new +0.20 pistons for my 66 Atlas

Bernhard · Dec 12, 2020

pierodn said:
Happened a strange thing that i cant explain.
Standard Hepolite pistons removed have the gudgeon pins of 18 tickness!.
So that i found the conrods eyes of 18.
The new GPM or Hepolite + 0,20 i will (like all the other pistons) have 17,2 pins.
I have never know of 18 pins pistons.
Now i must change conrods or can i make bushes to 17,2?.
Thanks
Piero

Look at the brushes in the small end of the con rod, someone else might have fitted a smaller size phosphor bronze brush. Can't tell without looking at it and measuring it up.

pierodn · Dec 13, 2020

Bernhard said:
Look at the brushes in the small end of the con rod, someone else might have fitted a smaller size phosphor bronze brush. Can't tell without looking at it and measuring it up.

Sorry but i am takking about the eyes of conrods not the bottom shells

Bernhard · Dec 13, 2020

pierodn said:
Sorry but i am takking about the eyes of conrods not the bottom shells

So am I.

dynodave · Dec 13, 2020

Not quite following the story?
piston pin at 17.2MM are for 500 or 600 engines with small rods with small end bushed to 17.2

not atlas which are big rods with 17.5 pin no bushing

18mm pin norton single ?

texasSlick · Dec 13, 2020

Per Norton Manual:
Gudgeon Pin dia. = 0.6868 / 0.6866 inches
Small end Con Rod Bore = 0.6878 / 0.6873 inches

0.687 inch = 17.45 mm.

Slick

baz · Dec 13, 2020

pierodn said:
Happened a strange thing that i cant explain.
Standard Hepolite pistons removed have the gudgeon pins of 18 tickness!.
So that i found the conrods eyes of 18.
The new GPM or Hepolite + 0,20 i will (like all the other pistons) have 17,2 pins.
I have never know of 18 pins pistons.
Now i must change conrods or can i make bushes to 17,2?.
Thanks
Piero

I would carefully measure the gudeon pins from your new pistons and see if they are actually 17.2 , they should be 17.45
Also if you're con rods are bored to 18mm I think you would be wise to change them for standard ones
Cheers

pierodn · Dec 14, 2020

Hi.
Yes, 17,45, sorry.
Pistons found in place are standard concave for Atlas but with 18 pins.
Conrods are the big of Atlas.
Seems that someone has enlarged conrods eyes to receive the 18 pins.
Question is if one time sgo they supplied Hepilite pistons with 18 pins?.
Thanks
Piero

dynodave · Dec 14, 2020

Some years ago some venders were offering non standard pistons and required modifying the con rod small end. Big wrist pins on hepolites with the norton or Hepolite part # pistons would likely be a local modification for NO good reason? IMO
I refused to buy or use them and ruin a set of rods.

texasSlick · Dec 14, 2020

Either change con rods or ream new pistons to 18 mm to use old gudgeon pins.
If the cases are split, the decision is easy.

I would think the pistons would tolerate 0.007" off the radius, but I would not like that approach.

Slick

pierodn · Dec 14, 2020

Hi.
Are you contrary to fit bushes to the conrods eyes to go again to 17,45?.
Thanks.
Piero

texasSlick · Dec 14, 2020

pierodn said:
Hi.
Are you contrary to fit bushes to the conrods eyes to go again to 17,45?.
Thanks.
Piero

Is it possible to fit a bushing with 0.007" wall thickness? I would let the machinists have the last word.

Slick

baz · Dec 16, 2020

texasSlick said:
Is it possible to fit a bushing with 0.007" wall thickness? I would let the machinists have the last word.

Slick

I'm not a machinist but that bush would be extremely thin! I reckon the bush would disintegrate ?
I have gone up .4mm on a gudgeon pin before when fittings a Yamaha piston to a BSA
I reamed the piston until the pin fitted and luckily there was still enough depth in the circlip groove to locate the clip properly

Bernhard · Dec 16, 2020

Any reason why you cannot open the con rod eye say .025 thousands inch and make a suitable brush to fit?

new +0.20 pistons for my 66 Atlas

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