Dommie cylinder lubrication

I always take advice from the component manufacturers rather than journalists. What do you think would be best for cylinder sealing - a 0.005" (5 thou') or 0.0002" (2 tenths of 1 thou') ring to bore gap? The workshop manual suggests a difference between the worn part and unworn part of the bore of 0.005" to be max. acceptable wear. However, this was written approx. 38 years ago. Why not have a rebore to current recommended tolerances? Rebuild the top end with 0.002" cylinder ovality/wear if you like, but don't be surprised if you have high oil consumption, piston ring blow-by, excessive crankcase pressure, loss of power, oil contamination of valves/combustion chamber/spark plugs and smokey exhausts. It's also false economy as you'll have to strip the top end sooner and have the required work done anyhow. I suppose it depends what your priority is - money, or doing it properly.

See www.cp-carrillo.com for relevant info.

Rob's carb jetting......

Single 1 1/16" 376 Monobloc, 250 main, 20 pilot, 3 cutaway, .106 needle jet, 5-groove needle in middle groove, paper air filter.

With the paper air filter in place this engine is probably going to be runninga little rich throughout most of the range to start with. The manual suggests just a 25 pilot jet. A 20 jet is used by bigger engines with generally bigger carbs.

Plus, unless all the bits are newish, the needle jet will probably also be worn, causing richness in mid-range running. This could be why the original plugs were sooty.

Plus...Plus....Plu... apparently the Ethanol, that has been addedto our petrol, affectsits overall specific gravity and causes original floats to sink a little. Thus causing the carb to be constantly flooding. Just wondering if this and a rich mix anyway, is washing/dilutingthe oil getting to the pistons. Thoughthis surely would affect both pistons.

Rob,

I'm beginning to see why you've had the problem since 1972.

Of course the valve guides could be to blame for an oily combustion chamber.My 1952 Land Rover would lay down a good smokescreen which was cured by replacing the valve guide seals. Of course pre Commandos don't have guide seals, but the guides themselves could be a loose fit in the head or simply just worn.

Fuel can do nasty things. Years ago Shell produced some horrible petrol which hadVauxhall ifI remember giving them the bill for a spate of engine seizures. I filled up my BSA with some out of necessity and it too seized - 12,000 miles after a rebore.

As a matter of course,if I have an engine reboredI go for the lowest available compression ratio pistons. It runs cooler, it's easier on the rest of the engine and it adds a bit of future-proofing. There's nothing like detonation for destroying engines. Gordon.

Rob,

I don't do rebores. I was pointing out that without measuring accurately and establishing dimensions you don't know, you are just guessing. Ditto oil pressure. The way you're going about the problem, it may be another 41 years before it's resolved. If ethanol was to blame then every reciprocating internal combustion engine in the uk would be seizing. As you have a single carb then I would have thought you can eliminate carburation, especially as you mention it's been running rich. If you get your barrels checked and also check your oil pressure, then at least you can rule them out as the cause. I'll leave you to go about it in your own way.

Previously rob_bradley wrote:

Previously simon_ratcliff wrote:

Why fit unknown rings? Do the engine a favour and spend about £40 on a decent set - after you've had the barrel checked for ovality and honed to suit the rings. Peoples opinions are one thing, but there is no substitution for accurate measurements when trying to eliminate possible causes (in my opinion), hope you get it sorted,

Simon.

Unknown rings in the sense that I don't know the manufacturer. +0.030" are hard to find so I've had to get what I can, from Cox & Turner.

What counts as excessively oval? Is 0.002" OK, front-back vs side-side?

Hello Well 0.030 oversize pistons and ring are the Norton motors bracebridge street works recommendations for there over size were plus 10 +20 +30 , the other sizes that are now available are after market over sizes pistons from +20 +40+60 these are the pistons that have been found to be giving Norton Owners some problems And Fred Swift found that Norton piston tolerances too be too tight in the bore and he had his racing 88 and 650s bore to 0.005 , my Norton Manxman is on standard pistons made by BHB and Honed to 0.003 and these work nice with no smoke or oil loss , to me 0.002 is tight in the bore and 3thou sound be good enough as long as the end gaps are right on the piston rings and do not forget to measure the lands gap as well .I have a document on Piston and Rings and there measurements , I will send a copy to MW for the roadholder and then you can keep this copy in your files for the future uses , yours anna j Dixon

Wassell stocks the "Hepolite" rings which are made in the USA by Hastings (see: http://www.hastingsmfg.com/ )& are of an excent quality & are offered in .010 incriments from stock to .060 oversize. If you cant find a dealer in the UK, PM me & as a Wassell dealer myself, I can get them for you also (I am in the USA).

