Vintage Dirt Bike Q & A

will a topend off of a 79 work on a 78 bottom end.

thanks

Yes. Check that the piston you use is drilled to lubricate the exhaust port bridge introduced with the 1979 cylinder.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

if i have a nos honda piston why would they make you drill the holes.

thanks

Wiseco recommends some oil lubrication holes for the 1979 cylinder because of the exhaust port bridge which wasn't present in 1978. Honda however offers the same part # piston for both 1978 and 1979 (13100-430-000), so I assume both don't have holes. I use the former, hence the advice.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

every 78 CR250R ive owned had a bridged exhaust port-the port that is different is the intake port-it has a bridge crossways in the middle of the main intake port not present on 78 CR250R cylinders-if there is something different i havent seen it-anywho the drilled skirt is a good idea on any bridged exhaust port

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1979 Honda CR125RZ
1982 Honda CR125R
1979 Yamaha YZ100
2003 Kawasaki KX250

Thanks for clearing this up (and the pictures you sent). I have cylinders with and without intake port bridges. Now I know that they are 1979 and 1978 respectively. I am sure that at one time I had a cylinder with an un-bridged exhaust port, but I let that one go with a parts bike so I can't check. I wonder if some clown ground the bridge out in a "porting" job? Or perhaps I am losing it If you have your 1979 cylinder sleeved check the smaller of the bridged intake port(s). I found that the hole in the LA Sleeve was much smaller than the port itself which can't be good for the air-flow. I assume that you drill holes in the NOS Honda pistons you favor?

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

i can tell you one thing i am not drilling holes in my piston.

And there lies the difference between well-intended advice and an instruction.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF