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05 CR250 cylinder on 03 bottom end?

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hi all,

does anybody know what's involved in putting an 05 cylinder on an 03 bottom end?

i know the exhaust is different, but how about anything else?

any help on this?

thanks

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I haven't tried it, but it should bolt right up.

You should compare piston weights if you intend on using the 05+ head and piston. Otherwise I would just use the 03 style piston and head with the 05+ gaskets.

Check the piston height in the bore and verify that it is close to the same as the stock 03 cylinder.

Ideally I think that the piston should be set level with the top of the cylinder, Euro bikes usually have multiple base gaskets to set this zero deck height. I believe this is how the factory intended it, and seems to agree with what tuners are doing in relation to shaving the bottom of the cylinder. If you also notice at least on the 02 (haven't look at my 07 yet) the exhaust port is lower and not level with a fully open PV, which is the other mod that they do after shaving the bottom raise the exhaust port.

I wish the factory would have released the blue print specs on these motors, so we knew what they were originally intended to be by the design engineer before the assembly line introduced acceptable tolerances. :ride:

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The 05 cyl will bolt up fine. You have to use the 05 head & exhaust. The bolt pattern is smaller on the 05. You have to use the 05 piston or cut the 05 head to match the earlier flat top piston. I think you have to mod the head stay slightly (open up the holes). If you use the 05 piston you will need the 05 piston pin also (it's shorter).

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thanks for the replies, i have the 05 cylinder, head and a fmf gnarly front pipe

i have looked at a few parts diagrams and it seems the o5 has different reeds 6 petals instead of 8 on the 03, is it worth using the 05 reed block?

also seems the 05 has a different cdi and tpm on the carb, can't see this would make

much difference?

also the crank is a different part number, it's the same stroke though

i think i'm going to use a new wisco piston kit, but there quite expensive here in the uk

the reason im doing this is my barrel needs replating and this 05 has come up, cylinder and head is cheaper than a replate.

thanks again for all the replies, great help

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The crank is a new design but is dementionaly the same. The 03 cdi & reed cage will work fine. If you use the 05 reed cage & boot you may need the air box boot also. They raised the carb up some. Can't remember how much (10-15mm). If you want to use the newer flat top Keihin you want to use the 05 boots.

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