Previously rob_bradley wrote:

Well, I did it my way and put it back together, and it's cured the left cylinder smoking issue. Success!

But now the right cylinder's smoking worse than the left one used to!

I don't know what's going on. But I've only ridden 20 miles so I'll try it a bit longer and see if it improves. Otherwise all I can think of is to tear it down again and re-hone the right cylinder, or stop messing about and have a rebore.

A few facts to follow up on some previous replies:

Oil pressure, taken from the plug on the rear of the timing cover: 80-90 psi cold. 30-40 psi hot, 20 psi idling.

Valve guides replaced in February, inlet guides Commando type with seals. Inlet rocker shafts changed to plain type. Valve stems max. wear 0.0005", within acceptable limits according to manual.

Fuel: Murco Super. This particular batch seems to be ethanol-free according to a water extraction test, but their technical man tells me they're being forced against their will to change their plant to add ethanol to all their petrol. He's been inundated with enquiries about ethanol from classic car, bike and lawn mower owners.

Compression test: 12 bar/175 psi both sides. Warm engine, without oil squirted down the plug holes, which would give a higher reading still. Healthy enough?

hello why have you got pain rocker shaft spindles have you fitted the 6 start oil pump, as you have not said, and if you have that amount of compression

then its not the bore, the oil coming for some were else like at the back of you valve guides , have you clean the oil ways in the cylinder head and at the back of the barrel in to the timing chest , these get blocked up and can mimic a oiling problem as all the oil ends up round the top of the valve guides and down the pushrod tunnels

yours anna j

Previously eugene_brolund wrote:

Wassell stocks the "Hepolite" rings which are made in the USA by Hastings (see: http://www.hastingsmfg.com/ )& are of an excent quality & are offered in .010 incriments from stock to .060 oversize. If you cant find a dealer in the UK, PM me & as a Wassell dealer myself, I can get them for you also (I am in the USA).

Hepolite pistons and rings are after market products and are not the originals pistons and rings,

Previously rob_bradley wrote:

Previously anna jeannette Dixon wrote:

hello why have you got pain rocker shaft spindles have you fitted the 6 start oil pump, as you have not said, and if you have that amount of compression

then its not the bore, the oil coming for some were else like at the back of you valve guides , have you clean the oil ways in the cylinder head and at the back of the barrel in to the timing chest , these get blocked up and can mimic a oiling problem as all the oil ends up round the top of the valve guides and down the pushrod tunnels

yours anna j

It's still got the original 3-start oil pump. I fitted plain inlet spindles when the smoking first appeared, in order to limit the amount of oil in this area, inlet guide seals for the same reason, in case this was the source of the oil causing the smoking. There still seems to be plenty of oil getting into in the inlet rocker box though. I did check the rocker box and barrel drain. Valve guides were replaced by an engineering shop as I wanted them to re-cut the seats afterwards, and they said they were a good fit in the head.

After 75 miles of riding the smoke from the right exhaust seems to be reducing and the plug isn't wet and oily any more, so maybe it's just that the oil rings are bedding in a bit slower than the compression rings.Now you say it well you need the scrolled spindles in and Clean OUT the oil ways from the inlet valve side near were the spring fits and check all the oil ways down from there too the time side were it come out , the is a dog leg oil way in the crankcase like a L shape this gets blocked ,so then if this is blocked the oil ends up running down the valve stem as there are on rubber valve stem seals on Dominator's

Maybe just maybe your crancase mouth is not parralel to your crankshaft. I have never found a 750 Atlas, Commando or G15 with the cases paralel with the shaft. The worst ones were 15 thou out left to right at the barrel flange. The engine would barely turn over with no rings once I tightened down the barrel. The cases were not mismatched. One set had never been apart. It's easy to check with the top off the engine, all you need is a piece of precision ground bar to go through the little ends and a set of feeler guages. Worth a try!

Bill