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MG MGB Technical - MGB Con Rods

I've got a set of early 18V rods (with raised balance areas) and would like to know which way round they go. There is an oil hole on one side designed, I presume to either spray the cam or cylinder wall. So, should this hole go on the cam side or not?

Many Thanks
Kemper

No - it should point away from the cam - it is designed to spray oil on the thrust side of the cylinder wall, not lubricate the cam.

Chris at Octarine Services

Cylinder wall it is then! Many Thanks!

Follow up question: These particular rods are assembled with pistons. Is there any harm installing the pistons "backward" (arrow pointing backward) if the rods were put on backwards. Obviously the correct thing would be to have them fitted properly, but for the sake of discussion.
Kemper

Do not install with arrows backwards!
The piston pin holes are bored slightly off center in the piston, to provide a constant side load against the thrust side, in order to keep the piston from thrashing about on the bore. Backwards will really confuse things, as the thrust will push the piston one way whilst the offset pushes it the other.

FRM
Fletcher R Millmore

"The piston pin holes are bored slightly off center in the piston"

Hmmm, not sure if I'm glad you wrote that or not! My V8 was assembled (quite possibly 100k ago) with the right bank con-rods the wrong way round. This isn't an oil hole thing, but the crank bosses are offset so the little end sits in the middle of the gudgeon pin, and mine are biased to one end. I *was* thinking that when I replace the bearings I'll simply turn them round, which will turn the pistons round. One authority says the V8 pistons can go either way, but you are making me wonder now, even though the engine has a completely different lineage. Mind you, I do have another engine complete with con-rods and pistons, I suppose I could measure those carefully somehow. As a matter of interest, what is the offset with the 4-cylinder pistons?
Paul Hunt 2010

Paul -
My statement is the general case, and not all engines use offset pin bores, but there will be some reason the pistons are marked. If it's not some obvious asymmetry like valve cutouts, skirt splits or such, then pin offset is usually it. If the V8 pistons have longitudinally offset bosses that would qualify, but it would be quite likely that they are also not bored on center. That might mean that R & L pistons would be different. Or markings could just be a way of ensuring that things go together correctly after the pistons are correctly fitted to the rods.

The OE mgb book says specifically that the press fit pin/rod IS offset, but only says of the floating pin earlier piston "It is essential that the piston assemblies should be replaced in their own bores and fitted the same way round: they should be marked to facilitate this." Kemper says he has an 18V, so the markings should rule.

Offsets would be something like .030-.060, but could well vary by piston design/manufacturer, since wall/skirt loading is the prime determinant. I have encountered something with .125 offset pins, but I don't recall what. I haven't measured many, since I adhere to the possibly quaint notion of "reading the instructions"! I take cast or clearly production stamped markings as law, hand stamped ones as suspect but possibly useful (or confusing!) info. I always look carefully to see if any markings appear to agree with all physical features - and sometimes they do not = quandary.

FRM
Fletcher R Millmore

Quandary indeed. For example when the rods are not marked "front" and come secondhand and are loose in a box with the new pistons.

My rods and caps were stamped 1-4, but only have the oil spray hole on one side. I was unable to find a reference to these holes when I assembled the pistons/rods. The arrows are forward in the bores, but that oiling question has bothered me since I built the engine seven years ago. Now I need to put the engine in service and I'd like the new cam, or as I now know, the freshly bore cylinders to stay oiled. I can't remember which way round I assembled them, so I'll have to remove the sump to check. If there's a wrong way to assemble them, and there is, luck dictates that they are backwards.
Kemper

Oil hole in rod should lubricate the thrust side of the piston/bore Arrows forward is correct for pistons, as mentioned above the gudgeon pin holes are offset to help quieten piston slap. Jim
jim soutar

Yep, book is most vague on the oil holes. Only good thing is a pic of the early slant split ones with a notation on the hole, not much good if you only have the straight split ones!
You must always understand any physical asymmetries when assembling.

FRM
Fletcher R Millmore

Press fit and circlip pistons for the B series both have offset gudgeon (wrist) pins. The offset is quite noticable if you look at the bottom of the piston.


Arrows can and have been omitted or wrongly stamped on aftermarket pistons - always check the offset against the marking.

Press fit pistons have a recess around one end of the pin bore - this recess faces forward in the engine. All pistons should be fitted so that the offset is towards the thrust side - oil hole in rod and offset AWAY from the camshaft.
Chris at Octarine Services

This thread was discussed between 31/05/2010 and 06/06/2010

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