Seized piston, do I have to rebuild the bottom end?

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redbug2
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Seized piston, do I have to rebuild the bottom end?

#1 Post by redbug2 » Fri May 03, 2013 10:20 am

Hello,

Working on an RD350 engine. Owner had driven without filling injector tank... Engine froze. When I got the bike I rocked it back and forth in gear, crankshaft was moving. Sprayed some WD40 in spark plug hole. Was able to get it kicking freely, sounded rough. Pulled off left cylinder, evertyhing looked great. Right cylinder, not so good. Heavy scoring on the piston and cylinder. Had to tap the piston out with wooden hammer handle.

Lower end looked fine, no up and down play on connecting rod. Wrist pin bearing was intact with no scoring. No bearing noises when I rotate the crankshaft.

What should I do? Can I put this together without rebuilding the bottom end? Was thinking of boring cylinders and getting new pistons, wrist pins, rings... the whole kit. It was already .50 over, but it looks like there are kits available up to 1.50 over.

Thanks,
redbug2

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steveloomis
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Re: Seized piston, do I have to rebuild the bottom end?

#2 Post by steveloomis » Fri May 03, 2013 10:28 am

Running with out oil is serious. Boring and replacing just the pistons without rebuilding the crank is like buying 3 new tires for your car.... Crank was probably damaged, why take a chance. You'd probably have to tear it all down again and rebuild it soon anyway. New crank seals etc. always a good idea. My 2 cents...
'81 RD350LC hybrid w/84 RZ engine. Zeel, Old style Toomeys, Yboot and filter 420 main 22.5 pilot, Air screw 2.5 turns out, DJ needle 2nd from top, Canadian VM26 carbs.
'85 KR/RZ US. Tank is seeping causing a blister in paint.....

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Bngt
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Re: Seized piston, do I have to rebuild the bottom end?

#3 Post by Bngt » Fri May 03, 2013 5:15 pm

If there is no play and not a lot of fragments I'd keep the crank. A top end rebuild is comparatively quick. If you later have to do the bottom end so be it but it is a chance worth taking in my opinion. 3 cents ;-)
F2 1986 project (ZX636R suspension)
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seahorse
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Re: Seized piston, do I have to rebuild the bottom end?

#4 Post by seahorse » Fri May 03, 2013 7:13 pm

agree with both, having said that if it is possible to get an engineer to measure the run out ( crank tolerance) whilst it is still instaleed in the bike may be an option, if the crank is still within tolerance you would be good to go. Just keep an eye on your temps and any oil leaks after though, as steve said damaged outer mains can cause all sorts of problems. (sucking air running lean, or leaking oil, or both.

As a side note if your in the US why dont you contact Roger he could probably rebuild the crank for you, and whilst your at it you could replace the seals. Will be an oportunity to have a look at the clutch, gearbox, and shifter forks.

Double note if the injector tank has been let run dry, id be having a squize at the oil pump as well.

Sounds like a big job, but the only real specialist tools you will need is a flywheel puller, a half decent socket set, a manual and a bit of luck.

Hint, if your going to do it, lossen off the stator and the clutch whilst the motor is still in the bike. Oh and if you do it, order a new clutch tab washer and water pipe o ring cause you will need the at the end.

It will be fun, and the LC is a very sort after bike, worth the trouble.

My 4 cents
81 RD 250 LC Hybrid .Widened frame with Aprilia RS250 Back end & Front end, strokd RZ 421, Kennys TSA's, 31KVM26SS Carbs M-360 P-25 ALBA Ndl Pos 3, Air 1.3/4
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dude99
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Re: Seized piston, do I have to rebuild the bottom end?

#5 Post by dude99 » Fri May 03, 2013 10:07 pm

At the very least split the cases and have it looked at like suggested above. You should replace the crank seals anyways so you'll be in there
1981 RD350LC

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nokturnal
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Re: Seized piston, do I have to rebuild the bottom end?

#6 Post by nokturnal » Fri May 03, 2013 10:43 pm

Where are you?
:smt017

redbug2
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Re: Seized piston, do I have to rebuild the bottom end?

#7 Post by redbug2 » Fri May 03, 2013 10:47 pm

Thanks for all of the information.

I'll direct the owner to this post.

Personally I want to get it bored and see how it runs. If it doesn't sound right I'll go all the way.

I've had a little experience with rebuilding cranks, did a YR1 once. Spent more time tracking down the bearings than rebuilding the crank. Would rather not have to go through that again. Hopefully the RD bearings are more plentiful.

It's a nice little bike. RD250 frame with RD350 engine. RD400 front forks and wheel. Lots of little extras all over. It was really done well.

I'm in southeastern PA.

Regards,

Redbug2

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nokturnal
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Re: Seized piston, do I have to rebuild the bottom end?

#8 Post by nokturnal » Sat May 04, 2013 12:00 am

so it's an aircooled bike then?
:smt017

dude99
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Re: Seized piston, do I have to rebuild the bottom end?

#9 Post by dude99 » Sat May 04, 2013 12:07 am

So your not even going to inspect it? Seriously, for the cost of a tube of case sealant just split the cases and have a look at the big end... At least then you can change the crank seals to protect your new top end... To just slap a new top end on it and ignore the bottom end seems unwise. But To each there own...
1981 RD350LC

gus
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Re: Seized piston, do I have to rebuild the bottom end?

#10 Post by gus » Sat May 11, 2013 11:31 pm

if 1 of those needles comes out of an overheated rod bearing and comes up through the transfer you will be pissed for sure,

siryamaha
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Re: Seized piston, do I have to rebuild the bottom end?

#11 Post by siryamaha » Fri May 31, 2013 12:34 am

It's a 2 stroke, they usually will run no matter what. How good, all depends................. Now with that being said, If it were me, I would do a complete rebuild. That way you know for sure all is good and you won't have to get stuck somewhere wishing you went through it.

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