Vintage Dirt Bike Q & A

hi
i bought nos 1st oversize piston and rings for my cr.the top ring groove is smaller than the bottom groove yet thr 2 rings nos are the same size @ rings in packet from honda via ebay.are the rings the same size or is my piston faulty and need to be machined and re pinned
cheers

_________________
Jim Garvey
1x honda cr250m 1973 in bits
1x honda cr250 champion dirttracker
1x yamaha tt500 dirttracker 1976
1x yetman triumph dirttracker
1x tt500 longtack speedway bike
1 x 1940 350 ariel speedway bike
1 x bultaco astro grasstrack bike antig

Well I went to check my rings and they are both the same size I then put them in my piston and they wont fit so I think I got the wrong piston as the rings are right at least the part number is. Anyone know if the 75-76 cr250 rings are smaller than the 73-74?

my reco man helped me today,he new straight away what the problem was.(he also used to race elsinors in 73 to 760.the top ring is a keyhole ring.put a straight edge on top of the rings and you will see 1 is tapered.the piston top groove is also tapered.mine had green paint on the nos on the ring so they couldnt be distingushed from each other.he also said one was green and one was red.so all is good now back to engine building

_________________
Jim Garvey
1x honda cr250m 1973 in bits
1x honda cr250 champion dirttracker
1x yamaha tt500 dirttracker 1976
1x yetman triumph dirttracker
1x tt500 longtack speedway bike
1 x 1940 350 ariel speedway bike
1 x bultaco astro grasstrack bike antig

Whoosh: 73-76 Wiseco rings are the same. Honda does have different part numbers for the 73-74 vs. the 75-76, but I've only used Wiseco since I had this bike, so I don't know if the dimensions on the Honda rings are different between the two.
standard bore on 73-74 is # 13011-357-003
standard bore on 75-76 is # 13011-381-003

I know the pin placement is different on the piston, but pin placement is different on the Wiseco pistons and they use the same rings with same part number, so maybe there is a difference on the Honda rings.

I am wondering if maybee the piston is wrong I can get one ring to fit in the top groove but not in the bottom groove. The piston is also missing the oil holes where the wrist pin goes. I guess I will just spend the extra money and get a wiseco piston this bike just keeps getting more and more expensive.

As sure as I can be, that is I don't know if someone screwed up piston swaps and rings before I bought the bikes: The 1974-1977 MT and the 1973-1974 CR use the same piston. The 1975-1976 CR use thinner rings. I had the same problem.

All the other measurements are the same AFAIK but I have heard here that the ring locater pins moved to allow for different porting. That is, you can move a 1973-1974 piston and rings into a 1975-1976 CR and go the other way too but you may have problems with ring gaps and where they line up with the ports.

Rick

Thanks rick and mitch I compared the pins on the new piston to the pins on a used piston from a 74 cr and they are different so I guess I got a 75-76 piston and 73-74 rings o well only cost me ten bucks.