Forum

[Presto]

Don't confuse the standard piston with the forged piston - the forged piston needs approx 6.5 thou. That is more than the standard piston - and needs to be.

[ChrisD]

Sorry guys, I may have assumed too much. My iron bullet piston kept seizing - no, not the rings or bearings but the piston itself. I know others have had similar problems with the iron barrel, so I assumed the same for yours, Devon John. I understood that 3.5 thou is way too small a clearance for an Indian aluminium piston in the iron barrel - especially if you want to ride it an anything over what I hear is the usual Indian freeway speeds of 40mph. All I ever wanted to do was to cruise at 100kph (~60-65mph) on our South African freeways and still have oomph left to escape a looming truck. Finally got there too (but I can still cruise like that effortlessly on my 1951 Matchless G9 (twin 500cc).
Cheers, ChrisD

[simon]

The interesting thing about my old bike seizing was that it was the piston seizing in the bore due to overheating but it was caused by lack of oil flow to the big end. The splash from the big end is a major source of cooling for an old machine. I took an age to pick it as the big end always felt fine and I assumed it was the original white metal one that would have run in a minute without oil. The fact that it had rollers meant that it was getting just enough lubrication from the oil mist in the crankcase not to fail. It had clearly been a bit warmer than it ought though.

[devon john]

piston broke up , cracking barrel , twisting con rod, bending valve and pushrod!!!!!!

not good and expensive

not sure of where to go from here ???

john

[Norm]

John, doesn't help much but something was not right in the rebuild. Any idea how many mls on that conrod?, possible crank not true twisting the rod

[devon john]

it was a alfa recon crank with new rod and alfa bearing .

catastrophic worst engine damage i have ever had!!!

so back to the drawing board .

engine out strip check then start all over again.

john

[Dennis C]

The only way you would get a bent valve and pushrod is for the piston to hit the valve, now this is pretty much impossible on a running engine unless the valve jams wide open, the resulting damage could well be as you describe.

On the rebuild did you fit new valves and guides is so and the guides were cast iron and the new valve was stainless as most are now that would cause you problem, stainless picks up in cast iron and you need to use either phossy bronze guides or plated valve stems.

[devon john]

Dennis the piston parted at the gluggon pin then contacted the valve ,and yes new valves and bronze guides
its a mess !!!!!

still got to get it back together as its worth nothing as it is.

[Phil Ashbrook]

The bore gets lubrication from the big end fling , have a look at the oil crank feed cork , I got the neoprean replacement as I find on strip downs the cork is cracked and loose .

[devon john]

Phil new neoprene was fitted,,, a engineer suggested the rings might have been to tight !
what ever it was it did a dam good job!!!!!

will take the engine out latter today ,then see whats salvageable .