Britbike forum Forums British motorcycles in general British motorcycles in general To bore or not to bore, that is the question

My A65 bitsa is down for low compression. Measurements show bore size is 2.9925-2.9930 in., both sides. According to the book, standard bore size is 2.9521-2.9530 in., so my bores are right on the money for .040 in. overbore.

The pistons measure 2.988 in., leaving .005 in. clearance. My machinist says he will have to take off .001-.0015" to clean up and hone the bores, so the clearance on those pistons would then be .006-.0065 in.

My question is, would new .040"-over pistons be bigger than the ones that are in there? The fact that the bores are "right on" for +.040" would imply that the excess clearance is due to wear on the pistons.

By the way, I measured the rings in the bores and the gaps are .022-.030", indicating significant ring wear, which may have in itself caused the low compression.

How worn do the pistons look, the rings wear a lot more than the pistons do. There are 2 tolerances, one the fit tolerance for new pistons in a fresh bore and then the wear tolerance on top. Your 6 to 6.5 thou is within the fresh fit tolerance for clearance but most go for 5 thou with a rebore as a rebore specialist can hold tighter tolerances than the factory could (hence the use of graded pistons). A worn bore would be at 10 to 12 thou clearance on a new piston. A worn piston with good rings will not lower compression, just give you piston slap.

Hi Mark, since I have entered the era of more time than money, I would go for HONE and new rings. If it didn’t work you are back to the more severe fix.
Hope to not hijack your thread, but is this a situation where folks would knurl the piston? I have found pistons like that in Motors, but never had it done myself. I actually don’t even know how you get it done! I believe it was intended as a piston slap fix.

KC had pistons knurled in my 58 matchless g12. Local machinist did work was very happy with process.

I have had pistons knurled many times over the years. It is one accepted method among many to expand the piston skirt and tighten up loose pistons. One of our remaining local machine shops still has the machine that does it.

Interestingly, the GO-300 Continental aircraft engine came from the factory with knurled pistons to improve piston lubrication.

All this to suggest it is a useful way to get more service out of a piston that hasn't been seized badly and still has good ring grooves and pin fit. Check with your local machine shops. Maybe you have one who still does it.

Many years ago in a Harvey Ferguson cookbook, mention was made of clamping the piston between two larger grained files in a vise, then hammering files to roll the piston between them to knurl it! The author claimed it was good for 100 or so miles to get you to the next paycheck.
So my question is: how many miles will professionally knurling a piston with proper equipment extend the service life?

Unless the bore really needs major tidying up, this seems the best approach.

Run a hone down the bore first and check for any low spots which are still shiny. If there aren't any, the bores are good.
It's also worth running a thumbnail up the top inch or so of the bore to check for wear ridges which might damage the new wings.

Mike's got the answer with piston coating-- it's not expensive either-- can build up to a max. of .002" is what I heard

Opened that link and it’s really impressive. I’d like to know what the cost would be for a small job lot, 2 old bike pistons. No Doubt the piston coating answer is a great modern day option, so not discounting that process. The thing is, I believe the flavor of the OP is how to keep it simple, reasonable cost, and in my way of thinking Local help. I figure if knurling worked on these old bikes and old cars at the time, it’s good for this particular issue. I hope Mark comes back with his choice and results at some point, just for the “data base”! Always fun and easy to watch the other Guy’s work! 🥸

My matchless G12 650 pistons were good for many hard miles and were still good 4 years later when I sold the bike

KC, the order form says $38 per piston. May be other charges. I did not read all the details.

I haven’t worked with A65s, but 0.0065” doesn’t sound like too much clearance in a 3” bore.

What I've seen is most wear occurs in the bores, not pistons. New pistons typically have a certain amount of shrinkage initially and then settle into a size that will stay constant for the rest of its life. Of course some sort of trauma can occur that can "resize" them....
With that in mind, I have coated pistons successfully I feel. The motors wound up running fine and quieter than before. A good hone (not a dingleberry) and new rings were necessary. And because not rebored, the cylinder will not need to be replaced as soon. A bunch of advantages IMO.
And TT, I agree that 6 1/2 thou will work, but I've found that motors get pretty noisy and ring seal seems to suffer. My thought process leads me to believe all the beating around in the bores accelerates wear too. But maybe I'm just nuts.....

According to the website, the max thickness is .020". The minimum is .0007".

I'm not likely to go for the piston coating. I'm hearing that .0065" clearance is not real bad, and like KC said, better to go with a minimal fix first.

I'd just like to know if new +.040" pistons would be a little bigger than mine. I don't know how to gauge the amount of wear on the pistons, except by the current piston-to-bore clearance, and the fact that the bores are .040" over standard bore, right on the nose.

I will buy new +.040" pistons if they are a bit bigger than mine. Otherwise I'll use the pistons I have and just refresh the rings.

Just use a set of 3 piece oil rings if you're worried.
They are a lot better at controlling oil consumption on
motors with a little wear.
OR as your bores are at +40 you can use a set of T140 standard rings
with the 2 piece oil rings, they are quite good too.

Just my 2c

https://vintagetriumphparts.com/Piston_Rings_T140/p1600

Are the piston skirts machined before the coating is applied? I often wonder this because if you have a bore which is a couple of thou over, its often at its upper end of service life but un-ecconomical to rebore a piston a further .020"