Con Rod Oil Hole

[bikerbob]

How important is this hole. I have a 1956 swinging arm A7 and on a recent strip down to replace timing side bush  and other work I did not at the time think that about the hole in the Con Rod I mistakenly thought that it was only used on the A65 of which I have a 1962 model. Now I do remember looking at the con rods and the big end shells and am sure that there was no hole in the left hand rod and also the big end shells definately did not have a hole in the top ones to allow the oil go through tey looked quite new as that would have made me think again. This bike has been overhauled by a previous owner and I have found some things that were not done very well at the time. I have checked with some dealers on ebay and some of them are providing big end shells without the oil hole in them. So how importent is it, I would if advised strip the engine down again over the winter and correct this I have found an article that shows how to do this the hole is 3/64" diameter at a 30 degree angle this article was showing how to do this on an A65 which said that some A65 engines were experiencing oil starvation on the left hand cylinder so I assume the A7&A10 would be the same.

[coater87]

 New forged MAP rods do not have an oil hole at all.

 I think you could get by with 1,2 or even zero oil holes. And I am willing to bet the ones that are there dont care if they are pointed left, right, or down.

 I think that hole made some engineer feel better, but it kind of seems like a waste of sharp drill bits.
 Lee

[duTch]

 There's already a whole(hole ) bunch of debates about this....but for what it's worth if I recall I bought some big end shells waaaay back that were marked left/right (hole & no hole)

[Butch (cb)]

Yeah - we've done this one for sure.

And you should worry - on the S7 'Beam I bought the PO had swapped the caps over on the rods. Shells were toast inside 100 miles. But then the oil pump body was cracked so that probly didn't help.

[RDfella]

Some say the oil hole is to help lubricate the LH cylinder. I can't see that, as if that were the case the rod should squirt oil at the thrust (rear) side of the cylinder, and to do that the the oil hole should be on the rod's shoulder (as with any other conrod with a squirt hole). And why would the LH cylinder need more lubrication that the RH one? I thought the purpose of the hole was to enhance oil flow through the crankshaft, which would make sense from an engineering point of view.

[Greybeard]

I reckon that at some time an engine or two failed on the left cylinder so adding that hole was a knee-jerk reaction. The left side, being further away from the oil supply may have been in need of slightly more jollop squirting about, especially on a worn engine.

[RichardL]

Whether you believe the hole is there to lubricate the cylinder wall (which I do not) or to promote oil flow to the left side (which I do), if the hole is there it WILL promote oil flow regardless of who thinks that's needed.

Richard L.

[Swarfcut]

   I have pondered this over the years. If you consider the bearing shells need to be supported to the maximum degree on the thrust side  of the rod this would mean the locating tabs would be towards the front of the engine. Positioning the rod like this means the hole points towards the flywheel. Is the hole necessary? Well if there is a hole it is there because of some engineering consideration in the past. Is it effective? Who knows? In all honesty a 3/64" hole flashing past a 1/4" hole for a nanosecond ain't gonna pass much oil.  To lubricate the bores, overflow from the timing bush onto the crank cheek may do the timing side, but most oil will come as overflow from the cam trough, onto the flywheel and up the bores. Perhaps this is why we have the hole as a back up on cold start up, the timing side overflow lubricating the right hand cylinder.

   Of far more importance is the area where to oil exits the crank on the big end journals. When new the oil hole in the crank was at the bottom of an oval shaped cup, machined into the journal. While this reduces the total bearing surface by a small amount, it has the effect of allowing a wider film of oil to be deposited on the shell surface. As the cup is in effect conical, those who did Calculus will be familiar with the relationship of surface area to depth, typically demonstrated by an old style Martini or Babycham glass. There is a huge reduction of surface area for a small reduction in depth. So, as the crank is reground, each grind removes more of the available surface area for oil
 to be spread across the shell. Also when you get your crank back it is treated with care, we avoid touching those journals.  But in fact the grinding process has left the oil hole with a razor sharp edge, ready to gouge a neat groove  in the shell. so if you are having a crank done, reinstate the cup with a Dremel Grinder first, get the crank ground and then make sure the edge of the oilhole has a nice smooth  lead onto the journal surface. I consider this to be a better way of improving oil flow.

    The Glyco Bearing Website has a wealth of information about crank grinding, journal polishing and bearing selection. After reading it you will wonder whether your  grinder really knows what he  is doing.       Swarfy.

