1958 Dommie 99 piston rings

John, inferior can only be quantified after use, unless the item, in this case rings can be seen to be not up to the job, in which case you wouldn't fit.

I couldn't find any original British made rings for my 1960 99, still on std bore and with no alternative, fitted Italian made GMP (I think)GPM? rings, new from RGM. I did find one oil control ring to be damaged but this was replaced. In a month or two I hope to start this rebuilt motor up. Only then and in the subsequent long term will I know if these rings are any good and therefore of acceptable quality. If they look the part there is a good chance they will do the job. Fingers crossed.

Good luck.

I have just purchased some 99 +20" rings from Ken White. Give him a call as he can help obtain rings for old British pistons. 01992 892537. Some people have found that GPM piston rings do not work properly in anything else but GPM pistons.

500 rings are smaller than 600 rings. 66mm bore as opposed to 68mm.

There are some Triumph rings that will also fit a 99 (600) that has been over-bored. The T100 model rings can be used for +40 and +60 rebores on Hepolite pistons or similar.

It's not uncommon for rings to stick in the pistons. There are two simple fixes.If you have access to a lathe, you can skim out the ring grooves carefully so that the new rings have 1 to 2 thou side clearance. If you don't, take one of your broken rings and grind a chisel point on it then carefully ease it round the ring groovesclearing out any baked on carbon etc. Check that your new rings are not being pinched and away you go.

Hi all,

Im aware that this is a bit of an old thread, but thought i`dpost this as a warning to those considering putting new GPM rings onto old stock pistons.

I too have an original pair of +040 Hepolite (AE) pistons. I bought some rings to suit (or so I thought). When the rings arrived, (GPM) all looked good. However on closer inspection I noticed that the rings weren`t seating into the back of the piston grooves properly - the rings were actually very slightly proud of the pistons! I then measured upthe old original rings I had and dicovered that there was a diffence of approx. 10 thou in the ring width - the GPMs being wider. If I had blindly installed the lot, I`m pretty sure it would only have been a matter of seconds before I had a siezure due to there being no clearance between the back of the ring and the piston groove.

Be warned, GPM rings DO NOT FIT onto OEM pistons correctly.

I can only say this is the case for the 600/ 650 pistons, but may be true for others as well.

Neil, assuming it isnt too late, it may well be worth chacking out what you have before installing.

PS Does anyone know what the correct back of ring clearance should be?

Hi Trevor,

I'm running a bit behind with this project but should have it up and running in the coming weeks. I will of course keep you posted and it can be said that I was warned.

My 99 is on the standard bore, a bit worn as the ridge at the top I had to work on will be witness. But given that there is a bit of wear in the bore, though within toleranceI doubt in this case that seizure will be likely!!! Hope I'm not wrong.

When I rebored my 99 at plus 10 there were no pistons around. I bought some NOS bare 650 pistons ,re-used my old pins and bought plus 10 rings from M Bratby. that was 20 years ago, the bike goes well and has as much compression as I can cope with.I suspect the Valve was open on your bad cylinder and the rings rusted and stuck in the piston. Do not use GPM rings unless you get the ring grooves machined deeper to suit. Someone somewhere will have the rings you need.

Hi Neil,

I wouldnt risk it!

When the rings are pushed hard into the piston groove, they are still 2 or 3 thou proud of the piston. With my barrel having been rebored, I would have been surprised if I could even have got the assemblies to fit in the barrel! Under the circumstances I didnt try it.

The problem is that once the engine is started and warms up, the pistons expand and, because the rings are hard up against the bottom of the ring grooves, all they can do is expand as well.Since they are already proud of the pistons, they havent got to expand much, if at all, before the whole lot siezes up.

As Robert says, the only solution is to get the grooves cut slightly deeper - easier said than done, buy complete GPM pistons and rings or preferably find some original rings.

I had the same problem when buying new aftermarket rings for a new old stock piston I wanted to use. The ringsinside radial diameters were a little on the small size resulting inthe rings being slightly wider andto not fully fit in the piston groove depth. This also meant that the rings had less spring in them which meant moreunnecessary cylinder wall to ringpressure.(increased wear)

While I waited at his counter, the ring supplier reduced the radial ring width in his latheby turning the I.D. of each ring out a few thouto fit the piston groove depth. This gave me a slightly less rigid ring which will be kinder to my cylinder bore AND fit the original piston groove. He had a series of different diameter turning jigs made for the specific purpose to do this ring alteration.

A lotsimpler method than setting up a piston in a chuckto deepen the grooves which would be fraught with danger of damaging the existing groove side walls.

