250x Engine Rebuild Help Needed

[ZSK]

Years ago I had a 300ex but never had to get into the motor at all, now I'm into a project with a 250x motor and I'm trying to figure out just what I have. I bought the motor used and was told it was "built" but when last run it just shut off.

I tore into it tonight and the culprit was a bent intake valve, the valve retainer(s) popped off and the valve hanging down contacted the piston. The valve guide was chipped and that needs to be replaced also. The cylinder looks like it could use a fresh hone and rings, so onto the questions....

I need to source a set of rings, an intake valve, and a valve guide but I don't know exactley what I need.

The bore measures 80mm, stroke is 57.5mm that comes out to 289cc. The valves measure 29.1mm intake and 24.5mm exhaust. The cam has the markings 12-03 (I'm assuming this is a date) and 16605 (I found a Megacycle datasheet online that lists this part number).

Thoughts?

[Bandit65]

Any markings on the piston?

The 80mm would be the 330cc for a 300ex, I do not know of anyone that makes overbore pistons and rings for the 80mm sleeve, if the sleeve isn't worn too badly perhaps a set of replacement rings for the 80mm piston will do for a while.

Stock stroke is 57.3mm, looks like you do not have a stroked engine.

Stock valve sizes are 25.5mm intake & 22.5 exhaust, so you probably have aftermarket stainless valves. Try Kibblewhite for replacements.

Depending on the lift of the cam you may need a shortened guide.

[ZSK]

No significant markings on the piston. The sleeve and piston are in good shape, fresh rings and hone would take care of it.

Would the rings be the same size for other manufacturer's or are they dependent on the company?

[Bandit65]

They may be a bit different from each manufacturer, dependant on the intended use. Full race pistons will use a slightly thinner ring set for less friction. I would measure the thickness of the rings and contact the piston manufacturers; JE, CP, Numara (this piston always uses a coating on the skirt, fairly easy to tell from the others), Wiseco, etc. Wouldn't hurt to call Powroll either, they have popular bore kits for the 250x-300ex. I don't think they manufacture their parts though, but could be wrong. I just finished putting together a 300ex build with an 80mm JE piston, I could scan the data sheet that came with the kit and post if you think that would help you out. Here's a couple of pics of the piston installed.

http://i111.photobucket.com/albums/n128/bandit1100/Quads/IMG_6416.jpg



http://i111.photobucket.com/albums/n128/bandit1100/Quads/IMG_6413.jpg

[ZSK]

If you could post it or email it to me, zkruske@gmail.com that would be great.

Judging by your sig, if that's 11.5:1, my piston must be in the 10:1 range. It has a flat top with valve cut outs, no dome to it.

[Bandit65]

I'll see if I can take care of that tomorrow evening after I get home.

I've got another 300 engine that's destined for a minibuggy that has a 10:1 first overbore (74.5mm) Numara in it.

http://i111.photobucket.com/albums/n128/bandit1100/Quads/IMG_6160_web.jpg

This is an 11:1 Wiseco,
http://i111.photobucket.com/albums/n128/bandit1100/Quads/IMG_6163_web.jpg

[Bandit65]

Here you go, sent an email as well.

http://i111.photobucket.com/albums/n128/bandit1100/Fluff/scan.jpg

[ZSK]

Measured my rings tonight and got .98mm and 1.2mm, it looks like JE rings might work if the groove is slightly oversized.

[ZSK]

My appologies for the cell phone pictures, top of the piston. Has an arrow pointing to the exhaust.

[ZSK]

Here's the side profile. The bottom of the piston has 79M on the bottom.

[ZSK]

I've browsed around a little more and I'm wondering if this is a Powroll piston kit. I've sent an email off in hopes of identifying the piston.

[Bandit65]

When you say the 79M code is under the bottom, as in turning the piston over and it's stamped (or cast) on the bottom side of the crown? That's sounds a lot like my JE, although it could also be a Powroll product as I don't have any experience with their piston kits. Hope you get to the bottom of the mystery.