Hole in nearside con-rod

[CheeserBeezer]

I am interested in the opinions of people with engineering qualifications about the the hole in the left hand con rod. I have some con-rods, manufactured in America, supplied specifically for A10, and these rods don't have a hole. I'm told it isn't necessary. I understand why the hole is there on the original rods, 1) to cool the inside of the barrels and 2) to guarantee flow through both big end bearings. If this hole is essential, do all bikes have them? Answer - obviously not, because I'm not aware of the hole on anything else; owners of Triumphs, Nortons etc. please correct me if I'm wrong. Presumably, if the running clearance is not too tight, the absence of a hole will improve pressure on the big-end bearing surfaces which, in my opinion, is desirable. The additional cooling to the centre of the barrels is, in my opinion, a bit of a non-starter, as I don't believe this squirt/splash will make that much difference. Opinions please!

[muskrat]

G'day Andrew.
I'm with you on this one.
The hole will reduce the oil pressure on both the big ends. Fluid will always seek the easiest path.
Putt a couple of holes along a garden hose. Now block the end and turn the tap on. Lots coming out the holes. Now un-block the end, how much is comming out the holes now?
Cheers

[Swarfcut]

 I reckon we need the opinion of a " lubrication practitioner" as we all think a higher pressure is better, but I would consider the volume of oil maintained between the moving parts to be just as if not more important. So how does high pressure, low volume vs low pressure high volume stack up?

 If a hole was needed, why not put it to aim a jet to the thrust side of the piston? Or is the jet purposely aimed to the side of the bore where there is greater piston/bore clearance? Was it a stop gap measure, poorly researched but maintained as it worked, the earlier A7 design does not have this hole, despite a larger linear stroke.

 Swarfy.

[groily]

One heck of a question I reckon, and very interesting.
I was always encouraged to believe that flow was as important as pressure, and that centrifugal forces were also a big factor in relation to lubricating big ends. Whether by chucking the oil out (bad!) or by increasing the pressure usefully, I'm not sure!
I have run A10s with and without the hole in the left side rod - the latter from necessity frankly when young and very poor and short of rods - but although nothing broke, that experience is  not meaningful because my engines in those days were shoved together in back gardens or at the side of the road and something else was bound to (and did) go bang every so often.

The very ancient article below is a bit tekkie - and a discussion rather than an attempt at being definitive -  but it does focus on pressure vs flow, and on centrifugal forces, especially in relation to the #4 b/end journal in the motor under discussion. But it doesn't have a drilled rod like an A. The bits I could understand are interesting, other bits passed well over my head.

Certainly other twins (including I'm fairly sure Trihards with plain timing side bushes) don't have the exit hole. So it's probably not a 'plain bush' thing although I wonder whether with an end-fed crank and ball or roller mains any of the considerations are different? Maybe just a workaround for poor cylinder oiling and nothing more? But my end-fed Notrun doesn't have or need, and my AMCs are built totally differently with a direct  feed to each big end from the plain-shell centre main, 50:50 equal volume and pressure split, with roller mains elsewhere.

With my own A, I just followed the 'book' and it ran well for a long time, showing almost no wear when I took it apart just the once to sort the crank end float.
My ha'pence says the hole is probably not essential - but I think I'd drill one to be honest, just in case.

Very interested to hear what better informed folk think, as it's a more-than worthy topic that has rolled around since for ever  . . . .

https://www.jstor.org/stable/44434014?seq=1

[Ted_Flash]

For info, the Service Sheet from 1950 (not applied to A7's?):

[muskrat]

G'day Fellas.
I was taught volume for rollers, pressure for plains. Both my bikes with roller big ends sit on 20Lb and as low as 10Lb at idle. If my plain bearing motors got that low (before going bang) I'd be rebuilding them.
Cheers

[Sakura]

My RGS has Carrillo rods. I queried this with Mr Carillo himself. His response was that his rods are narrower than the originals and there will be plenty enough lubrication. I did 7000 miles with no problems. I doubt anybody who uses them in highly stressed engines drills a hole in them.

[RichardL]

I'm going to spout a hypothesis and see if it gets any takers. It's similar to what I've mentioned here before. I'm not married to it, but I think it's fun to consider.

1. BSA identifies a lubrication problem to the left cylinder.
2. BSA believes they can get more oil to fling onto the left cylinder by drilling a hole in the conrod.
3. They drill the hole and left-side cylinder under-lubrication ceases.
4. BSA (or maybe just the rest of the world) assume oil from hole is flinging onto the cylinder wall, thus having solved the problem. 

All this said, the area for oil flow through the hole is only about 30% of that for flow around the circumference of the journal. Then, the hole in the conrod is only exposed to the holes in the shells about 2% of the time.  To me this seems more a case of promoting overall flow than counting on the hole to provide the missing flingitude. So, I asked myself, why would the feed hole at the far end of the sludge trap get less oil. The hole near the right end of the trap is small in comparison with the trap diameter, therefore, I feel that pressure along the length of the trap should be about equal, based on Pascal's law (yes, I had to look that up, being 40 years since physics class). So, why less oil flow at the far end? Now, the controversial part. What if oil foam or bubbles build up at the far end of the trap enough to coincide with the conrod feed hole? Very limited oil is going to flow, maybe just enough to lube the shells but not the cylinder. Add the hole, then, the air vents from the foam or bubbles and flow increases to equal that of the right rod. If this was the case, I'm thinking BSA wouldn't have known it, because, after #3 above. They didn't need to care about it.

