Piston Rings: early shallow/late deep

Currently working on 4 engines. 500, 2-750 and a 850.
Not readily finding information that I needed, yet eventually found results for the 13 sets of 750 pistons I am considering which sets to use. The pistons listed below are not likely all that exist and no judgement is made on those not listed.
My findings:

In early pistons, the oil rings/groove are shallow @ around .125" deep groove. In 750 they are ring ID as 18020. Piston brands like GPM734(dunstall), GPM738, Teikoku, MC, Norton atlas 24283, AE17510.

In later pistons they are deep rings/groove @ .175/.185". In 750 commando, the change "may" have occurred at the "combat" piston time. These are ring ID as 26260. Hastings #7007
Pistons 06-3229/AE18964, AE19145A, AE18964A(06-3353/combat)

This may not be entirely a very late change as I found one old piston for one of my 500/600/650 that had deep oil rings, all the rest were shallow.
I had known this general concept for a long time but not to such detail as I now have found.
The old shallow ring sets seem to be very expensive @ RGM.
There are some of these piston oil ring grooves that can be machined deeper to accommodate the deep rings...which may be technically better and more available.

dynodave said:

In later pistons they are deep rings/groove @ .175/.185". In 750 commando, the change "may" have occurred at the "combat" piston time. These are ring ID as 26260. Hastings #7007
Pistons 06-3229/AE18964, AE19145A, AE18964A(06-3353/combat)

Click to expand...

https://www.nortonownersclub.org/support/technical-support-commando/pistons-and-rings

"
Service Release No. N46
Nature of Release Introduction of S.E. piston rings (oil control)
Models Affected 1971 Commando- all models Engine Nos. 146584 - 147176 and 147259 onwards.............

........The piston bodies are revised to accommodate the spring expander type of ring and for this reason it is not possible to use the later rings on an earlier piston or vice versa."

Thanks LAB
SR N13 and SR N46 obviously state there was a problem and two changes had been implemented.
Details otherwise...nothing
I am trying to source more affordable technically correct ring sets to buy, or perform machining on existing piston sets, if appropriate and will accommodate the change.
Some of the piston sets clearly are lacking material on the back side of the oil control ring. These would need original type shallow rings.
The piston sets with "A" are the style that the pistons would need to be worthy of modification, like the GPM and Teikoku.
Interesting that RGM calls their shallow rings, narrow...500 thru 850 oil rings are all 5/32" wide.

Might be worth checking the amount of cam grind on the GPMs.
We found inadequate cam grinding on early GPMs made for Vincents.
There were a lot of seizures due to this.

Glen

I am wondering if those of us who are more haphazard about our research might accidentally put "deep" rings in early pistons and what consequence there might be? I have a '71 750 motor sitting on the bench which I have not opened up, but will be looking at the details of the piston grooves when I do (thanks to reading this).

I can absolutely tell you...follow this story
I was assembling my friends manxman 650. I was to discover that these NOS BHB pistons were original shallow oil groove pistons.
They had been prepped by Total Seal for their gapless rings. This was done in the early 90's (after the 92 rally) while they had an east coast R&D facility in Greenland New Hampshire. (It was closed by 1999?) Only the compression ones were done.
Engine sat unfinished for a LONG time, then recently turned over to me to finish the job.

Assemble the engine is attempted. Ring compressor went on, barrels would not go onto pistons.
I found they were deep oil rings in shallow groves. Total seal did not do the complete job.
This was the big AH HA wake up moment for me in this topic.

I had to custom precision mount the pistons on the lathe and cut the groove for deep oil rings. Pain in the butt!!!
Engine then assembled normally as you would expect, and is fine now.

Dave if you are uncertain about something why not ask Anna J ??
I'm sure she'd furnish you with all the information you could ever need!!!!
Ha ha
Ok I'll get me coat!!

I had some questions for Andover a little bit a go and was referred to the following chart.
This is what they carry.

dynodave said:

I can absolutely tell you...follow this story
I was assembling my friends manxman 650. I was to discover that these NOS BHB pistons were original shallow oil groove pistons.
They had been prepped by Total Seal for their gapless rings. This was done in the early 90's (after the 92 rally) while they had an east coast R&D facility in Greenland New Hampshire. (It was closed by 1999?) Only the compression ones were done.
Engine sat unfinished for a LONG time, then recently turned over to me to finish the job.

