NOS concave GPM +0,20 pistons

Joined
Nov 10, 2012
Messages
3,092
Country flag
Sorry, i am here again with my 66 Atlas restoratiin.
Bought A NOS GPM concave head pistons of 70 th complete of pins and rings but without circlips.
So that i have bought the wires circlips for GPM pistons from Andover.
I have tried to fit these wires circlips but i have seen that the piston grooves are not senicircular but rectangulars so that the circular GPM circlips dont copyied well even if they was tight.

Removed the circular rings, i have fitted the rectangular section circlips, in stsinless steel HO-68 SS 11/16 internal, those suggested by Jim Comstock.

Please, have you never cecognized this situation before?

Thank you.
Best wishes for the 2021.

Piero

P.s.: the circular SS circlips stay well in pistons place (Also the other tried circular circlips sold from Andiver for new Hepolite) but they are tight but turn easy if i moove them with the tool.
 
Last edited:
Joined
Apr 20, 2011
Messages
5,323
Country flag
Don't use round clips in those pistons.
They are not machined for them.
Go to your car factor or engineering shop, they should have a big box of those square ones.
 
Joined
Nov 10, 2012
Messages
3,092
Country flag
Don't use round clips in those pistons.
They are not machined for them.
Go to your car factor or engineering shop, they should have a big box of those square ones.
Hi.
Please, what you mean for “round clips”, the wires or the seager!
Thanks.
Piero
 
Joined
May 28, 2003
Messages
2,509
Country flag
the piston grooves are not senicircular but rectangulars
NOS pistons ? Ok good for you......GPM means nothing NOS means OLD style

rectangle/square groove need square seeger DIN 472 style LIKE JimC said
your circlip must fit the square groove type....
 
Joined
Nov 10, 2012
Messages
3,092
Country flag
NOS pistons ? Ok good for you......GPM means nothing NOS means OLD style

rectangle/square groove need square seeger DIN 472 style LIKE JimC said
your circlip must fit the square groove type....
Hi Dave.
Old style GPM, OK!
But they are an old manufacture.
I didnt know that old GPM had square groove.
The actual GPM that i buy from Andover for my commando have circular groove and wire circlips.
The square seeger in SS (the JimC suggested) fit well but i am bothed because they turn easily in the grooves.
Thanks.
Piero
 
Joined
May 7, 2005
Messages
4,876
Country flag
Try the nearest metric size up the way as GPM is Italian they may have used a metric seegar clip.

RELAXED OUTSIDE DIAMETER: .760" (19.32MM)

.035" (.89MM) THICK WITH A SECTION OF .038" (.95MM) AT ITS NARROWEST POINT (JUST ABOVE THE EYES) INCREASING TO A MAXIMUM OF 1/16" (1.6MM).

So a 20.5 mm relaxed OD for a nominal 19mm and maybe 1.00mm thick but measuring the groove thickness in the piston will tell you the thickness required

 
Last edited:
Joined
Jan 19, 2012
Messages
1,117
Country flag
dangerous?
Yes, of course (to the engine). Forget the AN circlips as they are made for a different brand and design of piston. Forget the Seeger circlip recommended by Jim as well (which piston was Jim referring to when he advised using them? Probably not a GPM piston). Give GPM Gondini a call when the business opens and ask for guidance. The correct type of rectangular section circlip should hardly turn in the groove at all.

Happy new year to all!

-Knut
 
Joined
Jun 1, 2003
Messages
2,070
Country flag
You also need to look at the end of the wrist pin. Is square or does it have a chamfer ? You need a squate end for seager clips and the chamfar for wire clips
 
Joined
Jan 19, 2012
Messages
1,117
Country flag
You also need to look at the end of the wrist pin. Is square or does it have a chamfer ? You need a squate end for seager clips and the chamfar for wire clips

Bill, I guess you looked at the Mahle motorsport video in this thread:


A chamfered wrist pin is optional for wire clips with no tangs, not a requirement. Fitting this kind of wire clips need modifications done to the piston and possibly to the wrist pin, see referenced thread. It's probably the best way to secure the wrist pin. I would not fit a wire clip unless it secures at least 330 degrees of the groove circumference.

However, a wire clip is not an option for Piero who wants to use old stock GPM pistons.

-Knut
 
Joined
Nov 10, 2012
Messages
3,092
Country flag
Hepolite rectangular section circlips 19,1 mm external, 17,46 internal, 0,8882 tick, are small.
GPM Gandini sells rectangular metric circlips 19 mm internal, 20,5 mm external, 1mm tick.
For a first check i have bought 20mm external, 18 mm internal, 1 mm tickness.
Ciao
Piero
 
Top