new +0.20 pistons for my 66 Atlas

pierodn

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Nov 10, 2012
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Hi All

I need to rebore my 66 Atlas barrells to +0,20.

I can fit new GPM concave or flat head.

What do you suggest?
 
Doubtless others will promote the benefits of balancing/lightening and other wallet drainers, but I would (and did) stick with the low compression pistons. For a bike with such a vibratory reputation I can't see why anyone would want to compound that.....
Though plenty do (plus for a long time Atlas pistons were like hen's teeth)
One opinion only :)
 
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What are these concave pistons?
Looked at GPM website there is NO picture of this concave piston crown, all it shows on another site is flat top pistons as per Commando.
 
If you fit either GPM or JP you would be wise to have the crank rebalanced, as both these makes & JP in particular are a lot heavier than standard pistons. Steve Maney recommends a balance factor of 75% for the Atlas.
I purchased a pair of JP +0.020" pistons for my 650 AJS & they weighed 90 grams heavier than the original BHBs. Need less to say they were put back in the box & I hunted around for a pair of BHBs. The only safe way to remove weight from after market pistons is to taper bore or grind the ends of the gudgeon pins, & this will only lose about 10 to 15 grams.

Martyn.
 
yes the JP ones are quite heavy though my experience with GPM's are that they are usually lighter than the stock pistons I have used GPM's in my ES2, 650SS and concave ones in my Atlas, so far the vibration is no worse than the 650SS and the Atlas is pleasant to ride though I am running a 20 tooth gearbox sprocket, be aware if using GPM pistons you need to check the skirt to flywheel clearance as it is common for them to touch.
 
possm said:
yes the JP ones are quite heavy though my experience with GPM's are that they are usually lighter than the stock pistons I have used GPM's in my ES2, 650SS and concave ones in my Atlas, so far the vibration is no worse than the 650SS and the Atlas is pleasant to ride though I am running a 20 tooth gearbox sprocket, be aware if using GPM pistons you need to check the skirt to flywheel clearance as it is common for them to touch.
Click to expand...
That's good to know. I do recall there were problems with GPM pistons quite a few years ago, but hopefully they are okay now.
 
pierodn said:
Hi All

I need to rebore my 66 Atlas barrells to +0,20.

I can fit new GPM concave or flat head.

What do you suggest?
Click to expand...
No concave pistons available in +.020 presently. You need to look for a set of used BHB pistons, or fit flat topped pistons (8.7:1). The latter is not ideal as the bike will vibrate more (perhaps a lot more). This may be countered by careful dynamic rebalancing of the crankshaft.

www.gandini-industria.it

Pistoni 2017 / Gandini Industria

Leggerezza, resistenza, durata e prestazioni.Le tecnologie sempre più sofisticate hanno esasperato la ricerca e l'applica
www.gandini-industria.it www.gandini-industria.it

-Knut
 
pierodn said:
Hi All

I need to rebore my 66 Atlas barrells to +0,20.

I can fit new GPM concave or flat head.

What do you suggest?
Click to expand...
On Ebay UK a pair of +0.040" BHB low compression pistons.
Item number
124182626757
 
Matchless said:
On Ebay UK a pair of +0.040" BHB low compression pistons.
Item number
124182626757
Click to expand...
Herein lies the problem, once you have paid extra for 2 rebores, you cannot put the metal back again to keep it +020 !!!
 
If he wants low comp original pistons he ain't gonna get any luckier!
 
What I meant was cam grind the pistons and use 020 rings years ago it was common practice maybe the old skilled engineers are no longer around.
 
mdt-son said:
No concave pistons available in +.020 presently. You need to look for a set of used BHB pistons, or fit flat topped pistons (8.7:1). The latter is not ideal as the bike will vibrate more (perhaps a lot more). This may be countered by careful dynamic rebalancing of the crankshaft.

www.gandini-industria.it

Pistoni 2017 / Gandini Industria

Leggerezza, resistenza, durata e prestazioni.Le tecnologie sempre più sofisticate hanno esasperato la ricerca e l'applica
www.gandini-industria.it www.gandini-industria.it

-Knut
Click to expand...
I have a new pair of GPM + 0,20 concave pistons with rings.
GPM is an italian brand manufacturing from Gandini company and are easy to find here in Italy.
Pictures sent for mail that you can post please.
 
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Matchless said:
If you fit either GPM or JP you would be wise to have the crank rebalanced, as both these makes & JP in particular are a lot heavier than standard pistons. Steve Maney recommends a balance factor of 75% for the Atlas.
I purchased a pair of JP +0.020" pistons for my 650 AJS & they weighed 90 grams heavier than the original BHBs. Need less to say they were put back in the box & I hunted around for a pair of BHBs. The only safe way to remove weight from after market pistons is to taper bore or grind the ends of the gudgeon pins, & this will only lose about 10 to 15 grams.

Martyn.
Click to expand...
Standard pistons found on my bike are GPM concave head
 
pierodn said:
I have a new pair of GPM + 0,20 concave pistons with rings.
GPM is an italian brand manufacturing from Gandini company and are easy to find here in Italy.
Pictures sent for mail that you can post please.
Click to expand...
OK but these are probably new old stock (NOS).

-Knut
 
possm said:
What I meant was cam grind the pistons and use 020 rings years ago it was common practice maybe the old skilled engineers are no longer around.
Click to expand...
Yes I get you.
Any U Tube videos of this work being done to a piston?
 
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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
 
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