Sachs 6B Engine Rebuild

[Todd Rovello]

Guys:

My current project is a 1973 Rupp RMX 125 with the Sachs 6B engine. The piston to cylinder clearance is right at .005 inches on a standard bore. The cylinder has been recently  honed. Should I just install new rings or re-bore and a install a new piston? I believe .005 is close to the wear limit and am leaning towards going one size over.

Also, has anybody had success repairing the right side case due to chain damage? The top flange that butts against the ignition cover above the counter sprocket and the circular flange protecting the Motoplat are chewed up pretty good, but the case appears otherwise solid.

Thanks for a great site.


Todd Rovello

[Daniel P. McEntee]

The decision to bore the cylinder would depend on the availability of  proper size piston. I gather just from general reading that some sizes are not easy to find sometimes. There is another thread here somewhere about having pistons coated, I think a ceramic coating, to improve the piston fit. I don't know if the cylinder would need to be bored to match or not, but worth investigating.
  As far as the case goes, it should be aluminum, and a competent TIG welder should be able to fill in material, and you then just grind away what you don't need. I've repaired cases that way. Holes can be backed up with steel so they can be filled up, and the TIG process gives you the control (plus some technique and a steady hand) to do vertical build ups to fill in gaps at the case seams. It's harder to explain that it is to do sometimes. I don't have any TIG welding equipment at my disposal right now or I would offer to do it for you.
  Good luck with the project. I think I have seen pictures of the Rupp 125MX, and should be pretty cool when finished.
  Type at you later,
  Dan McEntee

[Todd Rovello]

Dan,

thanks for the info. Fixing the case definitely sounds do-able. I happen to know a couple of good tig welders in this area so I will try that route. This is my second Rupp restoration. The first was finished last year and it was the bike I owned as a kid.

Thanks to POG for a great site and to the members for being willing to share their knowledge and give advice.

Todd