For my engine work I purchased a set of GPM 850 piston rings .020 oversize, to fit the .020 oversize pistons that are on the bike (by previous owner and stamped as such on each piston). The old piston rings had a .013 ring gap when inserted in the bore, 2" below the top - I checked them. The new rings I received as for the same .020 oversize have a ring gap of .016 on the top and second ring at that same location in the bore. The Norton manual calls for a top ring gap of .012 maximum. So per Norton specifications, the new rings have a ring gap that is beyond specification.
I'm not sure what the best course of action is here. I do not want to install new rings that already have an installed gap of wider than what the specs call for but am not sure of the best way to approach this. In my previous "life" as a auto performance engine builder, we used rings that were oversize (gap too narrow) and that had to be filed to obtain the proper gap. I assumed that new rings which were manufactured for the .020 oversized bore would have less gap than the maximum specified by Norton or at most, the maximum gap of .012. Installing these rings would, in my mind, be installing "worn out" rings. This is especially troubling since the old rings in the motor had a narrower gap than the new rings.
.016 ring gap is what we used on 4.04" bores in Mopar 340 motors - seems waaay to wide for a 3" bore - as the Norton specs indicate.
Does anyone have any thoughts about this - is there some current info that indicates Commandos should have a wider ring gap than that specified in the original shop manual? I am not happy about losing 100 bucks for a set of rings that are not right...
When I reringed my std bore 850 with hepolite rings the gaps were within the workshop manual limits using the 2" down the bore you used. I would speak to the supplier as the rings don't sound right and initial run in will make the gap even larger. Might be better to go for Hastings or Total seal rings if the thickness and depth etc match the GPM pistons.
Provided the rings are a good fit in the grooves and have adequate tension on the cylinder wall,you won't notice any difference in performance at up to 0.060" gap,maybe even 0.080" gap.There is nothing to worry about. It is a good idea to have a few thou extra gap on the second ring,compared to the top ring.This is a proven winner.
Mike, FIRST, you need to MEASURE the bore. Measure ˝” down in the bore. At 20 over you should read between 3.0515”- 3.0520” then measure about 2” down if this 2nd measure is 5 thou bigger,it's time for a REBORE if your bore is in spec, the rings should not have that much gap HTH
And Peter R said “Provided the rings are a good fit in the grooves and have adequate tension on the cylinder wall,you won't notice any difference in performance at up to 0.060" gap,maybe even 0.080" gap.There is nothing to worry about. It is a good idea to have a few thou extra gap on the second ring,compared to the top ring.This is a proven winner.”
I totally disagree with this statement. Rings with 80 thou gap will smoke like a train!
I did all the bore checks prior to ordering the rings - there is NO measurable taper in the bores and the dia is correct for .020 over so that's not an issue. I talked to Fred at Old Britts and he confirmed that some sets are tighter, some are looser. He said he'd be happy to take them back and exchange them for a set that he would personally check to ensure the gaps were OK. I also found that one of the ring sets produced .016 gap in either bore, the other produced .013 in either bore - both beyond Norton specs.
However, in the last hour I have gained a lot more info that convinces me all will be fine though obviously the factory GPM tolerances for piston rings is not all that great.
What follows is more info re the subject and why I have decided the rings will be fine:
After posting here and being quite annoyed with the whole deal (NOT with posting here, with the piston rings!), I decided to make an international call to a friend who I used to work with in my performance engine building "life" who was a "piston/cylinder guy." I described the situation, the type of engine, the specs, and the rings themselves and that the top ring gap was too wide per the oem specs and that the second ring would be just as wide instead of narrower like it should be.
He said, "Mike, I'm shocked, you've gotten way behind current engine building" He informed me (as DennisB noted in his post) that FIRST, the gap width recommendation re the 1st and 2nd ring is reversed from what we used to do. Now, 2nd ring gaps are WIDER than first ring gaps instead of narrower because they found that the "old way" induced ring flutter on the top ring. The ring flutter causes poor sealing - much worse sealing than does even a far too wide (spec-wise) ring gap.
