GPM Piston Rings Which Way Up?

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Samurai
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Joined: Wed Nov 28, 2012 9:53 pm
Location: Somerset, U.K.

GPM Piston Rings Which Way Up?

Postby Samurai » Sun Jan 12, 2020 2:04 pm

Some help please?

Last year I had my cylinder rebored and a suitable new GPM piston and rings supplied. These came in brand new packaging but without any instructions.

I was aware that rings usually have a top and bottom and looked carefully under a jeweller's loop to see if there was any dot/marking to identify the top or chamfer/step in profile etc. However, I was completely unable to find anything like that.

I double-checked the rings on the old piston (unknown make) and again they too looked completely symmetrical, with no obvious marks or profile to indicate the correct orientation.

I placed the 'double' oil ring at the bottom and the other rings on the top in no particular order as none was obvious. The bike subsequently runs and starts ok, but after a couple of thousand miles still smokes a fair bit when the throttle is whacked open from idle or very low speed and the plug is showing signs of burning oil.

The bike also leaks some oil from the clutch push rod and a bit from the sprocket, despite changing the clutch seal and the usual fixes for the front sprocket, so I'm wondering if one or more of the rings IS upside down, (despite there being no obvious way to tell!) and the combustion gases are pressurising the crankcases and forcing oil past the seals?

Before pulling the barrel off to take a look at the rings again, is anyone able to tell me if GPM rings actually DO have an orientation or order on the piston and how I can tell?

I'm entering this year's Moto Giro and the bike starts first time and appears to run fine apart from that, so I'm reluctant to start messing about unnecessarily.

Thanks for any advice.

Jason.

themoudie
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Location: Scotland

Re: GPM Piston Rings Which Way Up?

Postby themoudie » Sun Jan 12, 2020 10:57 pm

Aye Jason,

I haven't used GPM pistons, but have you contacted your supplier or GPM pistons for guidance? I would expect both to be able to offer specific guidance. The pistons and rings that I have used, have either a stepped or chamfered compression ring (top ring). The other rings are usually dimensionally different, or as you described, multi-part.

As for the 'running-in' process, we all have our own preferences. I use a straight and single viscosity oil (non-semi, or synthetic oil) for the initial running in mileage. Semi-synthetic and synthetic oil additives actively prevent the 'bedding in' of the two bearing faces between the rings and the cylinder.

If the engine "after a couple of thousand miles still smokes a fair bit when the throttle is whacked open from idle or very low speed and the plug is showing signs of burning oil", then I would suspect that for whatever reason the piston rings have not bedded in. Consequently, I would contact your supplier or GPM pistons for their guidance. I would also set the engine up at TDC, with all the timing marks aligned (the head and timing chest bevel gears and the timing gears in the timing chest), before removing the cylinder head and lifting the barrel to inspect all the components for any damage/excessive wear.

Depending on what you find and the information that you obtain from the supplier or GPM pistons; then you may have to replace the rings and de-glaze the barrel or more.

I hope you are able to sort the engine out, without more expense.

My regards, Bill

double diamond
Posts: 555
Joined: Thu Apr 28, 2011 1:20 am

Re: GPM Piston Rings Which Way Up?

Postby double diamond » Mon Jan 13, 2020 12:24 am

If the oil ring is a 3 piece (expander between two thin rails) there is no orientation as far as the way they are installed. If the two compression rings are rectangular in section, again, there is no up/down orientation. The only consideration would be if one of the rings is chromed, this ring would be placed in the top groove. Were the ring end gaps within specification? Ring gaps displaced 180 degrees apart? Was the cylinder properly honed to 60 degree cross hatch? Did you scrub the cylinder to clean any honing residue? The honing basically acts as a file to abrade the rings and effect ring seating. There are many opinions regarding ring seating but if you buy the “honed surface as a file” argument then necessarily you would want a cylinder preparation free of any grit and oiled with a light lubricant to allow the rings to be sealed immediately upon starting the engine. You can see if rings are properly seated by observing the contact surface of the ring after a short time running. There should be an obvious polished or burnished contact surface distinguished from the dull finish of the surface of a new ring. If all these considerations are answered in the affirmative, look to your valve guides. Introducing proper ring sealing to a worn engine can exacerbate any shortcomings in valve sealing.

