Conrod oil holes

These are the very early Atlas conrods. Probably from a 1962/3 model or even 650 engine. They can be used but are not the best. The later versions are much stronger.

It all depends on how you use the bike. I had some in my first Atlas, along with a 3 speed oil pump and had no problems. But then I never took this bike to Track Days or hammered the engine. When sold on, it had 128,000 miles on the clock.

If you are likely to get throttle happy then you need to get the later type of conrod. These have more metal at the big end with a 'V' shape to the shoulder as opposed to the 'U' shape of the earlier rods. The 'V' rods come with or without a bleed hole. If you use the bleed hole version then make certain that your oil pump has a 6 speed worm and drive gear to keep the oil pressure high.. Checkout attachments

I'm curious:

Was the oil hole there to let the oil in or let it out?

Michael

Bleed holes were for extra cylinder lubrication and helped to use up the extra flow from the double speed or larger pump.

BSA put an oil hole in the drive side con rod on their A10s to equalise cylinder oiling. Previously,the timing side cylinder received more oil as that side was at full pump pressure whereas the drive side was at a lower pressure due to oil bleed from the timing side con rod - which is of course nearer the oil pump. Have I explained this clearly?

Hmmm, just want to add that I believe the conrod oil hole was added to address a perceived problem of lack of oil on the cylinder wall. When actually it was a early batch of pistons were bad and caused siezures but Norton thought it was piston pin source so made oil jets aimed twice a turn to strike at the piston pin seam.

The NOC at http://www.nortonownersclub.org/support/technical-support-commando/conrods describes this feature, and details the issue of the pump speed.

Basically, don't use conrods with holes with a low speed pump.

Once the holes were added they were never removed. In my Atlas and Commando rebuilds I turn the bearings in the big end to block the hole as this increases the oil pressure.

What a BSA has to do with a Norton I have no idea!

What has BSA to do with Norton? Guess who designed the engines. It might explain why you can fit BSA A7 conrods to 88s and 99s

Without wanting to seem to nit pick, the pressure in a system is constant throughout the system. If the pressure is varying then it varies throughout the whole system. What does change is the flow rate, you can only get a change of pressure though a pressure reducing valve. An example of that is your pressure relief valve.

I have had Norton twins for over 40 years.On the road and I raced them for many years to and won a few champinships.

On my 650 I always used late Commando rods and turned the shells round to cover the oil holes, then fed the rockers on the return side. I never had problems with wear on the rockers or lack of lube upstairs. You don't need to flood the valve gear, it's not OHC.

I prefered to feed the bottom end with maximum oil preasure. If you think about it if you have 50 - 65 psi feeding the crank the only way the oil can escape is through the big ends. When the crank is flying round there has to be a mega amount of oil spraying out of both jurnals flooding the camshaft and both cylinder bores. Why lose big end preasure to spray more up the bores and starve the big ends. The Japs don't bother with there high reving multies.

I also changed the rocker shafts and tried a 6 start pump but found constant reving to 7k over oiled the top end and made the motor smoke so went back to the original system. This works for lots of other Brit bikes. Well, thats my humble opinion folks based on experience.

Cheers Tony

Not knowing the root cause and instead treating the symptoms is understandable but wrong once the root cause finally surfaces. BSA also added an oil hole on the A65/A50 to cure driveside piston seizures, root cause later turned out to be the 4CApoints cam profile casing random sparks. The proper fix to piston oil starvation is better oil (eg full synthetics) and directing the flow exiting the big end journals upwards using a groove on the outside of the con rod above the big end. Adding holes to the con-rod increases flow at the expense of pressure in the big end.