SECOND, ring gaps themselves are slightly wider than used to be recommended. .004" per inch of bore used to be standard. Now, .005/inch is the common recommendation for the top ring and .0053 for the 2nd ring and an oil ring gap of a MINIMUM of .015, regardless of bore size.
SO... based on all this and what I had told him about one set being .013 and the other being .016 he told me that since both 1st and 2nd rings are exactly the same configuration and material, just take the two 13s and make them the top ring for each bore and the two 16s and make them the bottom ring in each bore. "You'll end up with exactly what you should have for that engine!" So, I notified Fred at Old Britts that I wouldn't be needing to exchange the rings.
I had initially asked him in my call about using oversize rings, like .040, and filing them to fit. As was noted on that BSA link, He explained that doesn't work because different ring sizes - std, .010, .020, etc - are machined to be a different outside diameter. If you put an 040 ring into an 020 bore, even if you adjust the gap to be correct, the rings will not seal properly against the cyl wall because you are trying to make a circle of one diameter press tightly all the way around a circle of a different diameter. The rings will bear against the wall at a couple of points but not seal properly.
Obviously others here may agree or not with the above but this guy knows his stuff and I have known him for a long time though this was the first time we've spoken in a couple of years. So take it for whatever. At worst, it's just another opinion. But I will be fitting it all up as he suggested...actually it wasn't exactly a suggestion, more of a "Quit thinking on your own and just do what I'm telling you!"
Rohan, thanks - I wondered the same thing about the honing but the old compression ring, which showed .013 when I removed it originally was a bit over .014 after honing. So I expected new rings to still be well within Norton specs of 10-12 (top ring). So even with NO honing, one set of the new GPM rings would have been over spec at around .014; the other set would have been fine at .011. So the OLD rings had narrower gaps in the old bores than the new rings would have.
I have to admit that despite the input of my friend, I was awake half the night wondering about this. It just goes against my years of doing engine work to do as he said, even though he knows a lot more about this particular aspect of engine work than I do - that's all he does. But although he may be right, It just seems wrong to me...
FWIW, this morning I had an email from him with a link to a site with more info re all this. The link I listed below is to page 10 of the show. It's from JE pistons; he wrote the data for the show. Click on the link and then expand it so you can actually read the slide. The rest of the slide show is quite interesting, and reflects "current thinking" re piston rings.
MIKE.....If I remember right, the seccond ring has a slight angle cut in the ID and this must go up or you will burn oil. You can't use the 2nd ring for the top ring. I use Heppolite rings, so LOOK CLOSE and see if one ring has this champher.
The GPM 1 and 2 rings are identical in every way - same markings, same backside taper, same material, etc. Additionally, in the ring packet there is no instruction/information at all so if there was a difference in which ring should be where, I would expect that to be in the box. Frankly, If I knew then (two days ago!) what I know now, I would have hunted up some oem Hepolite rings...might still be possible though it would delay this by at least a month...
If it is any comfort to you I had similar concerns about using GPM pistons and rings but in the event things were fine.
I use GPM pistons in my race bike because I couldn't get anything else at the time. The rings seemed to have a very big end gap (Sorry I cannot remember how much but they were at the top end or outside the recomended Norton spec. I do remember that. I havnt heard of your friends wider gap at bottom and just installed them as I found them.
Anyway they seem to work fine. I replace the rings about every two years as part of a normal race maintance programme. The 500 Norton twin has seen 53 bhp on a dyno and will out run most normal Commandos.
They are crazy Italians but in my experience it does seem to work. No special running in programme. Take it easy for a few laps and then go for it. But I do use real castor oil Castrol R 30 which is not suitable for road use.
Interesting! The 1 and 2 ring in the GPM set are installed the opposite of that Hepolite diagram. If I read the diagram above correctly, it shows the bevel on the inside diameter of the ring and facing down. On the GPM rings, instead of the bevel down, the bevel goes up on both rings - the word "top" is inscribed on both rings and that puts the bevel at the top (on the inside diameter.).