Matt

Bevel bob
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Location: Bromley Kent UK.

Re: GPM Piston Rings Which Way Up?

Postby Bevel bob » Mon Jan 13, 2020 8:18 am

Pattern sprockets are not always correctly made and can't fit right. The oil level on my 250 finds the right level and stops leaking after a while. There is a built in oilway designed to let some oil out onto the sprocket. Hard to spot though!.These motors can take a lot of running in if set up tight.

Samurai
Posts: 137
Joined: Wed Nov 28, 2012 9:53 pm
Location: Somerset, U.K.

Re: GPM Piston Rings Which Way Up?

Postby Samurai » Mon Jan 13, 2020 6:02 pm

Many thanks for the prompt replies chaps!

With regards to the honing, the rebore and new piston was arranges by Brian Silver here in the UK, at the same time as rebuilding the crank for me (he's a well respected Ducati Single specialist). The bore came back nicely cross-hatched and it was oiled before assembly and 'normal' but not overly cautious running in carried out. I use straight 40w mineral oil as advised by Brian anyway, so I don't anticipate any problems on that front, but can't say I thought about scrubbing the bore, measuring the angle of the honing or anything like that. Perhaps naively I assumed that would have been done correctly as part of the rebore process :roll:

I did space the ring gaps evenly, but I can't recall noticing if one or them had a chromed surface (it is more than a year since I fitted them), but I would have thought I'd have noticed that at the time. However, you never know . . .

I've sent a similar query to Nigel Lacey, as well as asking when the 62.5mm rings will be back in stock, just in case I need to replace them. However, I'm struggling to find a website for GPM pistons at the moment, otherwise I would have gone down that route first. Will keep digging though.

Interesting about the oil level, I fitted the dip stick extension at the same time I rebuilt the engine, thinking that the ability to hold more oil was a 'good thing'! However, since reading several 'oil leak' threads on here and tried the common remedies, I'm steadily reducing the level to the point where I'm at the Minimum level on the dipstick, which seems to be just a little above the original filler level. Perhaps taking it off is the answer :?

Thanks for all the info so far guys, keep any suggestions coming and I'll let you know what I find.

ecurbruce
Posts: 299
Joined: Fri Apr 01, 2011 12:43 am
Location: Lakeland fl

Re: GPM Piston Rings Which Way Up?

Postby ecurbruce » Thu Jan 16, 2020 11:38 pm

Hi Jason,
It's just a thought,
Is it possible that you didn't get a top compression ring in your new set, and you have two second (scraper) rings?
I don't know for sure, but all the top rings I've seen have a different profile...

Just thinking outloud,
Bruce

Jon Pegler
Posts: 459
Joined: Sun May 16, 2010 6:19 pm

Re: GPM Piston Rings Which Way Up?

Postby Jon Pegler » Fri Jan 17, 2020 11:23 am

I've got a couple of new sets of GPM rings on the shelf and looking at them the top two compression rings look identical with no marks to say which way up they go, so presumably they can be fitted any way.
The scraper rings are not marked top, but a look at the ends of the rings (that butt up close together to form the ring gap), there is clearly a taper profile to show that they should only go one way.
Maybe your scraper ring is upside down, Jason?

Jon

Samurai
Posts: 137
Joined: Wed Nov 28, 2012 9:53 pm
Location: Somerset, U.K.

Re: GPM Piston Rings Which Way Up?

Postby Samurai » Fri Jan 17, 2020 1:32 pm

Jon, thank you so much for checking. Nigel Lacey got back to me too in the meantime and said the same, so I think you're probably correct and I've may have put one of them in upside down.

Guess what I'll be doing this weekend :roll:

I'll let you guys know what I find or if I'm just jumping at ghosts.


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