Oil Pump Plunger too long

[Dave T_LAPSED]

So whilst I wait for my cylinder to be rebored (Yes sir, end of this week or beginning next, no problem blah blah, etc.) I looked at the oil pump 'cos the pin only engaged about a sixteenth of an inch as the person wot last looked at it fitted a great thick copper washer to the holder and it also had a flat worn on it!
So, new pin fitted, new gaskets on end plates and turning the engine by the con rod it has a tight spot! Remove and refit pin with thin cu washer, same thing. Remove pump end plates, no tight spot?? The plunger was proud of the rear plate surface by a fraction, hardly visible but could be felt when the plunger returned. The thin paper gasket had a little hole squashed into it in the middle.
I thought of grinding a bit off the plunger but that felt a bit extreme, even for me, so I made a cardboard gasket with the middle removed and no more tight spot.
Has anyone else found this problem, if the cylinder was on the motor I'm sure I wouldn't have felt the resistance turning the motor with the kickstart and would have damaged the pump/pin/chaincase by running it like that.
Right, now where's my cylinder you lot at Demolin & Co??

[g80csp11]

if the guide pin is worn the pump will travel too far . fix the guide pin and every thing will be back to normal

[SPRIDDLER]

if the guide pin is worn the pump will travel too far . fix the guide pin and every thing will be back to normal

So, new pin fitted, new gaskets on end plates .......

It's a new pin, Peter, but if it's fitted the wrong way round i.e. the 'thin end' engaging the plunger, it might have the same effect as a worn pin.

[Dave T_LAPSED]

I think with the worn pin it didn't push the plunger to it's limit, the new pin showed up the fact that the plunger is too long by pushing to it's limit, if I had left it something would have worn/bent eventually but anyway it's fixed now. I just wanted folks to know this can happen and check for it.

[SPRIDDLER]

Yes I see that, Dave, but if the pin is too small in diameter (either with a wear flat or inserted the wrong way round in its holder) it will not be a close fit in the plunger's annular groove so not only will the lateral movement of the plunger be reduced but the slack between the pin and plunger groove mean that the plunger will be free to slightly exceed (overrun) its designed backward throw when being rotated by the driving worm on the timing side axle. 'Backward' because as well as rotating the plunger the direction of rotation of the timing axle will tend to draw the plunger towards the rear end plate. You did say that the protrusion of the plunger was barely visible......
You didn't answer as to whether you have inserted the pin the right way round or not. The larger diameter end (1/4" dia. for your '54 engine) should enter the plunger groove. The depth of penetration of the new pin into the plunger groove will not affect the 'throw' of the plunger nor stiffen up the movement so no point in grinding a bit off the end unless the pin is so long that it's acting like a grub screw by bottoming out at the base of the plunger groove.
These very basic pumps are pretty fool-proof so if all is otherwise O.K. then there's a manufacturing fault somewhere. The earlier pins had a smaller (3/16") dia. end than the later engines. Could you have been sent that smaller dia. pin?
Are you having the rebore done on account of wear that could have been due to a previously existing oil circulation problem?

[Mick D]

Hi Dave

I'm with you on this one - a worn pin or annular groove will only reduce the motion of the pump in a working condition, it sounds like the pump has been manufactured on the edge of tolerance, (assuming no work has been done to the crank case pump plate interface).

Regards Mick

[Dave T_LAPSED]

The pin is correctly oriented. As I said the plunger protrudes barely visibly but can be felt to protrude. Cranking the con rod by hand the turning became hard and stopped unless I forced it at the point when the plunger was at it's max rearwards motion. The obstruction caused would have worn or bent something if I had kickstarted the engine just until it stopped touching the back plate.
As I say, has anyone else had this problem, it would be easy to overlook, I found it by serendipity because I was turning the engine manually (once a year). I don't like things that are mechanically incorrect.
The rebore is because the GPM piston was fitted with 2thou clearance and had inevitably seized front and back, looked like new otherwise. It also weighs 440g whereas the Robbins I have as replacement weighs 380g. Again, I deem the GPM mechanically indesirable and don't want it in my engine.
Am I becoming a finnicky old bugger or what? Probably!
Just reread you comment Spriddler, no the grinding would have been of the plunger not the pin, the old one was not far enough in the groove anyway because of the cu washer which I have omitted now to let the pin enter to it's proper depth.

[SPRIDDLER]

Thanks for the feedback Dave. It would seem that it was either the flat worn on the pin or that the pin was only registering a tad in the plunger groove that prevented the plunger from contacting the backplate. More than likely a bit of both.
The 60gm. difference in the two piston weights seems quite a lot (2 ounces in feet 'n inches). Out of interest, was the Robbins piston supplied with recommended a clearance for the reborer?

[Dave T_LAPSED]

60 year old piston, all I got was a box but the piston is in really good, uncorroded nick!
I'm leaving 4.5 thou clearance and as always using 50sae Rockoil lube, nice and sticky! Monograde always for roller or ball big ends.
A friend just rang to ask me what paint code or name reference was quoted for miltary AMC 350s, his friend's dad has one he's restoring and he needs to know the proper paint. Can you point (paint) me in the right direction to find out?
Cheers, Dave

[Duncan]

needs to know the proper paint

Oooh, that's a bit like asking "what oil?", according to http://www.matchlesswd.co.uk/parts-database/ its Khaki-green No3, but the general answer is any green paint that came to hand.