AJS & Matchless Owners Club

Hi, a question for the engineers out there. Some while ago I bought a +60 thou 500cc longstroke piston of unknown make without rings at an autojumble - you never know when it might come in useful. More recently I found a NOS set of +60 rings. The 2 compression rings fit their grooves, but the oil control ring is a few thou too deep top to bottom for the ring groove. I need to take a smidge off the ring; I could take it to a precision engineers and pay them to put in on a surface grinder, or I could put if on the myford and skim it. I could glue it to the faceplate, or mount it in the 3 jaw using a backing plate - your thoughts on method? Andy

I'm no engineer, but are you sure there is enough material on the ring to allow for it to be reduced without weakening it as they are pretty fragile.. That is assuming it is a one piece ring with slots. I would have thought a better solution would be to machine the ring groove in the piston to suit. Better still there are a number of companies (Thorntons etc. ) that could probably supply the correct size ring.

Andy - I have no access to lathes and other engineering tools save for a vice and a cheap pillar drill, so I tend to think up simple methods to fix things. In this case I suspect I would put a sheet of wet n dry paper on a flat surface, wet it and work the ring around while pressing on with as many fingers as I could to keep it flat.

Crude? Yes, but such methods I use quite often particularly when trying to get things to fit. Of course, this assume I understand it is the flat surfaces that sit in the top and bottom of the piston groove...

I agree with Ron. You can glue the corners of a sheet of wet n' dry paper, on a rather thick piece of glass to lap the ring at the good size. To apply pressure evenly, you can catch the ring on a piece of plywood, with 3 or 4 nails slightly protruding, but short enough to avoid them rubbing on she sand paper.

To achieve the desired ring/land clearance of approx 2thou thats a lot of work with wet & dry! Also,are you happy with the ring depth.

the oil rings are a bit on the fragile side but i would think youd easily get it sorted on a surface grinder with magnetic table .

would probably have to grind both sides equally though and not take a chunk off just one side .

i think most would opt to machine the piston or find a ring from an engine of similar bore , i think quite a lot of engines might have usefull sized rings which can be bought very cheaply .

if you get stuck for a small surface grinding or lathe job you can try PJO engineering in swindon , as i know they have a surface grinder and lots of machinery and are a freinds of mine and a family business i have known since the early 1980s .
they are into old vehicles , hot rods , scooters and also drag racing in a big way , you should see the 1948 ford pop there with an ex funny car blown hemi running on alcohol that he built .
they also do welding and fabricating , no job too small .

Hi

Whilst it's easily possible to reduce the depth on a surface grinder, (or even by hand lapping), I'd be very concerned about the resulting integrity of the ring. Is it really worth taking the chance?

I'd look for a suitable replacement ring first - if there are no available options then I'd re-machine the piston, easy enough if you are adept with your Myford.

I wouldn't even consider trying to reduce the depth of the ring by turning - the interrupted cut is just waiting to break the glue bond and then the ring.

Regards Mick

Hi All,

Thanks for your help. I have considered turnig the groove on th piston, but I'm a bit worried about holding it. As I understand it, pistons are not round and are also tapered top to bottom. They are also fragile and possibly a bit on large size diameter wise for the 3-jaw chuck, and very easy to distort when centring in the 4-jaw (although I suppose I could make up a pad so I could bring the tailstock to bear on the top), hence my thinking to modify the ring rather than the piston. Plus, if I b****r up the ring, I can always look for another, but if I b****r up the piston, that's the end of that!

Any suggestions welcome, Andy

For turning the ring groove, you will need to make a jig to hold the piston. Inside the bottom of the skirt should be a turned area which is round, so you need a spigot that the piston fits onto. You will need to make a dummy gudgeon pin with a thread through it at the mid point. Make a hole for a bolt in the spigot and use the bolt to pull the piston onto the spigot. You can then hold the spigot in the chuck.

Rob C