Small end bearing replacement....any hints or tips?

I have started the rebuild of my 4.2 engine and want to replace the small end bearings in the connecting rods.
I presume they press out of the rods, but are there any guides/tips/info on this task?
I cannot find anything in the archives or you tube.

It requires a proper-sized mandrel to remove/install, then they have to be honed to size, to the gudgeon pins.

I’d recommend you let a machine shop do it.


Typically on a rebuild you have the rods at an engine rebuild shop where they check for balance, straightness, and any out of round conditions in the big end in addition to the determining the condition of the small end bushing. It’s not generally an expensive process. This is important in particular if you are fitting new pistons. Jaguar has the pin a press fit into the piston. Every rebuilder I’ve used has changed that to a floating pin. Never asked why - know only that I’ve never had an issue with it, and it makes assembly a whole lot simpler.

Press fit into the piston?..never heard of such a thing. Some engines have the pin interfering with the rod, and “floating” in the piston.

If I’m not mistaken, ALL XK-engines had a fully floating pin from the factory, with a bronze bushing in the rod, and a clearance fit in the piston, retained with circlips. The rods in my ‘67 were this way, as were the SIII rods I later installed in it.

Ditto. And, yes, the fit into the piston is not a press fit: closely clearanced, yes, but not a press fit.

Mahle were so close on ours that I did THIS:

to slip the pins in.

IIRC, the 120 Jag manual uses the terms: “palm push” and “thumb push” to describe the piston pin cleaances…considered to be full-floating.

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There was a recent video posted showing exactly that where the guy picked up a piston and rod and just palm pushed the wrist pin into the piston and conrod.

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Sorry Ben bad choice of words. My experience with stock pistons is that the pin needs some force to get it into and out of the piston, but not enough to put it on an actual press. Jaguar recommended heating the assembly to do the job. My machine shop experience is that they hone the bore out enough that it fits together without any force or heat needed.

I’d just toss the gudgeon pins in a cooler, with dry ice, and they’d fall in.

I put my pistons in boiling water and the pins slipped in!

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its the small end bearing which I’m most concerned about. Looks like they will have to go off to the machine shop for fitting

  • It requires a proper-sized mandrel to remove/install, then they have to be honed to size, to the gudgeon pins.*

Yep, Just went throught this with a new set of Mahle pistions…Paul is correct…push fit, but needs slight honing to fit (without binding). JS

Just be aware that the famous west coast parts supplier sold me a set of bushes that were so undersized they had to be bored, then honed to fit…meaning extra cost. Mahle pistons, btw.

My understanding is that the small end bushing always need to be pressed in by a shop and then reamed individually to fit for each piston pin. That’s what I have done for each rebuild

Dennis 69 OTS

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That’s correct. If the piston supplier is worth a damn, then they should have to “match” each bushing to a corresponding pin, but it doesn’t hurt to check them all.

My experience is that the piston supplier doesn’t supply the bushing - the machine shop does (like magic) and with the pins in hand fits the pins after honing the bush. But as said, again my experience shows that they invariably hone the pistons so the pin easily fits without heating the piston.

My point was the pins should be VERY consistent in diameter, so they shouldn’t have to hone the small end bushings to match each pin, and then carefully keep them together as a matched pair.

Chances are, they’ll measure the pins, find them all within .0001-.0003 of each other, and then shoot for the same target on all 6 small end bushings.

All pins are fully floating in operation, unless antiques bolted in the rod eye. I prefer snug piston fit cold rather than easy. If it’s too easy cold it gets sloppy faster when hot.

The thing I’m anal about is circlip type and fitment. A wire circlip needs a round section groove and a bevelled pin to press it down securely under side thrust. A flat end pin needs stamped circlips in square grooves, installed sharp edge out.

Small Block Ford rods, some MGB rods, and many others have the pin thermally fitted into the small end of the rod.

That type of arrangement remains fixed in the small end of the rod during operation, as the thermal fit is the only retention mechanism for the pin, and it would crash into the cylinder walls otherwise.

To install, you stick the small end of the rod into a furnace, get it HOT, then stick the rod into the piston, and shove the pin home before the rod cools.