Piston to bore clearance - 650SS

The 650 is not the most quiet of engines and some cold slap is to be expected ,I would not build one with less than 41/2 thou clearance,so a used motor with 6 thou would not be worn enough to need work.In my opinion. I had my 88 bored and it never perfomed as well as when it clattered a bit. Mr Hemmings has been looking for some Hens teeth 650 barrels for a long time, Save the rebore,you may need it later.

.006" is a good clearance for a 650SS, or so I am assured by those who know. Mine is .006", and has only done 2000 miles since reboring.

I don't think sleeving the barrels is straightforward, either.

What over-bore is it?

6thou isenough to make a noise. Apart from the cold start knock I find piston slap is worst after restarting a bike that has quite a hot barrel and thin hot oil but there has been time for the pistons to cool, something like 10 minutes, it manifest itself quite strongly as you accelerate and reduces after a minute or two.

Does the knocking sound as a "singular knock" because as you would appreciate a twin cylinder engine should produce the knocking from one side to the other alternatively and possible a very slight difference in volume from one to the other....you could have used the screwdriver stethoscope trick whilst someone blipped the throttle...too late now unfortunately.

There is a chance that the alternator rotor is loose on the main shaft or the rotors magnets are loose on the centre boss, this can clack as each firing pulse is felt by it....

One other thing you could check is the ignition timing...I'm not talking about "pinking" but if the ignition is too far advanced it will cause the piston to hit the cylinder wall harder and more noisilyon firing and worth checking before the head is put back on if you use piston position to set the timing....

Ring Gaps: 9-14 thou....Piston clearance is usually provided by the piston manufacturer as it depends on piston construction (cast or forged) and what alloy is used....Les

My big single piston has a recomended manufacturers clearance of 6-7 thou, everyone who rides them say not less than 9 thou and 10 is good. Unless you like to seize at 90mph.

Robert tuck says "My big single piston has a recomended manufacturers clearance of 6-7 thou"

Obviously a big piston is going to require a bigger expansion gap!....the 650 twin pistons arethe equivalent of a 325cc single cylinder enginewhich isquite small....Les

What I'm saying is ,its not as simple as it seems, this is an old, old engine and may well actually need more clearance than ideal due to alignment issues .I would put up with a bit of noise for the peace of mind that its less likely to seize if you get the timing,carburation or plug wrong , or even run weak due to a blocked petrol cap etc.

I am with you on this, Robert, for exactly the reasons you state. If they are bored too tight, you run the risk of it nipping up. The 1963 twins had the low pressure 3-start oil pump, which at times, particularly when ridden hard; which is what the 650SS was designed to do; could provide marginal lubrication, risking the result you mention, Robert. Hence the adoption of the six-start system, which had it's own issues.

Theoretical gaps are one thing, but it is as well to heed what has been learned in practice over the years. I think the chap who bored your engine, Clive, knew what he was doing.

I do not imagine your knock is caused by a .006" piston clearance, in any event. It is worth looking at the areas Les has pointed to, including ignition advance. Ensure the timing is the same on both cylinders.

I have to say, I think the other way on this Robert. I would never exceed the recommended piston clearance as I detest piston slap.

I'm not trying to be facetious or be a clever clogs here, but one really does need to set the ignition and mixture strength correctly and run the bike in progressively over 1000 miles...Short cutting the running in process by increasingpiston clearance is something that you will regret for the entire next 20-30,000 miles. Furthermore with too much clearance the bore life is shorter and ring sealing that much poorer too as the rock created wears the rings edges more so will burn more oil too....Les

Of course there are arguments for and against careful running in. With modern machining there is little roughness to wear down and modern lubricants give excellentsurface layer protection so there is almost no metal to metal contact. Buy a new car or bike and there is no mention of running in. Fit an overhauled piston and cylinder to a piston engine aircraft and you are instructed to give it full throttle. So perhaps you can be too gentle with an overhauled engine.

I think we have missed the point. I would not build a 650 with 6thou clearance and equally i would not re-build the motor with 11/2 thou of wear. The important thing is how does it go.

In reply to Gordon Johnston. You shouldn't overlook the fact that car engines and modern motorcycles are water cooled with thermostatic temperature control. This provides a near constant maximum temperature right over the engines head and all the way down the length of the cylinder. This lets engine designers close up tolerances enormously...Furthermore the water cooling jacket provides a sound block/reduction to piston slap.

A piston is ground slightly oval but that oval shape does not mean it will fit perfectly the bore when it heats up and expands, the exact shape needs to be developed over a period of time and with an air cooled engine its doubtful if the cylinder is ever exactly circular when running so the idealcomplex shape cannot be machined into the piston during manufacture...it needs to be allowed to be created and developduring the running in mileage. Yes you can make the piston a sloppy fit so it will never seize even if you don't run it in but you get a noisy engine which is OK if you don't mind a noisy clattering engine!....Far betterif one has the patience to run an engine inwhere the clearance is just on the verge of seizing if excessive throttle is used when fresh out of the box, but over a thousand miles of controlled throttle and different loads the perfect piston shape is developed and one that fits the bore perfectly with no hot spots* and absolute minimal running clearance resulting inan engine that is reliable at all throttle settings & temperatures and makes virtually no piston slap noise...Les

* A piston tends to seize, regardless of how good the lubrication is, at just one small hot spot area created by a mismatch of piston to thebore as the piston expands with heat. The enormous pressure on the cylinder at this small area rubs through the oil and then smears the cylinder which snowballs and removes more piston metal and creates a bigger seizure. If the piston fits the bore perfectly, which can be obtained over a longish mileage after the hot spots are slowly worn down, the side loading/pressure of the piston is spread over a bigger and bigger contact area where the pressure iscorrespondingly reducedwith it, substantially reducingthe chance ofbreaking through the oil filmto make metal to metal contact and seizing.

