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1975 DT250 Top End Rebuild Advice

16 Oct 2020 14:53
George56
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1975 DT250 Top End Rebuild Advice #1
I'm new to the forum so thanks in advance for any advise. I bought this bike a few days to use as a rider. It looks like it's all there and in decent shape. On the test ride it runs ok but not as responsive as what I thought it should be. Also at idle there's a loud knocking from the engine that goes away as you twist the throttle and increase the ram. I'm thinking it's piston slap and needs the top end rebuilt. I'm a total novice when it comes to engine rebuilding, but after reading some of the restoration projects on this site I feel more comfortable jumping into it.
So, should I just replace the piston, rings and connecting rod bearing or do I need to tear the engine all the way down?

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17 Oct 2020 02:36
MrBernzo
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1975 DT250 Top End Rebuild Advice #2
Nice looking bike!
No need to tear the whole engine down for a top end. The best way forward is to find a shop or an engineer who can measure your cylinder accurately. It may need to be bored out to the next size.
They will then be able to advise what size piston you need.
A top end rebuild is not particularly difficult. You should replace all gaskets too. You will need a torque as well to tension up the cylinder and head bolts.

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17 Oct 2020 03:55 17 Oct 2020 03:58 by Hammer. Reason: Missing an
Hammer
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1975 DT250 Top End Rebuild Advice #3
George56,

FIRST, measure the cylinder (measuring locations shown in the Tech Section of this site), then get someone who is familiar with boring air cooled two stroke cylinders to see if the next oversize will clean up the cylinder walls or not. Once you have decided what oversize piston you will need, get the piston & rings, and give them to the person who will bore the cylinder to match the bore job too. He NEEDS the actual piston to do the job correctly--DO NOT just bore it to a size, and ten go looking for a piston and hope that it will work. I've done a bunch over the years, and two on my current RT-3, 360 Enduro (the first job had to be done over after the bike was sabotaged (story on this site). It's now running great, and I have the next oversize top-end kit on hand for whenever I need it.

Hammer

"Who, More than Self, His Country Loved."

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17 Oct 2020 14:33
George56
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1975 DT250 Top End Rebuild Advice #4
Thanks for the reply MrBerzo and Hammer. I'll review the tech section for measuring locations as suggested. If the bore is clean would a new piston and rings fix the slap. I guess I'm asking if the part that wears first is the rings, and by replacing the rings the tolerance is back to normal. I understand that if the cylinder needs to be bored the piston will need to go with the new bore size.

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