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sorting pistons

Submitted by paul_standeven on Wed, 20/04/2016
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Heavy Twins

I'm preparing to put my Dominator engine back together, after many years stored in pieces. Consequently, I have forgotten details of what I did

The Hepolite pistons are stamped LH and RH, and look the same on Ex and In sides. The valve pockets are the same width on both sides. I haven't seen how to choose which way round to put the pistons

Rings are new Repco compression and Hepolite oil scraper. There are no marking dots or 'TOP' stampings. Even with a magnifying glass, the rings look the same top and bottom, no sign of a chamfer etc. So have I missed something, or can I just fit the rings and forget? I did gap them

The gudgeon pins are tight in the pistons. Do I heat the piston (I don't have a gas torch) or make up a press?

My dynamo drive gear (behind the cam sprocket) was worn thin, so I bought a nylon one - no-one was offering anything else at the time. I have heard that they absorb oil and expand, causing problems. Are they OK, or do I bin it?

Paul

Submitted by john_holmes on Thu, 21/04/2016

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Nylon does absorb oil, on garage doors with nylon guides I use silicone grease for that reason so I would discard and obtain the correct one.

On the rings if there are no markings or taper then fit either way is correct.

Yes heat the piston with a hot air gun to free the pin, lot less messy than rag soaked in boiling water. Using a press risks opening up the holes in the piston, not much heat is needed and then its easy to move the pin safely.

Submitted by phil_hannam on Thu, 21/04/2016

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Paul.....check out the attachment. Not too long ago several well-known Norton parts dealers (& others) were selling pistons with rings that were not marked in an way with regard to orientation. Especially, the old GPM brand. If you were lucky, the packet of rings contained a slip of paper with a diagram.......not all.

Worse still.......sometimes they sold pistons with the rings already fitted but occasionaly the wrong way.

John is correct in stating that many rings can be fitted either way up but not all. Use a magnifier to examine the ring ends.

Attachments piston-rings-jpg

Submitted by david_cooper on Thu, 21/04/2016

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Remember not to get your gudgeon pins mixed up. They may look the same and measure the same with whatever instrument you use to measure such things, but they are matched to pistons. If you mix them up, they might be more difficult to insert - and vice versa.

Submitted by paul_standeven on Thu, 21/04/2016

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Thank you for the various comments.

The pistons are original Hepolite, and came out of a friend's 1956 Model 88 in the mid '60s, having worn out the bores. They show light scuffing on the faces. I was actively rebuilding the bike in the mid '80s, and pistons for 500 Nortons were unobtainable in New Zealand. I got the barrels sleeved to suit the pistons I could get, and got the Hepolite oil rings from a local who had some nos scrapers, but not compression rings. That's why the Repco compression rings.

As you say, look for markings on the rings. There are no dots or 'top' markings on any of the rings. If one side is finer polished, that should be the lower face. Look under a magnifying glass at the edge profile - that looks the same.

So the conclusion is that the rings are symmetrical, and it doesn't matter which way up they go. Well, it's better to be satisfied than have a nagging worry that I got them wrong way up.

A further question - do you expect the front or rear face of a piston to show more wear?

 


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