Piston size

+10 is an abreviation for + .010" (or 10 thousands of an inch) oversize. So if an 88 is 66mm bore, at +10 is would be 66mm plus .010" (or near as dammit 0.5 mm, just to confuse) bore.

Early barrels had a spigot which fitted into the cylinder head. Later ones had no spigot - a flat top. To confuse again, some early barrels have had the spigot machined off. If you fit a spigot-less barrel to a head with a recess for a spigot, it will leave an annulus. You can fit a couple of alloy rings to the head to fill this gap. These can come loose - see another topic on this forum. A bike will run fine without these rings fitted. I know 'cos I have done just that.

Hi Isaak, Just wondering, was I right? was the "worm" in your engine one of the alloy ring inserts from the cylinder head?

Re. "limited engine turn" post.

Regards

Alan Throssell

Put the rings one at a time in the bore and check the gap. Should be 0.008 to 0.010". Gordon.

Previously wrote:

Gordon: thanks, very helpful! The reason why I asked this is because I have been looking to buy new piston rings. The piston in my engine measures 66mm, so I would go for standard rings. But how do I know if they need to be replaced?? They look ok to me, all three. Set of piston rings is not cheap, so prefer to skip if not needed! :/

Alan: YES, the ring is the worm! I thanked you in the other post.

IF the ring gaps are not huge, there is no blowby staining and no smoke or excess oil consumption then leave them alone ,they must not be replaced upside down or in a different position,a plus 10 piston has plus 10 rings, get a set of ring clamps and a helper to replace the barrel. The bores will need de-glazing and de-ridgeingif you fit new rings and you will have to wait for them to bed in,most likely one of the new rings will have no gap and with no expertise you will ruin the motor, why do it.

Previously wrote:

Put the rings one at a time in the bore and check the gap. Should be 0.008 to 0.010". Gordon.

It is the gap between the two ends of the piston ring when it is in the bore. Be careful when attempting to remove replace rings. They are very brittle and easily snapped. As previously mentioned, make sure they go in the right way up and in the ring grooves they came from.

I am not that good at Maths but can not see 0.010" as equal to 0.5mm???

I have just bought some replacement Commando 750cc which state on the box:-

Standard +20 (73.5mm). Implying that 0.5mm has been added to the Standard Piston size of 73mm. This in turn suggests that 20 thou (0.0202) is equal to 0.5mm. Grabbing a chunk of scrap paper and etching device, I number crunched as below.

if:- 1.000" = 25.4mm (approximately)

then 0.100" = 2.54mm afterdividing both sides by 10

and 0.010" = 0.254mm againdividing both sides by 10

Which surely means that 0.020" = 0.508mm (or roughly 0.5mm after doubling)

To add to the confusion, if a standard cylinder isreboredto accept +20 pistons then does this actually involve only removing only 10 thou from the inside? This could explain why some newly rebuilt engines give a good rattle when started.

Attached is a list of Norton Piston numbers for a range of engines. Perhaps somebody could fill in the gaps and repost this as a useful tool fo members with collections of old pistons.

Previously wrote:

I am not that good at Maths but can not see 0.010" as equal to 0.5mm???

I have just bought some replacement Commando 750cc which state on the box:-

Standard +20 (73.5mm). Implying that 0.5mm has been added to the Standard Piston size of 73mm. This in turn suggests that 20 thou (0.0202) is equal to 0.5mm. Grabbing a chunk of scrap paper and etching device, I number crunched as below.

if:- 1.000" = 25.4mm (approximately)

then 0.100" = 2.54mm afterdividing both sides by 10

and 0.010" = 0.254mm againdividing both sides by 10

Which surely means that 0.020" = 0.508mm (or roughly 0.5mm after doubling)

To add to the confusion, if a standard cylinder isreboredto accept +20 pistons then does this actually involve only removing only 10 thou from the inside? This could explain why some newly rebuilt engines give a good rattle when started.

Attached is a list of Norton Piston numbers for a range of engines. Perhaps somebody could fill in the gaps and repost this as a useful tool fo members with collections of old pistons.

Hello Phil I have looked at your list of Pistons , And I cannot see any BHB numbers on there like the ones in my Norton 650 BHB numbers 23270 and 23271 , +10 The STD size Are Numbered 23268 and 23269 Left and Right pistons The Early 650 was only re-bored to +30 over size for obvious reasons ,it was Doug Hele That said that the 650 should Not be re-bored over +40 Has The liner gets thin And it will split in time , this has proven to be true , late on in years , yours AJD

Previously wrote:

Sorry, just to clarify: is this the gap that appears between the ring and the piston, when no force or compression is applied on the rings?

Or is it the gap between the ring (without any stress) and the walls of the barrel?

Thanks!!

Previously wrote:

Put the rings one at a time in the bore and check the gap. Should be 0.008 to 0.010". Gordon.

The ring gap is not that important as long as its not too small, which it is not!, what is important is whether the ring is sealing against the cylinder, post some fotos of the pistons and we can tell you what we think. You should have read up more before getting into the motor,you are not giving the motor or yourself much of a chance. Hope I'm wrong.

If you possibly can, get someone with a bit of experience to come and give you on the spot advice and help - so much better than trying to do it yourself. GOrdon.

Good advice from Gordon. You were invited to the Thames Valley meeting a while back; did you make contact? I am sure there must be someone in that branch who can help with a Dominator.

Cheers

Alan

Have been in that situation myself, more than once. Broken Norton being my sole means of transport. Last tip. If things are not going well, take a break. Amazing how a solution arrives after a cup of tea or a good night's sleep. Gordon.

P.S. Took 4 attempts to post this.

So is anyone near Isaak in Windsor able to pop over and give him a hand? I am happy to bimble up from Storrington but can't manage it for at least a couple of weeks.

My advice is - don't go any further inside your engine. Why did you take the barrels off anyway? Too late now, but if you go any further without any outside assistance you will end up with a basket case! It is pretty difficult to put the barrels back on without assistance. Too easy to scratch or even bend a con rod - or break a piston ring - if you do it by yourself. (Unless others here know better?). By the way - I always put the pistons in the barrel one by one on the bench before lowering it over the rods - and then put the pins in last of all. Not everyone does it this way. (And don't drop a clip into the sump - fill it with rag and take great care.)

I would hazard a guess that there are very few amateur or professional motor mechanics who learned everything they know from a book. The problem today is that most motors just go on until they get scrapped, whereas when most of us were younger we or our friends were frequently taking broken down engines to bits on the kitchen table. So my son knows nothing about engines and has little opportubnity or reason to learn.