Vintage Dirt Bike Q & A

Sam, I looked inside a buddies 76 CR125 cylinder today & the exhaust bridge looks like it swelled up?? Run your finger down the bore and you can feel it too! He's running a Weisco Piston & had the holes drilled but what makes the exhaust bridge swell up like that?? Heat? It melted & grooved the top ring to the piston on the exhaust side. The intake side part of the ring was also grooved pretty bad. Not to mention it looked like a kindergarder with a loaded dremel did the chamfering on the exhaust & intake ports! The motor still ran too..just not good! How do you relieve the exhaust bridge??

Thanks

It probably has aluminum built up on it and that is what you are feeling. I have had them swell on the intake side too, but the engine got extremely hot, we were too lean, and you could also have a lean condition and the engine is getting very hot, you need to check that. I use a small stone to work the bridge down, you need a coarse one to rough off the material, then go back to a fine one. You could also have a problem with the bore not being round, a lot of shops can't get them round, and if they made a mess of the chamfers on the ports you can probably bet they messed up the bore too.

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400

Thanks Sam. Question..would you try & work the bridge down before or after a new bore?? I was thinking if you got in there with a stone "after" it was bored you could ruin that new bore??

If you were working on taking the bridge down "before" a new bore, the new bore might clean up any imperfections you may of caused by the grinding?


I know one must be carefull when it comes to something like this (kinda like a delicate surgery).

Forrest Rohrs
1982 CR250R
1974 CR250M
1974+75 CR125's in the making.
1973 LT2M 100 Yamaha

I do them after I hone the cylinder, and yes you do need to be careful and not scratch the bore, if you are not comfortable with doing it after honing you can do it first, but you are just guessing as to where you need to be. The best I can tell, Honda had a machine that did them, they were too perfect and too time consuming to do them in production by hand. When I say do them with a stone, I use a small flat stone and do them by hand, not with a grinder.

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400

Sam427 you seem very knowlegable on this subject maybe you could advise me a little,just having my CR125RA 1980 re sleeved back to standard and when I get it back I'd like to do a little port work on it,I have access to some suitable equipment,apart from to usual tiding up and smoothing out (not polishing) the ports,any recommendations on any other changes I could make to improve things,eg should I raise the exhaust port slightly etc,also maybe awayout question but has anyone ever removed an exhaust bridge altogether successfully ?

I wouldn't recommend removing the bridge, it won't run long like that. Basically on the bridge, Honda says put a radius that dips in at the midpoint of the bridge about .004" and feathers out at the top and bottom of the port. I have done a bunch of work on those cylinders, and I have seen everything from stock to wild on porting, lets just say that less is more. I would love to help you, but I don't give away my trade secrets on the engines I build. Good Luck!

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400

Thanks Sam,yep intended to do the exhaust bridge as you had recommended anyway, I was just curious about removing the bridge,don't worry I wasn't angling for any trade secrets just some basic's thats all,unfortunatly your on another continent to me so sending it for some work is abit of a pain,I like to do as much as I can myself (well everything actually)but don't want make a mess of it so just general tiding up it is,
thanks for taking time to reply though,just been told my barrel is ready so should have it back in a couple of days.