View Full Version : Cylinder Boring for Total loss oil system

11-24-2010, 01:32 AM
I have my 1929 French bike (Terrot, 350cc) taken apart and the cylinder is out to be rebored. I have the oversized piston from france and my question is this:

Since this bike is a "total loss" system, where all the oil ends up out the exhaust pipe, does anyone know if the tolerances or fit of the piston to the cylinder is different than a normal system that recirculates the oil?

I have a good mechanic handling most of this work for me while I tackle the rest of the restoration work. The machinist on the otherhand may need some direction in this project.

Thanks in advance for any help.

Steve Slocombe
11-24-2010, 04:13 AM
Dear Kevin, on the VL, also with a once through oiling system, I've eased off from the usual rule of thumb of one thou piston clearance for each inch of bore, and use 0.004" clearance on the rear cylinder and 0.005" clearance on the hotter running front cylinder, adding a couple of thou for racing. This is with a standard bore of 3.422" and using strutted aluminium pistons. Harley also specified 18 thou clearance for the solid magnesium pistons, and 1 thou clearance for the unstrutted T-slot pistons, so check carefully what is your piston construction before making a final decision. Slightly on the loose side is better these days, as no one has the patience for the long running-in process originally specified for these bikes.

11-25-2010, 11:16 PM
Steve, many thanks, I'll provide this info to my machinist prior to boring out the cylinder. As for my piston, it is somewhere down south, not sure where. My mechanic up in Southern Maryland sent it out to be downsized a bit, apparently the one I ordered from France would call for too much boring of the cylinder, making the walls too thin. My translating ability apparently let me down. Anyway, thanks for the info. I appreciate any and all help I get from the more experienced among us.

11-27-2010, 08:18 AM
The key to total loss is not the piston to cyl clearance, but the ring gap. You need a large gap so it WILL BURN as much oil as the pump feeds.

12-07-2010, 04:49 PM
Thanks for that.

I never would have figured that the ring clearance would be the main factor. I appreciate the info. I hope that my machinist is aware. If not, he will be when I show him the post.

Thanks again