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MG MGB Technical - Piston / rod fitting

Can someone please advise when fitting new pistons to new rods on a fully floating arrangement how tight the gudgeon pin should be on the con rod. The pin in the piston is a good tight sliding fit but is way tighter than a sliding fit to the little end of the rod. Is this normal?
In this arrangement what rotates most the pin in the piston or the pin in the rod or doesn't it matter?
Many thanks
Mark
Mark Dollimore

Mark,
which rod are we discussing?
On the floating pin, the pin is loose in the rod and is "held in" the piston (pre 18V I think) and later the pin is held in the rod ie tight... and the piston floats (this is what I understand as the "fully floating" piston).

regards

MGmike
M McAndrew

Two types of pin/rod fitting.

Early rods have a bush fitted and the pin is a hand push fit when the piston is warmed up and is retained with two circlips in the piston.

Late rods have no bush and the pin is a press fit into the rod and it is retained by the interference fit. Approx 3 - 5 tons is required to press the pin into the rod.
Chris at Octarine Services

My rods have the bush and circlips on the pistons. So pin is hand push fit into pistons but I haven't forced the pin into the bush end of the rodjust in case it gets stuck. Sounds like the bushes in the rods need relieving a minute amount?

Mark Dollimore

They should simply go together with very little effort. That's the whole purpose of the design, to allow a minimum amount of friction on all of the moving parts. Every set, that I've assembled, has gone together with a little oil and no resistance. RAY
rjm RAY

If you have new rods with, presumably, new bushes, the bushes should have been reamed to size.
Dave O'Neill2

Dave, do you know Chris with an early white roadster who lives in Solihull, possibly Park Avenue? If so I'd appreciate it if you contacted me offline.
Paul Hunt

Paul

Sorry, I don't know him.
Dave O'Neill2

"My rods have the bush and circlips on the pistons."

Yes, but that means nothing. Do the rods have bushes or not? That determines how they have to be assembled. BTW, if you do have press fit rods, don't bother sticking the circlips in as they will be superfluous.
Bill Spohn

Hi Bill
1 Rods - these have bushes
2 Pistons - these have circlips to secure the pin which is a hand push fit into the piston.
3 Should pin be hand push fit into rod?

I think it should but question I would like answered is how tight? I think my bushes need reaming out a very very small amount to achieve that tight sliding hand push fit, I just needed someone to say yep that's right or otherwise advise, Chris and RAY have suggested it should be a sliding fit.

Regards

Mark

Mark Dollimore

If the rods are new or have new bushes fitted then they should have been reamed to fit the pins - if it is not an EASY sliding fit when lubricated with oil then get the bushes reamed out - take the rods and the pins to a machine shop and get them to do it.
Chris at Octarine Services

I always make sure the pin has been lubed & I slightly heat the piston with a shrink wrap heat gun to expand the piston. Note: circlips must be installed with flat side facing toward the cylinder wall; and curved side toward pin.
Rich Boris

Sounds like the bushes were fitted but not reamed to final size, and/or were slightly belled when installing them.

Take them to a machine shop with the pins and they can quickly hone the bushes to correct clearance. Good you asked instead of just pounding them in as some would do ;-)
Bill Spohn

Many thanks all will get the machine shop to sort. Regards

M
Mark Dollimore

This thread was discussed between 30/07/2013 and 08/08/2013

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