Conrod oil hole

[RichardL]

Josh,

I think I will stay waaay back from saying that having a hole in each rod is bad and will lead to engine failure. Frankly, I don't really know and I tend to agree with you that it is unlikely to cause a problem. But with me, there is always a "however." However, I am somewhat sure that having holes in both rods will not increase the amount of oil getting to the left side and having a hole only on the left side will increase oil to the right side (without decreasing oil to the left side). All of these volumetrics are, no doubt, very small in differences, but it is a little fun to torture the fluid dynmaics of it all (and at least three people here, who I know to know more than I about fluid dynamics, are probabaly about to tell me the level to which I am full of it).

Richard L.

[Brian]

In Munro's book of BSA twins he states that using a rod with the oil hole on the right journal will cause oil starvation on the left journal. Being BSA's technical manager at the time I think his advice is sound so anyone doing so does so at their peril.

He says that the oil hole in the left rod was introduced early in 1951 but doesnt say exactly why, just that it was done to promote oil flow to the left journal and also to aid with the lubrication of the left cylinder. Wether there had been some failures in the early engines is anybody's guess.

[Josh Cox]

In Munro's book of BSA twins he states that using a rod with the oil hole on the right journal will cause oil starvation on the left journal

That is exactly right if you have only one conrod with a hole and it is on the RH crank pin ( as mine was before rebuild ). But I believe you are missing the big picture.

If there is a no hole in the LH conrod, there will be no flow through the sludge trap and into the LH big end ( ergo oil starvation ), the oil hole through the LH conrod is there to promote oil flow through the sludge trap onto the lh big end and crank pin out onto the cylinder wall. If there was no oil hole, the only oil flow would be through the .001 crank pin / big end gap.

As there is no channel machined into either the crank pin, big end or conrod, oil will only flow through the hole when the conrod hole lines up with the crank pin / big end hole, i.e. at the top of the stroke (TDC).

When oil does flow through the hole, it lines the bore and keeps the cylinder cooler and the oil ring wet.

Doing this to both conrods is a good thing, remember in one 360 degree rotation of the crank, oil will only flow through the hole for approximately 5 degree (TDC- when the holes line up).

[Brian]

He states that the rod with the hole in it must be used on the left side only.

It is of course your choice if you decide to use rods on both sides with oil holes but BSA's technical manager advises against it.

As with all modifications to old bikes be prepared to live with the consequences if it goes wrong.

[Josh Cox]

Well Brian , there it is in writing, when was this written ?.

First there was no oil hole,
Then there was oil hole, near side only,
Then in the A65, which by my calculations is virtually the same engine, both rods.

Other examples of evolution and the relationship between your bike and when it was made in the timeline, I believe in the early bikes for example, there was no oil to the top end ?.

[Brian]

The first edition of this book was 1955. I dont know if he was still the Tech manager at that time or not.

Its a book worth having if you can find one.

The very early A7 engines didnt have any oil feed to the rockers, then they put a oil feed to the exhaust only and finally to both. A friend has a totally original 48' model and it only had oil to the exhaust but my Star Twin (1950) has it to both so I guess somewhere around 49' they must have introduced the oil feed to both.

I cant remember what oil holes where in my A65 or A50 rods, it was too long ago. (memory's going!)

[1660bob]

Josh-I think you misunderstand the purpose of the sludge trap. In your post, you talk of using oil mineral detergent to clean it out-I assume you mean with the engine running? NO! don`t do it-will mince your big ends. Its called a sludge TRAP for that very reason-it traps any sludge present in the oil that is being pumped through the crank and thus PREVENTS it (the sludge, not the oil)from passing through the shell/journal gap.Thus we should only get clean sludge free oil at the big end bearing surface.If you loosen all that crud (accumulated over years/1000`s miles) with detergent it will all be forced out through the b/e bearings and trash them in an instant.The only way to clean out a (BSA)sludge trap is to strip the motor and clean it  by hand.....
A little on how the sludge trap works will be useful: In the A10, the trap is a thin walled metal tube about 1/2"bore dia.It has a step up in that diameter at the "feed" end to about 3/4". There are two small holes halfway along the tube, these holes are where clean oil emerges on its way to the b/ends, and another single larger hole diametrically opposite the above pair (for the locating bolt).When the trap is correctly mounted in the crank, the two small  holes from which the oil emerges are positioned so they face inwards i.e. towards the crank centreline/axis.The principle of operation is : the trap is mounted in the centre of the hollow b/end journals in the crank.It is concentric to the big ends, but of course is not on the axis of the crank itself.Oil enters the sludge trap tube directly from the pump,at the larger diameter end,(the larger dia end matches the bore in the crank itself and prevents any oil sneaking past the trap and by-passing it, thus this fit in the crank here should be close).As the oil enters the trap it is subjected to high centrifugal forces due to the crank spinning, and is forced outwards, gathering along the back of the sludge trap wall, at the opposite side to the two small holes.Any sludge/debris in the oil is thus "pinned"  against the back wall of the trap tube,away from the two "exit" holes.The trap tube fills with oil,which is forced by sheer volume out of the two exit holes(against the centrifugal forces) into the crank journal leaving behind any crud "trapped"in the sludge trap..More later, Bob

[terryk]

The A7 longstrokes didnt have a conrod hole and they used the same type timing bush. Oil flows though the sludge trap and out the conrod bearings no problem as all other types of motorcycle and car engines that use this type of conrod bearing. The oil splashes up the cylinders on both sides and over all the internal moving parts.
Sludge traps are used on Triumphs etc as well. They are there to collect sludge. Oil flows through the sludge trap if there is a hole in the left rod or not.
I definitely would not have a hole on the right side conrod. This would reduce oil pressure to the left bearing just the same as if the timing bush has too much clearance it would reduce oil pressure to the bigends.