Hi Paul,

That sounds very interesting. I have been onto thorntons for the correct size of rings (in +040) but they dont have any left.

Im fast running out of options. At this rate I can see myself having to buy a complete set of GPM`s, although Im not all that keen on the idea having read some of the other posts on the forum.

Paul, who ground down the rings for you?

Did they also grind down the backs of the oil rings???!!

Previously trevor_francis wrote:

Hi Paul,

That sounds very interesting. I have been onto thorntons for the correct size of rings (in +040) but they dont have any left.

Im fast running out of options. At this rate I can see myself having to buy a complete set of GPM`s, although Im not all that keen on the idea having read some of the other posts on the forum.

Paul, who ground down the rings for you?

Did they also grind down the backs of the oil rings???!!

.............................................

G'Day Trevor,The inside edge of the rings were turned, not ground, and only the two compression ones needed doing, the one piece oil control was O.K.This was done by J.P. Products Adelaide South OZ, a long way from you if you are in Europe! But surelyJ.P. are not the only ones who have these skills to perform this type of work? From memory, the jig for holding the ring was a simple home made affair and I was under the impression, the operator had made the set of holders himself. One thing I was made aware of by the 'ring bloke', is that somepistons have ring grooves machinedto suit a metric size ring, while others have an imperial width ring groove. This was evident when I bought a set of ES2,presumably N.O.S. rings, from U,S.A. to fitmyN.O.S. 16H piston. Besides having to widen the piston oil ring groove in my lathe, the two compression rings had a little more side clearance than I thought there should be. At this point, I shelved my n.o.s. rings and gave the job to J.P. to supply the set ofrings to suit.(of which the comp ones needed the machining done due to rather shallow pistonring groove depths)

Paul

just measured up the pistons and rings

Piston groove depth 0.106, GPM Piston ring depth 0.112.

The rings are approx. 0.006 proud of the pistons!

I believe there should be approx. 4 thou clearance behind the ring, so it looks like I need tomachine the grooves 10 thou deeper.

I can get it done, but the machinist is going to have to manufacture a jig to do it and the costs will be prohibitive. Does anyone know of someone out there who already has a setup to do this?

Any help /information would be gratefully received!

I would hold on and keep looking ,Rings do come up for sale and its possible another private owner will have a clear out. I have some NOS std rings that are not likely to be needed (unless I blow the bores and need a re -sleeve.).I am keeping my eye open for some +10 rings and BE shells to store for my next re-build, not paying silly prices though.

Hi Robert

I bit the bullet and machined the pison grooves on the lathe after posting. I was a bit worried about doing myself but having a decent myford, thought id give it a go.After making a jigit wasnt too hard - ive got them all to within a thou or so and seem to have avoided touching the sides (too much). The rings fit a treat, so now just need to fit them...................... problem is, I have an unmarked oil ring, which i assume can go in either way up, an unmarked bright ring which is also unmarked and anotherdarker coloured ring with "top"marked on it.

I dont know which is the top ring, and which is the middle ring. There were no fitting instructions supplied with the rings.Any ideas anyone?

If no=one with the knowhow chimes in I will dig out my spare rings and study them .

The bright coloured ring would be a chrome ring to go at the top of the piston. The dark ring marked top goes underneath with 'top' facing uppermost...... Oil rings either way.

With these rings, ensure they do not have excessive clearance in the piston grooves, otherwise they will pump oil up. Refer to the workshop manual.

As I have said, I've been caught.

Hi Folks,

I have used the stock Italian rings and decided to stick with it as there is a bit of wear in the bore, they went over the barrel OK. Unless anyone is offering a set of standard rings then this is where we are. Other jobs are waiting.

However, I take note of what has been said and will now run the first 50 miles around the local villages so that I am within pushing distance from home.

This begs yet another spares issue and I wonder what is being done to rectify it?

I will of course let you know how I get on.

I have been tempted to fit the next size up rings (regapped if needed) when the right size is unavailiable ,has anyone done this and how did they fare?.

Been there, done that and it worked out fine. You will need to regap of course. Careful work witha fine file and you should beOK.

Previously robert_tuck wrote:

I have been tempted to fit the next size up rings (regapped if needed) when the right size is unavailiable ,has anyone done this and how did they fare?.

Hi Robert

There are some +020 rings on ebay at the moment at £30, item number 201387493022. They are being sold by MCS who used to be in Leytonstone High Rd but closed 3 or 4 years ago, but now trade on ebay.

You could always use these and regap them?