Motor Advice

Crankshaft Balance Factors 750/850 Norton Twins

For solidly mounted motor eg Atlas, G/N15/33 and P11 use 84%, for Commando’s use 55%.

These figures assume crank is clean and dry.


Can you supply shim cup 68-0638 for BSA A65′s and what exactly does it do?

After being asked this so many times and being unable to find anyone supplying it we are now set up to manufacture it ourselves.This .020″ thick lipped cup holds the crank end float shims in place behind the drive side bearing inner race. In actual fact, when everything is bolted up with the correct thickness of shims in place, this cup is not entirely necessary. However it is useful to locate them when assembling the motor and keeps them properly in place should the alternator rotor nut ever come loose.


Can you supply the needle roller coversion for the right hand side of the crankshaft to replace the bush?

We can help you with this through our relationship with SRM in the UK. The biggest expense is shipping your crank and right side case over there. Getting it back is not so much of a problem because we can have it put in with one of our regular air freight parts shipments.

What do I think of it? My local police dept in the UK in the mid 60′s used and thrashed police A65′s. They changed the oil regularly and got very big mileagles out of the standard bottom ends. My recollection is that most of their problems were electrical. If however you are going racing with a big bore kit, HC pistons and maybe an A65 crank, the bottom end conversion would be a must!


Customer seized his motor because his mechanic, after completing the rebuild, installed the feed and return oil lines the wrong way around.

In my 20 years plus in this business, I have seen this happen several times. As a matter of course I regularly take the oil tank cap off after starting the motor to watch the oil returning to the tank, this is especially important right after a rebuild. It shouldn’t come as a continuous stream, more as spurts with air bubbles because the return gears in the oil pump obviously have to be bigger than the feed ones. If nothing is coming back, shut the motor off immediately and investigate. On our race bikes we would put them in gear, remove the plugs and push them around a bit with the ignition switched off to get the oil circulating before starting. Owners of some bikes, with rocker gear fed from a restriction in the return line, were advised even into the 1960′s to start the bike up and put their fingers across the return oil orfice in the oil tank for a short while to divert oil to the valve gear!


What oils do you recommend for my bike?

Opinions differ on this one and if you look in the original handbooks/workshop manuals, especially for the older bikes, you will find lubricants recommended which are no longer available. I gave some thought to the bikes we most often sell parts for and came up with the same recommendations for all of them. If you have one of the following, this is what I recommend. BSA 250,441,500 Unit Singles, BSA A50/65 Twins, Norton 750 & 850 Twins, Triumph 250 Singles, Triumph 500,650,750 Twins.

For the motor – 20w50 Grand Prix Motorcycle Oil. If oil consumption is an issue or in hot weather Kendall GT-1 straight 50 is recommended, except in Triumphs which breathe through the primary chaincase. If running a straight 50 warm the engine up to get the oil around well before riding off. Always warm it up using as little choke as possible – unburnt gas washes the oil off the cylinder walls.

Primary chaincase – late Triumph twins use 20w50 as in the motor. For all others I recommend Kendall non detergent straight 20w or even ATF.

Transmission – 90w hypoid extreme pressure gear lubricant – lots of different makes available.

Front forks – 20w fork oil – the modern fork oils have seal swelling properties and are much improved over those which were available years ago.


I topped up the oil in my oil tank, ran the bike for a while and oil started spewing out everywhere – the oil tank was overflowing.

Everyone with a dry sump bike (that’s most British ones), Harleys and CB750 Hondas (K Series) etc finds out what wet sumping is all about. When the motor is running the oil gravity feeds from the oil tank on the side of the bike, down to the oil pump. From there it is pressure fed into the crankshaft and around the motor. The oil which falls to the bottom of the crankcases is scavenged up by the return gears in the pump and returned to the oil tank. There isn’t much oil in the motor, it just circulates through. The cavity in the bottom of the motor is mostly dry – hence dry sump. When the bike is left standing oil gradually drains past the oil pump and fills up the sump, especially where the oil pump is worn. There are various valves available to restrict this flow of oil into the sump.

If you start the bike up with the sump full of oil the pump will work overtime returning it to the oil tank, which needs to have room for it. Also with the piston/s flying up and down a lot of the oil can get squirted into the primary cases and out through any weak gasket etc.and out through the breather.

Bottom line, make sure the sump is empty before topping up the oil tank.


Low Expansion Pistons

Low expansion pistons are not totally desireable. In order to reduce the heat expansion, the silicone content in the alloy is increased. The problem with this is that the conduct of heat from the top of the piston to the lower areas is substantially reduced with the top running very hot. This causes greater expansion of the pistons rings than with normal pistons. I have always used Phil Irving’s “four thou per inch of bore” rule for gapping piston rings however manufacturers’ recommended gaps for some of the modern low expansion pistons are now up to three times that figure. If the rings are gapped smaller they will expand, and could lock up in the bore and break the top ring land.

Courtesy of Answer Engine Works, London, Ontario.


Fitting Valve Guides

We get occasional problems with valves seizing in the guides, especially with bronze guides. It is very important to ensure that an inference fit of .001″ is achieved. Putting in guides too tightly will cause them to narrow down in the centre when they get hot. For clearance use .003″ for inlets and .0035″ for exhausts, as per Triumpg factory specs


I ordered valve guides for my 650 Triumph by 2 different part numbers (two inlet and two exhaust) and you sent me four all the same.

For years PM Kibblewhite and several other manufacturers have been selling the same bronze guides for inlet and exhaust for 650/750 Triumph twins. We find these give good service although some mechanics are set against bronze guides and insist on cast iron. Speaking with guide manufacturers, all say that the inference fit is very important. You obviously don’t want the guide to be too loose but also if they are installed too tightly, another problemn occurs in that the centre squeezes in when the head gets hot and the valve can seize. Best to go for an inference fit of .002″, reaming the head and fitting oversized guides if necessary. Guides should be reamed after installation of course and the seats cut. Triumphs in general do not have good valve geometry and the mushroom headed tappet adjusters (either the Norman Hyde type or the USA made ones) we sell will vastly extend the life of the valves and guides. Another point worth mentioning is to examine the point of contact of the adjusters on the ends of the valves. If the valves have sunk into their seats the adjusters may not be striking the end of the valves. We have been presented on more than one occasion with a warranty claim where on examining the valves it was obvious that the adjuster had been striking off to one side of the end of the valve. A customer once asked me to supply lash caps and I referred this to Norman Hyde who said these would only be required on poor quality valves, something he would certainly never sell!

While I am on this subject there’s another matter Norman brought to my attention several years ago. On original 650 heads the valve seats are set back in separate hemi pockets but on 750 twins the seats are flush with ther main hemi shape of the combustion chamber. This is why, even though they both use the same valves and springs, the spring seats on the 650′s are stepped, so as to maintain correct fitted length for the springs. The problem is some of the replacement 650 heads made later on don’t have the original style of seat pockets so you need to use 750 spring cups or too much tension will be placed on the springs.


Triumph 1970 T120/TR6 push rod “O” rings.

The parts book shows 4 round section “O” rings, 2 at the top and 2 at the bottom of each tube. All the 1970 bikes we have seen also have the steel wedding bands with the square section “O” rings at the bottom – this is shown in the 1971 parts book.