I recon but I could be wrong, that BSA with their new A7 and A10 design in 50/51 wanted to be sure that there weren't oil pressure problems to the left side and therefore introduced the oil hole. I haven't heard of left side oil starvation on the longstrokes but I'm no expert and I havent raced them as some others may have. I have quite a few longstroke engines and cranks and the left side doesnt seem to be a problem.

I have also over the years pulled A10 engines apart that didnt have the oil hole in the left conrod or it was blocked. I cant comment if the hole makes a difference though. It usually was the same old reasons that you strip an engine it was blowin smoke but maybe previous owner had given it a quick re-ring and valve job and didnt do the bottom end. You would think if it had a total rebuild then the person doing the engine would see in the manual about the oil hole. If it had a total rebuild and not used a conrod with the hole then it doesnt make a difference whether the left rod has a hole or not. If its been running that way till the rings wear out or guides depending which one is the cause of the smoke then obviously there is enough oil getting to the left side because that would be after several years. 

Another issue Im thinking about is making a trough for the cam on the longstroke engines like the later design models. These longstroke cams always seemed to be worn badly. The latter design crankcases have a cam trough so I think some sort of one would be benificial to reduce cam and follower wear. I do have a few ideas to make a cam trough. Whats your thoughts?

[trevinoz]

Just my thoughts on this subject.
It seems that the hole was provided after the "new" engine configuration had been in service for a while and either problems had come to light or Bert Hopwood had a revelation and decided that oil flow to the LH journal had to be enhanced.
Surely the oil pressure at both journals will be the same with the hole acting as a relief valve for the chamber.
Providing a hole in the RH rod shouldn't affect the LH rod. Two holes in the chamber will tend to lower the pressure but the pump should supply enough volume to overcome this.
In the late A65, both rods were drilled but by then the pump had a wider gearset thus higher output volume.
I have seen many engines which were worn badly on the LH side or thrown the rod on that side, but this was caused by the sludge trap being blocked.
I feel that it doesn't matter whether one or both rods are drilled, it is essential that the trap is clear and the oil pump is in good nick.

Terry,
           Regarding the long stroke engine cam and follower wear, have you considered having the bits nitrided?
Triumph did just that to stop premature wear on the exhaust cam.
They even eliminated the oil feed to this area. It seems to have worked OK.   

   Trev.

[terryk]

Gday Trev, yes thats what I will do with the worn cams and followers that I have when I get them reconditioned. I do have one really good longstroke cam and a new set of followers that I want to use on the 48 so I thought a trough of some sort would give them a longer life.

[Josh Cox]

Thanks for your input Trev.

Sorry to keep banging on about it, I think it very important to remember that the oil hole/s only line up with the crank pin holes for a very small part of a 360 degree rotation of the crank ( i.e. 5 degrees or there abouts ), so I do not believe any one part could be adversely effected by the RH conrod having a hole.

If it was a serious concern for you, make the LH oil hole bigger than the RH.

Where's Olev, that guys usually got an opinion on most things......

[A10Boy]

Sorry to keep banging on about it, I think it very important to remember that the oil hole/s only line up with the crank pin holes for a very small part of a 360 degree rotation of the crank ( i.e. 5 degrees or there abouts ), so I do not believe any one part could be adversely effected by the RH conrod having a hole.

Sorry Josh, but I have to disagree.

The position of the hole is irrellevant as there is a film of oil under pressure between the crankpin and bearing surface and oil is being fed in there under pressure through 360 degrees rotation, so oil flow through the hole will be more or less constant. Also, there are two holes on the crank, [from memory], each perpendicular to the axis of the crank throw, therefore, the holes will only line up about 87 degress before and 87 degrees after TDC.

Simple laws of physics dictate that if you introduce a hole [or vent] into a pressurised area, you reduce the pressure within that area. It might slightly increase the flow, but it will reduce the pressure, as you are helping the pressure escape. If you make the LH bigger than the RH hole, you will reduce the pressure even further. As terry points out, other MC and car engines using plain bearings didnt have holes fitted and BSA engines dont need them either, apart from oiling the cylinder above. The BSA hole is so small to be insignificant.

Any fluid under pressure finds the easiest path to escape. Therefore oil being pumped into the crankshaft via the TS bush would escape by the easiest route before a more difficult route, this would lead to say 60% of the oil going out via the RH big end, and say 40% escaping via the LH big end. It is this imballance which caused starvation to the LH cylinder and piston. This is what BSA tried to correct by introducing the hole in the LH conrod.

I think its best to leave the set up as STD.

Sorry, I know most of that had been said before.