Replace pistons or not?

Re: Replce pistons or not?

General rule of thumb for ring gaps is 0.003" per inch of bore, so 0.009" for a 3" bore

Re: Replce pistons or not?

Hobot,
I think we are talking about two different things. I'm talking about the difference in ring gap measurements done in the worn upper part of the bore and the relatively unworn lower part. It doesn't matter what the actual ring gap is just the change between the measurement in the two locations. If I follow what you are saying an overall increase in ring gap from when the piston were originally installed to a later measurement could also be use either to measure bore or ring wear given you know the initial measurement or assume it was close to specs.
Htown

Re: Replce pistons or not?

Yes ring gap is valid way to judge bore wear and taper. A real bore guage needed to examine accurately while correcting. Let us know what ya end up doing and how it works out. Takes a lot of miles to tapper Commando bores though rather less if poor air filtration.

Re: Replce pistons or not?

hobot said:

... consider Bore Tech treatment so barrels last a few more pistons/rings before new bores. About $350 and weeks processing though to cover a couple decades extra cylinder use, maybe.

Click to expand...

Should triple bore life.

Re: Replce pistons or not?

Found a machine shop here in Malta. Bore measured and found to be out of shape. Piston to bore gap would have been .0075" if I dropped new standard size pistons in that bore. Decided to go .020" over. I gave the machine shop one of the new .020" over pistons and asked that he bore the cylinder with a .004" gap between piston and cylinder. They did a good job. Here's a pic of the freshly machined bore:



New .020" over JCC pistons with Hastings rings with installed ring end gap checked and found to be .010":



CNW barrel mounting kit made the barrel install easy:



Just waiting on a new set of pushrods from Norvil to install before the head goes back on.

Interesting to note that the pistons sit slightly proud of the barrel. This can be seen in the last pic above. Apparently this is correct for Commandos.

Cheers,

Joe

Re: Replce pistons or not?

I would say you did a nice piece of work there. Where were you getting the pistons from and did you return the std ones?

Standard pistons return to Norvil with +.020 purchased from there also. Hastings rings purchased from Fleabay.

Re: Replce pistons or not?

Do Norvil recommend 0.004" clearance for these pistons ?
From previous chat here, thats on the tight side of a normal 4.5 or 5 thou clearance. (?).

When I asked the reborer to bore tighter than recommended for something,
he said 3 hail marys and refused to do it.
I subsequently saw others aftermaths that suggested he was right.
Loose is good, Tight is Trouble.

Re: Replce pistons or not?

freefly103 said:

Click to expand...

Could be the picture shows a distorted view but if not the pistons must be rotated 180 degrees (large inlet pockets on the backside) :!:

Re: Replce pistons or not?

freefly103 said:

Found a machine shop here in Malta. Bore measured and found to be out of shape. Piston to bore gap would have been .0075" if I dropped new standard size pistons in that bore. Decided to go .020" over. I gave the machine shop one of the new .020" over pistons and asked that he bore the cylinder with a .004" gap between piston and cylinder. They did a good job. Here's a pic of the freshly machined bore:

New .020" over JCC pistons with Hastings rings with installed ring end gap checked and found to be .010":

CNW barrel mounting kit made the barrel install easy:

Just waiting on a new set of pushrods from Norvil to install before the head goes back on.

Interesting to note that the pistons sit slightly proud of the barrel. This can be seen in the last pic above. Apparently this is correct for Commandos.

Cheers,

Joe

Click to expand...

What base gasket have you got fitted?

Base gasket is standard Norton part from Norvil UK.

Nortonspeed wrote: Could be the picture shows a distorted view but if not the pistons must be rotated 180 degrees (large inlet pockets on the backside) :!:

Click to expand...

Nortonspeed is correct about this!! I don't know why but JCC got the L and R marks mixed up my Norton pistons. My supplier wrote over the stamped mark with a sharpy marker. On yours it looks like the L and R mark are correct and spinning them 180 degrees will do the trick. The small cutout goes in the front and the larger cutout goes in the back. I am pretty sure that the intake valves will hit if you leave the smaller cutouts in the back.
Dan.

Yep, they need to be rotated 180 deg. Good spot.