Mystery knock on start up cold 948+

[niftyrodman]

All new rebore new pistons crank grind etc etc, and all was terrific, then after running a few days I noticed a slight knock, at start up when the engine was cold, but as soon as it warmed was quiet as a mouse. Sounded like a noisy big end. Pulled off the sump and strainer, and checked the big ends one by one. Shells all looked good. Checked the big end journals clearance with the plastic thread gauge, and all were from 2.5 thou to 3.5 thou, and one appeared to be 4 thou ish So I put a piece of A4 paper(003) under the cap and tightened it as an experiment and couldn't turn it over by hand. So I now know that my big end bearings all have less than 4 thou clearance so what can that knock be. Surely big end bearings oiled are not going to rattle at startup with less than 5 thou Clearance? Im at a loss.

[oliver90owner]

I don’t think I would be happy with 4 thou. clearance on a big end. Half that would be my top target, I think (not looked at the specs).

Were the plastigage checks carried out with dry shells and rods?

[niftyrodman]

Of course. Well probably not the top of the rod, but how much oil would remain withstanding. A quarter of a thou, or less, pressured? I mean the plastic measure is not absolutely gospel, is it. Getting back to the journal showing the biggest clearance, if 3 thou locked it up, it could not have had 5 thou prior to that or it would have freely turned. So if you packed the cap 3 thou it cannot have more clearance than that as it would be free. Has to be less, and thats my point. So Im assuming all my conrod bearings have less than 3 thou clearance, and that aint going to rattle. Or is it?

[philthehill]

It is perfectly acceptable to have a diametrical clearance of 0.0017" - 0.0032" at the big end bearings.

[niftyrodman]

When you are in the middle of restoration and the mechanical is done, with new everything including stainless inserts in wheel cylinders and Master, that fussy, there is one thing that saps the enthusiasm to keep going if something you have done is not quite right, and to neglect investigating, is a killer of inspiration, or would have been. Im convinced that I do not have a big end over clearance that would very soon damage the shaft, and if the tap is a piston or a gudgeon pin, or some other moving bit then that is not nearly as critical( Interesting to note the spell corrector on an engine type forum, doesnt like the word gudgeon) Onward ever onward,restorers need blinkers, or blinders.

[philthehill]

With a 948cc 'A' Series engine if you have gudgeon pin knock you are in trouble because the 948cc engine has a pinch bolt small end.
You may well have a slack gudgeon pin in the piston which when warm closes up.
Gudgeon pins are selected fit to the piston - did you move the gudgeon pins between pistons?

[niftyrodman]

Wish I has asked before I removed sump, as I did inspect the bores that I could see and the gudgeons, but no never did that test, as I didn't fit the pistons up. Macihne shop did that.The thing is, the noise goes out within a second or two, and I was wondering if I might have a tight pin to piston fit, and that might encourage a momentary slap of the piston, but I wish I had checked that out. None of the rods felt sloppy I can remember that.Anyway the sump is a piece of cake to remove if the noise increases. I dont want to be shaking hands with a rod thank you very much. Good call Phil.