Dunstall GPM pistons?

B

Bruce MacGregor

Joined

Sep 15, 2005

Messages

175

• Aug 23, 2017

• #1

Hi , All,
Digging thru my pile of Norton parts I found a pair of pistons marked GPM Dunstall that I believe came out of my new stock 1971 Norton 750 because I purchased a Dunstall 810 kit, cam & big valve head and installed them all before I had 500 miles on the bike.

My question is. Are these the bad pistons that come apart? If not then,
Does any one in the U.S want them? Pay for shipping!

Thank 's
Bruce MacGregor
Wakefield RI

 

L.A.B.

Moderator

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Nov 20, 2004

Messages

21,090

• Aug 23, 2017

• #2

Bruce MacGregor said:

Are these the bad pistons that come apart?

Click to expand...

The original 1971 Norton factory pistons would have been (AE) Hepolite with the slots behind the oil control rings, front and rear, as below:

http://www.norvilmotorcycle.co.uk/techtalk34.htm

If yours are marked "GPM Dunstall" then I doubt they can be original factory pistons.

 

gripper

Joined

Dec 3, 2010

Messages

644

• Aug 24, 2017

• #3

The engineering company that did my rebore in Brierly Hill (UK) showed me a pair of domed Commando pistons they had lying around. They may have been dunstall but they were very heavy and with the current trend for light pistons and long rods I would not bother with them.

 

robs ss

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4,232

• Aug 24, 2017

• #4

Found this on a French British Bike Forum - http://www.motos-anglaises.com/
It is meant to be a 10:1 Dunstall piston c/w standard. Anyone here know for sure?
Does it look like your's Bruce?

 

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dynodave

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May 28, 2003

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2,591

• Aug 24, 2017

• #5

robs ss said:

Found this on a French British Bike Forum - http://www.motos-anglaises.com/
It is meant to be a 10:1 Dunstall piston c/w standard. Anyone here know for sure?
Does it look like your's Bruce?

Click to expand...

I have some NOS GPM pistons marked as GPM 738 under the crown. exactically the same as my (68 atlas) Dunstall GPM except marked as Dunstall on the bottom of the crown. Both have a shallow dome of about .100".... therefore, nothing like the pistons in your pix.
These both may have been the 11:1 pistons
I would expect 810 pistons to be different.

 

B

Bruce MacGregor

Joined

Sep 15, 2005

Messages

175

• Aug 24, 2017

• #6

Hi Again,
Pistons have 734 GPM Dunstall on them and mike out @ 72.90 +- mm skirt & crown. 3 piece oil ring
So are 750 pistons, no slots.

Have holes behind oil ring & below maybe 1/2 inch apart except in way of wrist pin boss holes = 3/32 " dia

Bruce MacGregor

 

B

Bruce MacGregor

Joined

Sep 15, 2005

Messages

175

• Aug 24, 2017

• #7

Sorry Folks,
Dyno Dave spoke up for them first, Then John Tickle.
Bruce

 

A

ando

Joined

May 25, 2011

Messages

157

• Aug 28, 2017

• #8

The 810 Dunstall piston shown above (raised crown piston on the right) by Robs SS is as depicted in the Dunstall catalogue. The 810 piston is a very light piston.
The 750 piston shown in the Dunstall catalogue, part no 3023, is a flat top piston 10 to 1 compression ratio. There are no thermal slots.
ando