Small End / Gudgeon Pin Lubrication - Science, Luck or Magic

[oldtuckunder]

As work progresses towards the rebuild I am considering what caused No5 Big End to fail, what was cause and effect, so my mind is pondering all lubrication issues, which is how I have come to the conclusion that I have no idea how small end bushes actually get sufficient lubrication, and why different rods have different lubrication holes.

 

I get the concept, i.e the oil is pumped through the crank emerges out through the Big End and Main Bearing Journals onto the crank and then get thrown up inside the bore and piston, 1 to cool the piston crown and 2 to provide lubrication to the bore 3 to provide lubrication to the Gudgeon Pin and the Small End Bush.

 

There is of course some oil being thrown around inside the block from the cam lubrication, but I'm assuming that the bulk of the piston/bore lubrication relies on oil pumped out the crank and not any picked up by the crank lobes rotating in the sump oil? If the latter is an important factor then I'm worried as when cornering, braking, accelerating the oil level is all over the shop and especially as there is almost no sump pan depth under 1,2 & 3 they could get way less lubrication at times.

 

But to some extent how the oil actually get thrown inside the crown of the piston isn't the issue that has me pondering.

 

So we have oil being thrown up into the crown of the piston, some will hit the exposed surfaces of the gudgeon pin and get carried into the small end bush and into the piston journal, some will hit the top of the piston and then fall onto the top of the connecting rod in which we have a magic little hole that is drilled through the rod and bush to lubricate the gudgeon pin and bush.

 

OK this is where I get to the Science, Luck or Magic question. The Hole(s) in the rod are all over the shop on different rods. For example the early 2L rods I was using have a hole in the Top around which is cast a 0.5" 12mm bowl that protrudes from the surface of the rod and is obviously designed to act as a collector to feed more oil into the hole than would fall directly down it. A later set of rods I have from a MK3 GT6 still have the hole in the top but only a small chamfered cup. I can find pictures of Rods that have been lightened, polished etc (by reputable shops) that only have the hole left. If I look at pictures of very expensive Carrilo or Arrow rods I can see that they have two holes one on each side in the lower hemisphere (all be it that they both choose different places to locate the holes). In fact the more pictures of connecting rods you look at the more it appears that it was pure whim on the part of the manufacturer where the hole or holes were put.

 

So its either a case that there is so much oil floating about up there it just doesn't matter where the hole(s) are or some rods are far more prone to wear than others.

 

As far as I can see the only scientific (or could just be voodo magic) improvements to this see to be from Carrilo who whilst not offering it on their $1300 rod set! will for extra money machine 3 little notches on the side of the Big End journals to promote more oil being thrown up to the piston crown, or for even more money will drill a hole up through the connecting rod so that the small end bush get a direct oil feed from the crank.

 

So Guys, small end lubrication, Science, Luck or Magic?

[Motorsport Mickey]

You know I think I'll have to reorganise my drawers under my workbench, the first one constantly has items which stick up and get trapped when I close it. How does the rest of the TR owners prevent this happening ?

 

Mick Richards

[BlueTR3A-5EKT]

You know I think I'll have to reorganise my drawers under my workbench, the first one constantly has items which stick up and get trapped when I close it. How does the rest of the TR owners prevent this happening ?

 

Mick Richards

Resist the temptation to put hammers in at the top of a drawer, their rightful place is in the bottom, where I also have a TR3 con rod (you know the one with the oil supply hole from BE to SE) that I use with a length of threaded rod to hold pistons on the lathe when skimming crowns for deck heights. The hole is just the right size for tapping to accept a length of studding from B&Q

Peter W

Edited March 15, 2015 by BlueTR3A-5EKT

[Peter Cobbold]

The Carillo mods might be intended to throw oil under the piston crown to cool it, rather than lubricate the gudgeon pin.

- easier than making dedicated oil squirters.

[Motorsport Mickey]

Lol :lol:

 

Mick Richards

[Peter Cobbold]

http://koti.mbnet.fi/kolonen/sierra/kausi_2011/oilsprutarna/asennettuna.jpg

[oldtuckunder]

You know I think I'll have to reorganise my drawers under my workbench, the first one constantly has items which stick up and get trapped when I close it. How does the rest of the TR owners prevent this happening ?