[Greybeard]

   I have pondered this over the years. If you consider the bearing shells need to be supported to the maximum degree on the thrust side  of the rod this would mean the locating tabs would be towards the front of the engine. Positioning the rod like this means the hole points towards the flywheel. Is the hole necessary? Well if there is a hole it is there because of some engineering consideration in the past. Is it effective? Who knows? In all honesty a 3/64" hole flashing past a 1/4" hole for a nanosecond ain't gonna pass much oil.  To lubricate the bores, overflow from the timing bush onto the crank cheek may do the timing side, but most oil will come as overflow from the cam trough, onto the flywheel and up the bores. Perhaps this is why we have the hole as a back up on cold start up, the timing side overflow lubricating the right hand cylinder.

   Of far more importance is the area where to oil exits the crank on the big end journals. When new the oil hole in the crank was at the bottom of an oval shaped cup, machined into the journal. While this reduces the total bearing surface by a small amount, it has the effect of allowing a wider film of oil to be deposited on the shell surface. As the cup is in effect conical, those who did Calculus will be familiar with the relationship of surface area to depth, typically demonstrated by an old style Martini or Babycham glass. There is a huge reduction of surface area for a small reduction in depth. So, as the crank is reground, each grind removes more of the available surface area for oil
 to be spread across the shell. Also when you get your crank back it is treated with care, we avoid touching those journals.  But in fact the grinding process has left the oil hole with a razor sharp edge, ready to gouge a neat groove  in the shell. so if you are having a crank done, reinstate the cup with a Dremel Grinder first, get the crank ground and then make sure the edge of the oilhole has a nice smooth  lead onto the journal surface. I consider this to be a better way of improving oil flow.

    The Glyco Bearing Website has a wealth of information about crank grinding, journal polishing and bearing selection. After reading it you will wonder whether your  grinder really knows what he  is doing.       Swarfy.

Wow! Lots to ponder there. I can see that you are going to be a leading engineering expert on this forum.

[Swarfcut]

 
   Thanks GB. If there is  debate as to whether the hole lubricates the bore, or increases the oil flow do you reckon anyone has the courage to drill a SECOND HOLE on the other side of the rod?  That way you get more oil flow (if the hole does that), more oil to the bore and a nice oil shower to the roller bearing.   Plus no oil pressure and a weaker rod.        Stand back I reckon its about to hit the fan!

   Swarfy.

[Steverat]

But we aren't going to drill that second one are we? So, for us weaker brethren - hole pointing outwards? (Drags) - hole pointing inwards? (Jeff Clew/Haynes)

[trevinoz]

The numbers stamped on the rod and cap go to the rear of the engine which makes the hole on the inward side.

[bikerbob]

Thanks for the replies I think I will leave it for now as I have done about 2000miles on it without any oil starvation problems and I do not know how many miles the previous owner did. The previous owner did have problem with a timing side bush turning in the steel housing because it was not pinned he gave the job of fixing it to mechanic who stripped the engine and renewed the bush and did some other work renewing the big ends and reprofiling the cam shaft he did not in my opinion do a very good job as he left it with about 14 thou end play and I have since found out  the bike would have been fitted with a 334 camshft but it now has a 356 camshaft no problem with that other than the valve clearances are different. The timing side bush that he fitted has only lasted about 2000miles so when I stripped the engine down I have replaced it with a solid bush and when I replaced the ball spring in the crankcase the hole was partially blocked with sludge which could have accounted for the short life of the buush. In regard to comments about the purpose of the oil hole in the conrod I also own 1963 A65 which does have that hole, but when BSA first introduced the A50 and the A65 the very early ones did not have the oil hole and after certain engine numbers  the hole was introduced the reason given possible oil starvation on the left hand cylinder and a service note was sent out to dealers telling them when any of the early engines needed engine strip down then they were instructed to carry out the modification to the left hand con rod. I find that strange as it could have been years before an engine needed a strip down.

[berger]

I  have got some thunder rods for the norbsa project, lets open the can of worms . the engine will have new timing side bush and srm oil pump and relief valve so do I drill the drive side rod or leave it as is. anyone who says its my choice will not be going to the Christmas party  . I need to hear no's and yesses but not necessarily in that order. opinions of why not and yes do will give me lots of PUB pondering time

[kiwipom]

hi guys, Berger i got billet rods from SRM who told me that the hole was not necessary and that the intention was to promote oil flow through the crank, cheers