Well, I'm sure glad I'm out of throwing distance for rotten tomatoes.

Richard L.

P.S. I drilled my Thunder billet rod.

 

[Colsbeeza]

I presume that in normal operation, some of the leakage from the Timing Side bush would fling and add some oil to the RHS cylinder wall, so perhaps BSA wanted to match that on the Left side?? Their observation certainly was that the LHS cylinder got less than the RHS. I agree with Richard that oil exits the sludge tube hole equally to each bigend. So the LHS drilling might compensate for the T/S leakage. Given that the A65's have a larger oil pump, the A10's were always light on lubrication. Col

[BagONails]

It seems that BSA had an issue with cylinder lubrication and this was their somewhat half arsed response, although it can be deemed to have been successful as these machines don't now have a reputation for piston seizures on the nearside pot.  It is possible however, the issue was down to some other manufacturing variable and nothing to do with lubrication, the change being a late , knee jerk reaction maybe but once a change like this has been made they would never go back. This might explain why some have run engines without the drilling and not had an issue.

Speaking as an ex Ford manufacturing process engineer who cut his teeth on making con rods, over 750,000/year in the 80's, we drilled 'oil spit holes' in the majority of our rods, petrol and diesel variants. Those holes were very small diameter and a bugger to do in drop forged steel components as the drills being so small tended to break too often.  Those holes were all positioned in the centre of the H section, angled up to spray oil directly at the underside of the piston crown and the thrust side of the bore when the crankshaft drilling lined up directly with the hole in the shell and the drilling through the rod on every rotation of the crank. Cooling the piston and lubricating the cylinder wall.  Although the oil supply, pressure and volume, on a 80's car/truck engine would have been far superior to our 50's Beezers there must be some effect when you have 4 or more rods all venting oil like this but clearly worth it!

I can tell you from experience that making a change to introduce an extra hole in the left hand rod in a volume manufacturing situation would have been an expensive exercise for BSA and not a decision taken lightly. They must have had a big problem and deemed the change worthwhile back in the day. As to it's true value, I can't comment any further than what has already been said unfortunately.

[CheeserBeezer]

Thank you all for the replies. As usual, a wealth of information and hypotheses from people who know what they're talking about. Sifting through the information, it would appear, according to the service sheet, that the hole was introduced to improve lubrication to the left hand cylinder. Information I have had from another source is that A10's were prone to seizing on the left hand cylinder and consequently breaking the con-rod. When I say 'prone' I don't mean it happened a lot but, if the bike was going to seize it would be on the left hand side. This ties in with the purpose of the hole as described on the service sheet, so obviously something BSA tried to rectify. I have also been informed that the R+R rods I have are racing rods made in America and are very good quality, but R+R are reluctant to drill them due the 'stress-riser' this would cause. If I'm building a new engine, it would make sense to drill the left hand rod to avoid a seizure on tight bores. Given that BSA drilled the rods, and these R+R rods are better quality, I don't foresee a potential rod breakage being likely. Many thanks chaps.

[Tomcat]

RichardL got it, 1/ BSA identifies a lubrication problem to the left cylinder.
In Whatever happened to the British motorcycle industry Bert Hopwood told of continual left piston seizure in the new A10 engines but no one could explain why. Someone said 'if only we had a window in the engine we could see what's happening'. So a hole was cut in the engine case and a perspex window was fitted and then they saw it was insufficient lubrication to the left piston. A hole was then drilled in the LH con rod and the problem went away.

[CheeserBeezer]

RichardL got it, 1/ BSA identifies a lubrication problem to the left cylinder.
In Whatever happened to the British motorcycle industry Bert Hopwood told of continual left piston seizure in the new A10 engines but no one could explain why. Someone said 'if only we had a window in the engine we could see what's happening'. So a hole was cut in the engine case and a perspex window was fitted and then they saw it was insufficient lubrication to the left piston. A hole was then drilled in the LH con rod and the problem went away.

That's very interesting. Where have you got your information from regarding the perspex window? It would also make sense to ensure the hole in the con-rod is facing inwards as the hottest part of the cylinder will be in the middle. Thanks again.

[JulianS]

An aside - I bought R and R rods around 10 years ago and quickly disguarded them as they did not have the oil hole and because the had been made with 6.5 inch centres, some 1 mm longer than the correct BSA dimension.

I fitted Thunder rods which had one drilled and were the correct dimension.

Are they making the R and R rods to the correct specification now?

[Ted_Flash]

A quick Google search for "oil hole conrods" shows that (until recent improvemnets in manufacturing) this was a standard feature in Jags, Fords, MGs, Lotus, tractors, Norton, Studebaker, Alfa Romeo etc.  Mostly to do with piston, bore lubrication, with few mentions of shell bearings.