I used to believe, when I was younger, that all this stuff was black and white. Now I find that like everything else, it's all grey!
yes the bevel goes up in my expierence. gas pressure on compression and power, slips across the top of the ring towards the piston body. since there is a chamfer there, the positive pressure against that chamfer wedges the ring outward from the piston to the cylinder wall making better ring contact to the cylinder wall.
OK, one last question (yeah, right! :)) before I put 'er back together tomorrow.
THe service manual shows the method for installing pistons in the cyls using two ring compressors and the cyl is pushed onto the pistons. I read a procedure elsewhere whereby the pistons are inserted in the bores first and positioned where the piston pin boss is below the bottom of the bore; then the cyl/pistons is held above the crankcase on blocks or whatever; then the crank is rotated up and the rods attached to the pistons. THis sounds workable - especially since I only have one ring compressor.
Has anyone used this method and if so, would you recommend it? If not, Sears is up the street and I could buy another ring compressor tomorrow if I had to...
I did it once....had a base gasket go bad and the rings had just seated in nicely. Didn't want to pull the pistons from the bore and move the rings to avoid another berak in. I cut 2 pieces of wood, one 1 1/2" X 1 1/2" X 7 1/2" and one 1 1/2 X 1 X 7 1/2 to fit under the pistons.
I pulled the wrist pins and lifted the cylinder off with the pistons in place. Had a buddy help me lower the bore in place and replaced the wrist pins. A bit of juggling but it works. Went out to the shop and measured the wood... been about 3 years since I pulled the bores. In the picture it looks like the wood blocks are 1 1/2 square, I think I trimmed the one to 1" to fit the 750, studs were in the way.
Forgot to add... warm the pistons a bit, the wrist pins are a tight fit and will slide out easier. If I remember right (did this trick over 8 years ago) after removing the clip I slid a thin rod through the wrist pin of one piston to push out the other. The blocks will hold everything up while you do this but I can't remember if I removed them to put it back or not. Might be easier to get the ring compressor, You will find out.
Mike, the picture Alan posted is exactly how I do it (wood blocks under pistons, wide hose clamps, slide barrel down over ) I install the base gasket to the cylinder with permatex I turn the barrel over, and stick a pipe cleaner a short way in the oil drain hole, then paint the sealing surface with an acid brush. (dipped in Permatex) Pull out pipe cleaner and install gasket. This will hold the gasket up and out of harms way. After the pistons are in, and pushed a little ways up, have a spare set of hands hold the barrel, or string works to hold it up clean the surface the wood blocks were on, and paint with permatex here using the same trick with the pipe cleaner. After the barrel is on, wet a pipe cleaner with denatured alcohol and run that down the drain hole to make sure it's clean and unobstructed. But that's the easy and fun part! Getting the HEAD back on, now that is the part of the job that tests my mettle. HTH Cheers Dennis B
THanks guys, good stuff. Just as an aside, my 850 did not have a base gasket and the parts book does not appear to show one which I found odd. It was just sealed with what appeared to be silicone. As I mentioned in an earlier post, the oil drain hole from the int rocker box was partially obstructed with it. I will be extremely careful with this on re-assembly and use the pipe-cleaner method to ensure it is clear; I'll also blow some compressed air through that port from the "back side" down at the timing cover area to help ensure it is clear. I will be using the Permatex version of Yama-bond (Super gray) which has worked well for me on such applications.
MIKE, if you haven't already done so remove the front bolt that goes through the isolastics and let the engine drop and hang on the rear mount. This will give you over a inch of extra clearance to install the head. Very tight clearance here.
Re ring gaps...I discovered in a Norton service bulletin that the shop manual's gap specs were changed in a tech bulletin from .009-.012 to .010-.019. So the gaps as provided on the GPM rings is well within the revised specs from Norton and their revised specs are more in line with the larger gaps that are recommended by some ring manufacturers nowadays (greater than .004" per inch of bore)