 

Mick Richards

Hi Mick

Am I missing some very subtle inference to the magic lubrication of small end bushes in your comment, or did you go off at a tangent?

[john.r.davies]

I'm most suspicious that a drilling up the con rod would have any effect at all.

By the time oil reaches the big end, it has already travelled through the main bearing, and its journey to the big end is assisted by centrifugal force.

Is there any actual pressure left?

There is no centrifugal to help it up the con rod passage, as the con rod moves reciprocally.

 

And that little hole in the top of the small end is very close to the underside of the piston.

Won't it be shrouded by that?

 

So, is there some other mechanism that lubricates the small end? Is the reciprocation the clue?

At the small end, rotation is slight and reciprocal. Pressure on the bearing alternates from above and below. Could this somehow pump oil through the bearing, either from the crank oil mist or ujp from the big end?

 

John

[Peter Cobbold]

Some piston bosses are drilled to make an oil drain hole under the oil scraper ring, so collect oil from there.

Edited March 15, 2015 by Peter Cobbold

[oldtuckunder]

Some piston bosses are drilled to make an oil drain hole under the oil scraper ring, so collect oil from there.

My AE's have two such holes in the skirt along with a split between them within the groove of the oil control ring, but that still doesn't explain what the hole(s) through the rod end do, and why it appears they can be in any position around it but still "work"

 

 

 

And that little hole in the top of the small end is very close to the underside of the piston.

Won't it be shrouded by that?

 

 

From the cast cup shape around the hole on my old rods it looks like oil dripping from the underside of the piston is supposed to collect in the bowl and feed through the hole to the bush, but given that that rod could be going up and down 100 times a second its hard to imagine that it really works, possibly explaining why most other rods don't have such a cup arrangement around the hole. I'd love to know if there are any rods used that don't have any holes at all.

[john.r.davies]

The standard advice for lightening Triumph con rods includes grinding off that cup.

I follow it when race engine building, and while I've run main and big end bearings, I've never had a small end fail.

 

John

[RogerH]

I have an idea that it works like standard moisture on any part of the car.

As you know every part of the steel body work is prone to rust - even 'sealed' areas. Because moisture gets in because the atmosphere is full of the stuff (and other things)

 

 

Inside the crankcase with the crank cranking and the pistons pissing pistoning the trapped air in the casing must be chaotic.

Every part of the case would be covered in oil no matter where.

So any opening in the con rod will eventually become full of oil - but does it go in there or!!!

The gap between CR small end and Gudgeon pin attachment moulding will be dripping with oil.

Capillary action will draw oil into the small end from both sides of the small end. The hole could be there to free up any pressure to allow the capillary action to work.

 

The actual movement of the small end about the gudgeon pin is very small so lubrication is not done on industrial levels.

 

In fact it could be looked as though the con rod is sat in a bucket of oil.

 

Roger

[oldtuckunder]

The standard advice for lightening Triumph con rods includes grinding off that cup.

I follow it when race engine building, and while I've run main and big end bearings, I've never had a small end fail.

 

Likewise, which is strange given that the compressive forces on that little bronze bush per sqmm are far greater than on the big ends, all be it that the small end only occilates a bit whilst the big end rotates, which implies that its the rotating forces that do all the damage on big ends and very little to do with pressure loading from combustion.

 

So what it looks like is that regardless of how many or where the lubrication hole(s) are in the rod end the bush and gudgeon pin always seem to get enough lubrication, which doesn't answer the question was it just luck that the arrangement works and nobody worked out why on the basis that if it ain't broke don't bother finding out why, or is there some science involved in how lubrication occurs under the piston crown. But I think its safe to say that magic isn't involved.

 

I'll just stop thinking about potential top end problems and just assume the new rods will work like all others seem to!

[RogerH]

The big end has quite a gap between the rod BE and the crank journal. This is to allow a layer of oil to sit (under pump pressure) and separate the metal bits.

 

The little end bush does not have the same problems as the BE as it does not move very much at all.

If there was no gap between the small end and the gudgeon pin then there would be no need of any lubrication.

There is in fact a very very small gap so it requires just enough oil to to stop the metals picking up on each other.

 

The tighter the gap the better the capillary action. As stated above.

 

Roger