Just had my beloved Bonnie rebuilt at vast expense.Ran it in for 500miles, and on first trip out taking to 3500revs she seized, top of Piston disintegrated. Mr Mechanic says did this coz' of my abuse!! But it was pinking & no way I would abuse my Bonnie(owned since new)He put new Pistons in-but it is still Pinking and I am worried same thing will happen again. Using Leaded/SuperU as before-Timing spot on. My mate recons I should retard couple of degrees and should solve problem ANY IDEAS PLEASE
While the "book" states you should time the T140 motor to 39 degrees, Brian Jones, Triumph's engineer found in extensive tests with gasoline provided by MIRA (English automotive testing grounds) that emulated what was available in the US at the time, that the bike would ping less if timed to 37 degrees.
While T140's are prone to pinging, they are most vunerable to detonation damage during the break-in period. There are two things working against you: 1. the rings are not broken in with reduce capacity to transfer heat out of the piston and 2. The oil that is getting by the rings into the combustion chamber reduces the effective octane rating of even the best gas available.
Further, there is a trend in the vintage trade to emulate a lot of cylinder preparation techniques used with modern cars and motorcycles that use steel or ductile iron rings. These rings are pre-lapped round at the factory and do not require the break-in of grey cast iron rings.
Grey cast iron rings are lapped in situ during break-in. Thus using a modern 300 plus grit surface finish, made worse by planar honing techniques, reduce the ability for rings to hone themselves round.
The old rings of the damaged pistons, if not roughed up too much during the seizure, can often tell the tale. Look at the ring there should be a shiny silver wear mark 360 degrees around the BOTTOM of the two compression rings. If it is not there or intermittent the bore was too "smooth" and the rings didn't get "honed" or broken in.
Also the wrist pin can give you a lot of information as to the piston's temperature during the seizure. For example: if the wrist pin is still silver colored, as it came out of the box, the seizure would NOT have been heat related and I would check the piston clearance.
There are other basic assembly techniques that will toughen a motor to detonation. Being sure the head gasket doesn’t stick into the bore, the valve seats aren’t narrowed to racing spec’s ( we like .075 inch or wider for the street). Remove all of the sharp edges from the piston. Be sure the valve’s exterior margin isn’t too thin, etc… Good luck… John
#114648 - 04/24/073:17 amRe: Pinking on a 78 Bonnie
Joined: Jan 2007 Posts: 57Floss
BritBike Forum member
Hi all another possible cause is that if you had the cylinder barrels nicely powder coated during the rebuild, you would have drastically reduced the effectiveness if the fins to disapate heat. All the minute pore in the casting are filled in and greately reduces the fin surface area.
I have seen a bike sieze after two rebuilds as a result of this.