Hello from Germany

3 years ago, in a fit of madness, I bought a DT250 MX in parts , frame and engine from a DT400 MX with YZ400 top end and made a DT400 out of it. The engine got a complete overhaul (piston, connecting rod, all bearings and seals) by a "specialist" and caused a lot of problems.
After the first start, I noticed a crack in the housing near the oil pump, the clutch was slipping and rattling and there were strange engine noises. Thereupon I replaced housing and clutch basket with parts out of the DT250 engine.
The engine has run 300 kilometers since then, but unfortunately I think it still doesn't sound good.

www.flickr.com/photos/201801744@N03/5414...um-72177720321945356

Any suggestions where the noise comes from?

Sorry about my englisch, I am a native speaker...
 

Hello from Texas. I am not as experienced as others in this group but my understanding is the noise can be from excessive bore to piston clearance.

Brett

Yep certainly sounds excessive clearance. Only run about 2 thou on the skirt. Only other thought is piston just touching the head. Sounds bad enough to be an undersized piston, like a standard piston in a first oversize bore which would give 5 thou clearance on the skirt if you ran a feeler blade up beside it. [Does that add up? Or should it show 10 thou if the bore is 10 thou bigger] OK I'm not thinking straight this morning. Try 10 thou blade then. Rattle sounds "that bad".

Thanks for your feedback.

I will remove the jug and have it measured.

Update
Bore to piston clearance was 6 thou on top, bottom clearance was 2 thou. Piston and jug are standard but the liner is not dimensioned correctly on top.

If you used the "feeler gauge" method, I hope you were using the same area of the piston skirt to take measurements?  The piston itself is not perfectly round and has a taper from top to bottom.  So even on a brand new bore you'll measure significantly more clearance with a feeler gauge at top of piston than at skirt.

Feeler gauge clearance is always less than actual clearance as well

 

Measuring was done in a repair shop with an "Internal Diameter Gauge"...

My rough method without the gear would be to feeler the skirt base with piston near bdc on the bench. Then invert the piston & feeler the skirt base near the top of the bore just to get an idea of wear above the exhaust port. Maybe down a bit say halfway between top & exhaust port. Rough as i know haha but that's me when no other easy way. Of course getting a machine shop to measure is the way to go.