the longer you hold it the more likely it will be that the engine siezes.

As for why the exhaust side: that is the hotter side of the cylinder. The intake side gets cooled off from the fresh air/fuel entering the cylinder. The exhaust side just bakes.

The piston/wrist pin should be able to slide back and forth on the rod a considerable amount. There should be a small amount of side to side motion of the rod on the crank.


Rod
 
T

tnutcher

Member
Jan 10, 2008
55
0
Jbgabriel said:
Were there holes drilled in the piston where the exhaust bridge would be? If not you found your problem. Those holes are crucial for the lubrication and cooling of the bridge. Without the holes you experience the problems you have now.
Click to expand...

Crap no the piston is solid. That would explain it along with holding WFO too long on my part. Is that something that comes already in a new piston or are the holes a modification to a new piston? Thanks
TNutcher
 
T

tnutcher

Member
Jan 10, 2008
55
0

WOT was maybe 20 seconds max. But this had been the fourth time i'd done it.

Yeah I just didn't think it would score the walls and pistons like this. . . i guess it just got too hot and the rings started melting and piston cracked then started scoring when it cooled off rapidly??

Both bearings on the rod ends seem pretty darn good. The crank side does have side to side but i felt no up and down play. The piston end had the same.

The bottom seems to be solid still just the screwed up top end. . . what are my options with the cylinder and getting a new piston??

Thanks a lot!
TNutcher
 
B

bwood

Member
Mar 21, 2004
135
0
If there are no deep gouges in the cylinder walls a light honing may be all it needs. Take a straight edge and make sure the exhaust bridge is not eaten away and is still flush with the rest of the cylinder wall. If it is not, you will need to look into your repair options. I'm sure the guys on here know of some good places to send it. I have always gone back to stock. As far as the top end I checked MotoSport Outlet and it looks like they have a standard Wiseco kit in stock for $156. This is everything for a top end job (piston, rings, wrist pin bearing, gaskets, circlips and wrist pin). I would go this route. Follow the directions in the piston box and check out this link for a previous discussion on the purpose of the lube holes and the exhaust bridge.

One more thing, double check your fuel mix (fuel to oil) and make sure it is approiate.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
I have to disagree, that bridge needs more relief than the bore, at least .0015" to .002". Wide open did not cause the seizure. No holes in the piston/ bridge relief, is what caused it. A fresh engine will last 30 minutes plus 2 laps at WFO! Clean the bridge with a green scotch pad. It will remove material.
 
T

tnutcher

Member
Jan 10, 2008
55
0
I agree that sounds right. Though riding at WFO for those times just accelerated the wear.

so just rub the bridge with an abrasive cleaning pad to get that .0015-.0020" relief on it?? There isn't a more exact way to machine the relief gap??
Thanks,
TNutcher
 
J

Jbgabriel

Member
Feb 23, 2006
31
0
Relieve the bridge as said above and when you get your new piston make sure to drill the holes in it. They really are crucial to preventing this from happening again.
 
T

tnutcher

Member
Jan 10, 2008
55
0
sweet that's easier than I thought it would be. I dropped the cylinder off at a local shop who will over bore it and get a matched piston for me. Does the cylinder head need to be machined to match the over bored piston size or how is that done?

Thanks for the continued advise and support guys, it helps a lot.
TNutcher
 
R

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
No, the cylinder head should be fine. It is possible that the power valve may need to be ground down but usually it is okay up to the second or third overbore.

Pay careful attention to the directions that come with your new piston. It is common for the piston to come in a plain configuration that does NOT include the oil hole to lubricate the exhaust bridge. In this case you will need to drill a hole in your new piston.

Also be sure to check the end gap on the rings. Slide a ring into the cylinder, then use the piston to push it down a ways and make sure it is level. Use feeler gauge to verify the end gap. You may need to file the end of the ring a bit. Failing to ensure proper end gap can lead to another sieze as soon as the engine gets hot.

Also be sure to install the rings right side up. There will be a little pin in the ring groove, somewhere opposite the exhaust port, which locates the ring. The end gap of the ring will be skinner on one side to clear the pin.

If you find yourself in need of a ring compressor a simple hose clamp will work. I recommend putting a few kinks into the band so that the cylinder will push on it better.

Also be sure to clean the cylinder really well before you start assembling. They often come back from the shop all gritty from the final honing process and you don't want that grit in the engine.

Another tip: when you are installing the piston on the rod, stuff rags into the crankcase opening so when the circlip pops off it doesn't land in the lower end!

Rod
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
If a trusted shop is boring the cylinder, it must have a sleeve in it? When you get it back, wash it in warm soapy water, dry and spray wd-40 on all the steel surfaces. Like it has been said, double check your clearances and end gap! Doing this again in a week REALLY blows! Wiseco sells piston kits for liner overbores and kits for plated overbores. I would double check that also. Since it seized and you are not splitting the cases, I would flush the bottom end at least.
 
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