Rebore mileage.

Hi Les,

Mine was on +010 at 30,212 miles but this was badly worn and the bores pitted; you could see where the rings had been sat as the acid released when old oil decomposes had made a mess of both bores (this is why you should turn them over regularly when in store!) yet it did run sweetly. At least it did until last timeIrode it round the field when it developed a severe misfire then died completely. 40 years off the road and a BAD attempt at re-commissioning 9 years ago by a past owner who failed to find a seized pressure relief valve and an oil pump that did not do much (both of which I had diagnosed and sorted BEFORE trying to ride it). What didnât help was the slip ring in the magneto had a canyon worn in the Bakelite while the metal contact was at normal height!

The moral here is that it may run after decades of standing BUT it runs a high chance of serious damage if you just use it without the proper checks, compression is NOT a good indication of how good the engine is and nor is the fact the mag seems to have a good spark! You can cook a newly rebuilt engine with a duff mag as if it does not spark at the correct temperature your engine will overheat and be a pig to start when hot.

If you do need to do a rebore DO NOT use replica pistons as one likes to seize and the other to fall apart. I got lucky and found some new old stock Hepoliteâs for mine, they are out there!

If your trying to estimate the mileage on your bike, this is a difficult way to do it on such an old bike. Rebores are a funny thing. Considering normal running wear and not a blow up....I know quite a few people who will pull an engine down and get it rebored because they have the engine apart. They will very often have absolutely no idea why they are getting a rebore done other than they think it's the right thing to do. I've seen people waste the metal in their cylinders just so they can say it's been rebored with new pistons. Did they measure it up? Did they know what they were looking at? Did they know why they did what they did? Many engines cop a rebore way before they really need it. All going well if you could get at least 35k out of your cylinders before a rebore, then I think this is not a bad figure. If your bore looks good, then you might estimate it's half way along it's 35K, so are you estimating the bikes done up around 50 something K. How does the rest of the bike look.

The best way to check the bores is to measure them for ovality & taper, measure the pistons & inspect the rings for wear, then you know where your at and you can make the right decision.

Regarding pistons, mmmm, I've known so many people who go to great lengths to get NOS pistons and have trouble. They come up with pistons in tatty 50 year old boxes and are quite proud to have found the genuine original item. It's a lottery. The damn things have probably been kicking around the shop, shifted from one spot to another dropped on the floor, bought & sold as businesses have gone to the wall over the years. I've got my doubts about them. I've certainly seen a few blown up engines with you beut NOS pistons!! I've seen JP pistons bagged out on this forum a few times, but I've had JP pistons in my Commando for 15 years, I've a JP piston in my Model 18 and another in my 16H. I have had no trouble from them and I don't go easy on them either. Compare an old piston with a doubtful storage history, 50 year old metal technology, to a brand new freshly made item with current metal and machining technology. It's the piston that does all the work in the motor....it's your choice.

Bob

Back "in the day" (i.e. Mods & Rockers in the 1960!) I don't know of anyone who could get away with more than about 15000 miles without a rebore! When I bought my bike at only 14000 miles it had already been bored out to +.030"!This was the result of abysmal or zero maintenance of course - there lies the rub! Change the oil at leastonce year WITHOUT FAIL! It's the same with cars - if you see a smokey car exhaust on any car less than 15 years old it's because of poor maintenence - more specifically, not frequent enough oil changes. That's the cheapest and most effective maintanance you can do on any internal combustion engine.

I once owned a 1986 MkIII Ford Granada and I gave it away to a friend after it had gone over 220,000 miles. It still used no oil and didn't smoke. Conversely the only gauge still working on the dash was the revcounter! Difficult when trying to estimate when you needed petrol! When the anti-lock brake warning light refused to go offI disconnected it! Otherwise it would have failed the MoT! Its oil changewas every 6000 miles. As I was commuting around 600 miles per week, it had new oil and filter every 10 weeks! OK, not perfect maintenance but the dash electronics weren'yt really a DiY job and I was more a mechanic so the engine, gearbox and running gearwere ACE!

The traditional method of using a piston ring in the bore to check the wear is the best indicator - at the top unworn ridge section then near the centre and at the bottom. Any bad scoring means a rebore is essential anyway. Just remember your bike is from the 20th century and was never intended to go long distances at much more than 60 mph! When they opened the first section of the M1 motorway it soon became littered with 1950s cars which had never gone more than 70mph for more than a few seconds at a time in their life!

Previously wrote:

Back "in the day" etc etc...................

............. Just remember your bike is from the 20th century and was never intended to go long distances at much more than 60 mph! When they opened the first section of the M1 motorway it soon became littered with 1950s cars which had never gone more than 70mph for more than a few seconds at a time in their life!

This is an extremely good point and one I often ponder as I'm belting along on the Big 4. Roads are very different now. When my Big 4 was made in 1950 I don't think it had the opportunity to get along hour after hour on roads that we have today, enabling it to maintain higher speeds for longer times. It does it though, a few more oil leaks on the garage floor at the end of the run!!

Bob

Service manuals have set dimensions for acceptable wear and make it pretty cut and dry about when parts in an engine need replacing. You do have a service manual for your bike right?

Previously wrote:

Hi chaps.

I've now decided to completely renovate my Model 7 as it would not be practical to get it back on the road as a runner, it's in far toopoor a state.

Whipped off the head yesterday to find it has +20 thou pistons. The bore looks good with very little ridge (Mostly carbon) at the top.

QUESTION: What mileage would be expected for a Norton 500 twin to cover before it's first rebore was required?

I know this can vary, but with average use, average dusty roads and average usage with oils that were available in the 50's and 60's, there must be somepersonnal experience with club members as to what mileage they found a rebore necessary.

Would 35K miles seem about right? I would imagine the lack of air filter does not help in extending the mileage.

Many thanks

Regards.

Les H

i would say it depended on whether dad rode it to work and back or teenager rode it out at night to impress the girls, 500 twin or not, had a 350 nh ariel once , +30 bore when purchased ............... milage recorded 78126 when sold , dads ,till i got to 30 years old, and with very few oil changes too,, just dont thrash it, and it will last and last.

There are several factors whichneed to be considered when attempting to maximize the life of the cylinder bore. Piston speed, choice of lubricant and air filter just to start with.

Engine design and revs affect the speed the piston travels up and down the bore. A long stroke engine tending to have a higher piston speed than a shorter stroke motor of the same capacity. Throw in lots of regular, highengine speed in a long stroke engine and the recipe for fast wear is well under way.

Old and unfiltered oil contains huge amounts of rubbish that will rapidly accelerate the wear problem. Under load it will quickly break down, which in turn can lead to hot spots and then seizure. Likewise, cheap or the wrong grade of oil does not help either. Fit an oil filter kit and change the oil at least every 2000 miles. Always use a high quality oil. The sensible owners use a good 20/50 as this is suitable for most of the year and will out-perform any cheap monograde oil.

Fit anair filter.Even a basic wire mesh type can double the life of the cylinder bore. You need to stop grit from the road being sucked into the bore and basically sand-blasting all the parts.

With no air filter, most engines will struggle after around 20,000 miles and certainly need new rings by then. With all the rights safety features in place, 40,000 miles of bore is easily achievable.

Talk to Commando owners who have owned their bikes for a good length of time and whose engines have always usedboth air and oil filters.Most will admit that the valves guides, main bearings and camshafts usually die before the pistons wear out.