Check me here: con rods, non-directional?

On many old British cars the con rods need to be installed so the little squirt hole sprays a mist into the bore, and up to the wrist pin on the up-stroke.

As the XK con rod as no such squirty-hole, can I assume there is no left/right?

Oh… Big end down though, right???

Re-machined. They were .0015 ovaled. Plastigage now shows a solid .0015 on all rods/shells now. New pin bushings too. Sweet.


Yes, they are. The main caps are stamped with a three letter + a number code that identifies their position and their original engine, XYZ-1 through XYZ-7. (Same three letter code is found on the side of the timing cover). The number stamped on the side of the rods was stamped from the same side of the block as the main caps.

FWIW, I don’t know that it makes any functional difference on these engines. The pin oiling hole runs right up the middle. Everything is symmetrical. The pistons on most engines have valve reliefs that make them handed. You used to see some flat top pistons that had the pin bores offset to minimize skirt slap as the crossed TDC. Some stock class racing engine builder-wise guy figured out that if you installed them backwards it improved the rod-stroke ratio just enough that you could measure the difference on the dyno.

The Service Manual mentions it 3 times, but doesn’t really say why.
There may be a thermal expansion factor involved here, and they may also be assuming the piston and rod have not been separated.

The thrust applied to the slit side would likely propagate a stress crack at the end of the split. Been so long since I’ve used a split skirt piston I forgot they existed.

The slit OEM pistons are marked ‘front’, so I’m wondering if they suggest installing the rods via the stamped number in order to make sure the piston is correct (assuming you didn’t pull the pin). The rod itself… I can’t see any difference. My new pistons have no slit and aren’t marked ‘front’, so I assume they don’t care. But, I’ll install them so the writing is reading correctly (oriented reading from the front) just for symmetry’s sake.

1 Like

I’ll hold off installing the rods to the pistons til this thread wears thin.

The rods are not handed (in the Jag: not so with many others): split-skirt pistons are, with the split going on the non-thrust side (exhaust).

Out of convention, I installed rods with stamped numbers the way I found them.

1 Like

Yep, I do the same…

Funny how we go round and round and round with these topics …

I don’t consider this going round and round so much as stopping periodically to check the map. (You remember maps, those fold up paper things we had before we had GPS and smartphones.)

1 Like

My XK120 FHC has a drawer for them, British size though, the long US maps don’t fit in it.

Here is a Brico semi-split skirt piston out of an XK140 engine.
The round hole is intended to prevent crack propagation.

I checked all six and the cylinder number is on the same side as the split on all six.
Note the specific wording in the Service Manual. With the semi-split it is “MUST be fitted with the split opposite the thrust side”, i.e. number on the left, where with solid skirt it is “should be…” and “On initial assembly…are fitted…”
BTW note the one-piece oil ring.


Yep, I seen plenty of pistons like that. The oil rings are always frozen in the ring lands and packed so full of crud I never bothered to take them off of the pistons. Straight to the scrap bin. Never noticed they were one piece with a gap.

Is the finding that the multi-piece oil scrapers stay a bit cleaner? I’d guess because of all the moving parts.

Yes: back in the old days, it was very common to see stuck and gunked-up oil rings.

I cannot recall seeing the same with the multi-piece rings.

The early rods do have a oil squirt hole on the exhaust side to spray on the inside of the piston for cooling. The stamped numbers are the easiest way to tell as that side faces the exhaust side of the engine. Later rods do not have this hole and should not matter. Once reconditioned, order position is not important either.

1 Like

Thanks: I’d plumb forgotten that detail.

Are anyone else’s rods and caps stamped twice? Which one is ‘official’?

I’ve only ever seen factory stamps on the sides (bottom pic).

Or, at least noticed!

1 Like

I wonder if there was more than one supplier, one of them marking an “N”+…, others just the side mark.

Also, could rods and caps have been paired to come as close to the ideal weight and then marked, and marked again by the engine assembler?