Piston aero engines are mostly air cooled - at least the one I have had any dealings with. Again, the instructions are to avoid low throttle openings with a freshly overhauled cylinder and piston.

I quite agree that hammering a freshly overhauled Norton engine isn't a good idea, but you do need to load the engine increasingly over the running in period to get it to bed in.

Hi Gordon regarding the Aero engine....

1) I would imagine that they have a very large piston clearance to start withto avoid any chance of seizing. For obvious safety reasonsthere is no other option since full power is needed every time to get the plane off the ground even on the first flight. With open un-silenced exhausts and the pilot inside the cockpit, you are never going tohear any slap anyway so it doesn't matter.

2) Running an engine too slowly fromnew can result in glazed bores. This occurs when the combustion pressure is very low and does not force the rings hard enough against the cylinder walls to create a good seal. The result is that oil is burnt into the cross hatching and turns to an extremely hardglassy substance filling the grooves which then prevents further bedding in of the rings to match the bore. For this reason if a running in procedure is undertaken special running in oils should be used which are formulated to avoid this happening...Apart from avoiding too much low power running with the aero engines the majority of high power running prevents theglazing from occurring so top quality oils can be used right from the start too and the rings will still bed-in.

So summing up, if you want an engine to run with the least piston slap...do it the old fashion way with the correct minimal piston clearance with the right oils and take 1000 miles to run it in...OR...If you don't mind piston slap choose a big piston clearance, ride it with hardly any running in, and perhaps use ear plugs and kid yourself how nice your engine sounds for the next 20,000 miles until the next expensive re-bore time comes around and you have another choice.

Les

BTW Clive, have you decided what to do now then? If you don't find any thing wrong elsewhere you surely have no other option but to re-assemble the engine as it was?

Les

The most important thing here is to use the manufacturers clearance. They don't provide the figure for fun!

With regard to running in there are lots of theories, I race two stokes, and after a rebore it will get about half a dozen 10 minute runs talking up to full operating temp and letting it cool completely down, during the run I will accelerate and decelerate making the engine work at about half its power, this is essential to load the rings and get them to bed in. It's no good having a perfect piston fit if the bore is glazed and the rings are not sealing.

once I've done that it will get a fairly gentle ride in practice at the first meeting then ..... I ride like I stole it! I've had engine failures yes (one barrel let go which was exciting!) but not had any siezes.

i don't necessarily disagree that long run in periods are not beneficial although I'd question how much a piston would wear in that period, but I would make sure that you do accelerate through the gears and load the rings.

my twopenceth worth anyway!

In a recent conversation with Les Emery of 'Norvil' about my 650SS, he said quite categorically that long running-in periods are a no-no, and they load them up and drive them hard from very early on. They have built far more twin engines than most of us on here put together, and they are not going to build them in order to b#gger them up in the first few miles, are they?

Talk of the next 20-30000 miles is pretty irrelevant in this day and age, where most classics do less than 1000 miles a year. Given the age of most owners, we will never get near that mileage! They are not daily transport for all but a tiny few, and most owners possess several bikes, so their leisure mileage is divided between them. The two differing approaches voiced here are as much to do with how they are ridden. A 650SS is not the ideal mount to buy in order to ride in club runs at 45mph, when a nice, tight, quiet engine may provide the owner with bragging rights. When they are ridden, they invite the rider to explore their performance, so a slightly looser motor is favourable; in my humble opinion, of course

Like I've said Ian (several times)you either build the engine to be quiet or take the quick, blast it in method which you are going to need big clearances...you do it your way, I'll do it my way...I'm not fussed one tiny bit....Clive has a noisy engine...he stripped it down..he obviously doesn't like that 'orrible knocking sound...because someone like you made the bore too big for his brand new pistons....what's he going to do now?

If you and Les Emery are trying to tell readers here who are not sureand thatyou can have a correctly sized piston and blast it in, I hope they don't come knocking on your door complaining they've seized their engines because I know it will happen, whereas they should knock on Les's door ifhe sold him the pistons...Strange that Norton advised new owners aboutthe correct running in procedures...they didn't have a clue did they!

Anyway, I've got no more time so you can have the last word on the subject...I'm not fussed... you can do what you like with your engines....and I'll carry on doing what I know will create a nice quiet reliable engine.

BTW...I only mentioned the 20,000 mile figure to point out that you will have the noise (and a bit more) for the rest of the engines life...I wasn't saying you would actually do it...so not irrelevant!

Les

Les. You are making an unsubstantiated assumption that Clive's noise is caused by piston slap. That has not been established, and mine does not suffer from piston slap.

Norton advised owners of running in procedures 60 years ago. Since then technology has improved, along with improvement in oils.

I am not trying to tell anybody anything, just putting a point of view, alongside others, and individuals can decide.