piston rings

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Jan 21, 2009
hi just got a combat month ago been reading up on your topics finally figured i needed to pull barrels check rings. lot of smoke and fouled plugs. one compression ring broke original rings had everything measured a ok.got standard rings and other parts from old brits .heres my question finally.compresion ring and scraper rings fit perfect .011 gaps. trouble oil ring doesnt fit in the groove. to late to call old brits . i even tried fitting old 3piece oil ring back the old and new ring fit only above the piston pins not the front or back of pistons. the new ring is 2 piece 1 wire spring and 1 solid with grooves from andover. do i have to heat piston to put ring on ? all carbon was cleaned. tried both new rings whats up? thanks
Hi there, may be it's not the right answer, but bear in mind that in 750 commando pistons there were two different oil rings and consequently two different grooves, the only difference but of most importance was the depth of the groove, the later one was deeper , if interested can go in the shed and measure the dimensions through the pile of scrappy pistons......
preston said:
oil ring doesnt fit in the groove. to late to call old brits . i even tried fitting old 3piece oil ring back the old and new ring fit only above the piston pins not the front or back of pistons.

If there is enough clearance for the oil ring to fit at the sides of the piston, but not at the front and rear, then it sounds as if the ring groove has closed up, somehow?
The rings certainly must be able to move freely in their grooves.
Did you use a ring groove scraper tool on the piston to scrape the carbon off? You used to be able to rent these at auto stores or else they can be had for cheap at places like Harbor Freight. If you measure the oil ring groove width, is it the same all the way around? Also, there were some poorly designed pistons used for a time which had very long oil slots behind the oil rings. If you have these it is possible that the top of the piston has been hit and closed up the oil ring groove. I seem to remember they were pre-Combat though but someone may have found some and put them in later on.
By now you could have a number of replacement pistons some of which might not fit your rings. It's always tough to know what a previous owner has done on an old machine,
Pistons are just basic stuff and any good local mechanic or machinist should be able to help you. I also would echo make sure the groves are clean and don't damage them. The age old trick is to use an old ring to clean the grove if you don't have anything better.
I suggest you get a motors manual or such and read the ring fitting section to start with, just to give you a bit better feel for this job.
If you want a long running bike that isn't an oil burner ring fit is critical.
I have found in the past that you cannot mix one brand of piston with another brand of piston rings for the size issue. I haven't had to research pistons for my 850 yet so I don't know if there is only one brand or 10. For example my CR500 had a Pro-X piston and I tried to put new wiseco rings on it and they did not work as the ring groove alingment pin was different between the two.
If you are still using old style pistons with slotted oilgrooves toss them and start again.It looks like your ring goove has distorted. It sounds like you are using a one peice cast ring with an expander? Place the ring backwards in the groove it should slide freely around the groove and sitting below the edge of the groove, as it doesnt fit all the way around look very closly for fractured ring lands or cracking pistons.
Here's why Splatt is telling you to chuck those pistons with the slot/groove.

piston rings

piston rings
problem solved

talked with original owner of 36 years never had barrel off. 2 valve jobs with guides last one sept.2008.you are right original slotted pistons groove collapsed.i was thinking to machine the groove wider till i say your pics. and told me they suck.machinest said i was close to limit to have to bore. since i have to buy new pistons should i go up a size? you guys are the best!! thanks p.s. by the way got this bike because it looks just like the yellow snorton norton i had when i was 19. and here i am working on it again just like then.wouldnt have it any other way therapeutic


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piston rings

piston rings

Theres the 72 norton, 69 trail 90 from original owner and my 103cu. screaming eagle 113 h.p.rear wheel fat boy talking about freight train.handles like a PIG compared to norton


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My Goldwings feel like trucks compared to my Norton. I'd forgotten how tiny they are since they were so big a bike when I first got one.
I'm glad you are working with a good machinist as that makes a ton of difference when doing a job like this. I have lost track of the number of engines I've assembled, but when somebody gives you good parts all you have to do is check the basics and you get a good engine.
Hey Mike, so that's what it was huh?
Well hope you get her back up and running soon- we should go take 'em for a ride soon.

I'm taking mine to the CHP for a vin inspection on the 25th, then I'll have a license plate/reg.

Let me know when you want to go for a cruise! Take it easy,
I've got to get around to the VIN inspection too. Bit of a pain since I have to go to Redwood City.
aydin what up! you thought you had the broken motor surprise surprise..ordering pistons monday overnight get them tuesday fire her up wendsday sunny day thursday go for putt good plan cookie should i bore or leave standard? chow for now
These days I like to keep the bore as small as I can as long as I can. When I was a kid 20 over might mean you win on Saturday night dragging your friends. Now it pretty much only means that's one overbore you can't use later.
If you are doing classic racing it may be a different story and you want every quarter horsepower you can get.
I ride my Norton in the hills of San Mateo County and nine tenths of bikes I see on those roads are super bikes that handle better, stop better, and go a lot faster than my Norton. It seems somewhat pointless for me to try to get to 58 horses when the competition has 100 or more.
I did hold off a herd of Hyabusas in the corners last fall that blew by me with a wave when we hit the first straight. Just think, if I'd been running a big bore I could have held them off another ten feet.
Drop a ring in the bore and push it to the bottom with a piston to keep it square. Measure the end gap. Move it up to the top of the wear area and measure the gap again. If it is within specs I'd just use new pistons or an excellent set of used ones if I had them.
I remember having issues with the oil rings on my 750 some years ago. The ring groove was not deep enough for the rings I got from the Norton shop.
It's a long time ago now, but I think there are different oil rings for different years.
The original rings are almost the same as Lotus 1600cc twin-cam rings. You have to buy 20thou oversize ( I think, check the bore size) and get the oil ring groove machined out to take the lotus rings. Then you just buy a set of Lotus rings each time you need to do a rebuild.

Disclaimer: I did this a long time ago and didn't take notes, but if you check the details in a pistons ring book, you should be able to get suitable rings at a good price.
Preston -

Looks exactly like my bike did new in 1972. Yellow combat. Totally stone stock. So nice to see one that is not all modded up.

I needed new pistons in mine also. Bore was OK but ring groove clearance was excessive. This is what wears. I kept it stock bore.

Depending on what pistons and rings you get, find out from the manufacturer and specify the grit used when having it honed. (Have to break the glaze on the cylinder wall.) For example, the original Hepolite rings want to see a coarser hone job than most automotive machine shops do today.

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