Vintage Dirt Bike Q & A

Hi, I have a CR250M I'm rebuilding and possibly thinking of doing some Vintage Racing.

I managed to get a new 1st over piston, and thats what i took out but it has pre-ign damage. But I'm starting to rethink how much clearance there was supposed to be. The Clymers manual I have isn't specific enough about the piston to cylinder clearance measurement. I'm hoping that someone may have the factory specs. Does someone have the piston to cylinder clearance for a rebore, and also for the service limit. Does someone have the specs out of the factory manual? Or have a link or file of the factory manual.

Thanks for your assistance

The '73 CR250 manual indicates about .002" minimum piston clearance. The manual is available here, for the other specs:

http://www.alp-sys.com/honda-elsinore/m ... cr250m.pdf

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

don´t know what kind of clymer you have, mine says 0,06-0,07mm (0.0024-0.0028 in.)

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

The Clymer manual does state the piston to cylinder clearance as you noted, but it does not indicate if .0024-.0028 is the clearance from new(or when re-boring), or the service limit(wear limit). To me those specs are not clear enough.

before are the std sizes and service limits for the cylinder and piston listed.
take a calculator and you will know what is new or worn clearence size.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250