Assemble the engine is attempted. Ring compressor went on, barrels would not go onto pistons.
I found they were deep oil rings in shallow groves. Total seal did not do the complete job.
This was the big AH HA wake up moment for me in this topic.

I had to custom precision mount the pistons on the lathe and cut the groove for deep oil rings. Pain in the butt!!!
Engine then assembled normally as you would expect, and is fine now.

Click to expand...

I had the same experience on a 750 engine. I think it was a'71. Turned out they were the later type pistons with the series of drilled holes but with the shallow grooves. I questioned Mike partridge on it and he had never heard of that. Simple solution was, obviously to fit the narrower ring set.

obviously to fit the narrower ring set....

I would say "shallow"
500 thru 850 oil rings are all 5/32" wide...who makes narrower piston oil ring grooves for nortons .....AN?????? LOL
I have unfortunately lost a feeling of credibility in JS

baz said:

Dave if you are uncertain about something why not ask Anna J ??
I'm sure she'd furnish you with all the information you could ever need!!!!
Ha ha
Ok I'll get me coat!!

Click to expand...

good one
LOL
Can I borrow your coat too?

What bore size are you after Dave? I'm pretty sure I have a set of early 750 rings somewhere.

pommie john said:

What bore size are you after Dave? I'm pretty sure I have a set of early 750 rings somewhere.

Click to expand...

PM sent

Perhaps there are two considerations - deep rings might be stronger and give better sealing, but deep ring groves might increase the likelihood of leaving the piston crown behind ?
I would be looking at rings from modern car motors. A while back, my brother wanted to fit an oil ring into a Jawa speedway piston, he found a very narrow one from a diesel engine.

acotrel said:

Perhaps there are two considerations - deep rings might be stronger and give better sealing, but deep ring groves might increase the likelihood of leaving the piston crown behind ?
I would be looking at rings from modern car motors. A while back, my brother wanted to fit an oil ring into a Jawa speedway piston, he found a very narrow one from a diesel engine.

Click to expand...

I'd think you're right about ring depth. The early narrow ones were supposed to "flutter" and cause poor sealing. There were certainly cases of piston crowns leaving the skirt behind on some occasions too.

However, I raced mine with narrow rings for years and never had an issue with burning oil.

As for trying to find car rings that fit, I spent many hours looking at piston ring catalogues with no success finding rings to fit the early pistons. I bought some from a place in South Australia who made one piece slotted rings to fit, but after I bought them, they informed me they were only guaranteed to 5000RPM.

Found one set of 750 STD shallow rings on ebay they are 3 piece oil. Only needed one set.

I realize that few will spend the effort to modify some pistons to use the late deep rings.
The thing I was focusing on was to be sure the inside of the pistons had meat on the inside to hold the head on even if I cut the groove deeper. The AE 17510 is the bad news full slit type piston. Now the 18964 has the exact inner profile as the 17510 though it has not been slit, though it does have the deep ring pack.. However the 18964A has full backup material. This is the profile I am looking for on my aftermarket hi compression pistons that I want to upgrade to be the same as the later deep grove pistons and will allow use of the better ring pack.
I also have 3 sets of GPM 750 standard domed high compression (11:1?) only $55 a set new in box. However they need upgraded deep rings because they came with one piece cast iron shallow oil rings.

dynodave said:

obviously to fit the narrower ring set....

I would say "shallow"
500 thru 850 oil rings are all 5/32" wide...who makes narrower piston oil ring grooves for nortons .....AN?????? LOL
I have unfortunately lost a feeling of credibility in JS

Click to expand...

I think they mean narrow width. You wouldn't call them shallow because you are describing the rings. But if you are describing the pistons you might call them shallow.

swooshdave said:

I think they mean narrow width. You wouldn't call them shallow because you are describing the rings. But if you are describing the pistons you might call them shallow.

Click to expand...

Why call them narrow?????
ALL norton heavy twin 1949 thru 1975 oil rings are all 5/32" wide. It is the depth that is different...both the ring and the groove. Must have been a girl or salesman that picked the adjective. Certainly not a machinist or manufacturing engineer. On one of my very early twins (1957 99) I discovered that some one had put in a "narrow" oil ring. It was maybe from a small car... lawn mower?? It had cast iron 1/8" wide oil rings. You had to see it to believe it ROTFLMAO

If you look at the rings laying on the table are they wide or narrow in comparison?