Cr 250 exhaust bridge help

[Jakeyboi]

Hi guys. Just signed up to the forum looking for help. I decided to do a top end rebuild on my cr 250 02 , it was running spot on. But when I took the cyclinder off , the rings and piston were in a bad way. Also the top of the exhaust bridge was cracked. So I've sent my cyclinder to a local engineer who's goin to put a sleeve in my cyclinder and bore it to suit my piston. I've got a wossner piston , I've heard about drilling holes on the piston , and relieving the exhaust bridge, how would I go about doing the exhaust bridge. Thanks in advance

[DirtRider500R]

Dremel, CAREFULLY. Or have your builder do it.

[Jakeyboi]

It's still at the engineers ATM. I'm taking my piston down tomorrow so he can bore it to suit. So I'm goin to ask him to do it for me. What should I say to him. Like how much needs to come off the bridge ect. If the bridge gets releived will I still need to drill holes in the piston

[frdbtr]

It's still at the engineers ATM. I'm taking my piston down tomorrow so he can bore it to suit. So I'm goin to ask him to do it for me. What should I say to him. Like how much needs to come off the bridge ect. If the bridge gets releived will I still need to drill holes in the piston

 

The holes are to cool the exhaust bridge so Yes, you will want to drill them if you are using a forged piston (I am assuming so since you said it was a wossner). 

[Jakeyboi]

I'll get it drilled aswell then. Is there anything else that needs doin that can help while it's on bits. How much does the Bridge need taking off. Thanks for the fast replies guys

[Jakeyboi]

Sorry to be a pain. But I'm dropping my piston off later today and goin to ask him to relieve the exhaust bridge. How much do I need to tell him to remove.

[DHS_sponsored]

On the cr500 with a steel sleeve it is recommended to relieve the bridge, in the middle, (deepest point) by 0.08-0.10mm. Gradually arcing back to the regular size at the top, and bottom of the bridge. Extend the relief 5-7mm above and below the port. For example, if the port/bridge height is 30mm, the whole relief would be 40-44mm long. I hope that makes sense. It would be very similar I am sure for the 250.

[GEOF]

Sound interesting. 

I like to allow 3 thou on the bridge and as mentioned above allow a little say 5 mm at the top and bottom for blending back into the original size. 

You can do this with wet and dry paper. Start with a 100 grit and work out to a 400 grit. Do it slowly bit by bit stopping to checking the measurement. You can use a dremal as mentioned above but be careful not to move off and cut into the cylinder.  

I could be wrong but I would have thought the piston would have been supplied with the bridge cooling hole/s in it.

Allow 1 thou for each inch of bore size for the piston to cylinder clearances. I like to allow and extra 0.50 thou for clearances using a forged piston as the thermal expansion rates are faster for the forged piston. Once the sleeve has been installed you will need to chamfer the edges so the ring or rings don't run the risk of snagging. A very light 1 mm chamfer will do with small files or 400 grit wet and dry paper.

If you are planning on doing some match porting to blend the cylinder to the sleeve be careful on how thin you make that bridge for the sleeve. If it is too thin it could cause problems. Try to keep it the original size.

Don't forget to gap the ring/s and clean the bore in hot soapy water and use a clean white lint free rag to remove all the dirt. Shoot it with brake clean and then lightly coat the cylinder with 2 stroke oil. This needs to be done within 1/2 an hour. Don't leave it for a day then coat the cylinder oil as soon as you can after cleaning. Your engine builder should know most of this stuff anyway so just get him to do it. If it is a machine shop then they may not have the same experience as an engine builder. 

 

Good luck.

[Jakeyboi]

Ok thanks a lot. It sounds a little confusing for me so I'll take this and just let him read it. The holes didn't come pre drilled but that's something I'll tackle my self. Is there any info on the net about sizes and position of the holes. I know they need to be inline with the bridge. But I dont no how far down to drill the holes and what size they need to be