650SS piston specs

jseng1

jseng1

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I'm specing out a 650SS piston so I can make lightweight pistons for a customer (my 2nd set of lightweight 650ss pistons so far). The photo below is a piston he sent me. It says in the manual that the 650ss has 9:1 C.R. piston but according to my calculations it would be closer to 10:1 with this piston assuming the following.

The dome on this piston is 1/4" high and the compression height is approx 1.590" at the periphery (.050" higher than a stock 750 commando piston). I'm assuming that the commando has the same combustion chamber with the same depth and valves - just offset a bit with the resulting squish band and that the 650 and 750 have the same cylinder head height relative to the wrist pin at TDC.

Were alternate pistons available or is this a stock 650SS piston and what is the C.R.?

650SS piston specs


See the lightweight version of this piston at:

http://www.jsmotorsport.com/technical_u ... istons.asp

Look for 650ss Dominator
 
Jim, will these only work with your new rods or will you have a version that can be used with the standard 650ss rods?

Glen
 
My Norton manual shows only 9.0:1 CR for 650 .

650SS piston specs

Slick
 
Last edited:
Worntorn
The JS pistons use my longer rods which are avalable in an "ultralight version" which was designed for Dominators and solid frames and can handle 750s up to 80HP. The longer rod is what makes it possible to make a shorter lightweight piston. The original Hepolite brand of pistons as shown by the photo at the top of this thread weigh in at 230 grams compared to 170 or less for the JS lightweight jobs.

Rohan
The inside of the pistons casting has the raised lettels AE which would make it a Hepolite piston. I don't see anything resembling BHB. Do you think this is an aftermarket racing piston?

Jim
 
jseng1 said:
Do you think this is an aftermarket racing piston?
Click to expand...

Could be, or just a performance road piston.
There were all sorts of hot-up goodies in catalogs back then.
Does look a little high-domed...

Not sure how long those BHB pistons went for either, Norton manufacture switched to AMC manufacture,
would have been a change of piston maker in there at some point.
 
Can someone chime in with a stock 650SS compression height and dome height?
 
Off the NOC website are listed these 650 pistons

Manxman ( BHB) 23268 LH & 23269 RH

650 8.9:1 17839 /17840

650 10.2:1 18553 (Later 650SS Piston)

650 8.9:1 24022/24023

650 9:0:1 25387/25388

No help really, but the concept of a single type of piston for 650's just isn't so...
 
Jim, it could be a 99 piston. One of the favourite hop up tricks for the 650ss was to substitute 99 pistons for racing. This gives a CR of about ten to one, which agrees with your measurements. 99 s had a different head/combustion chambre and required a taller piston to make their CR., even though it was quite a low CR.
When I first purchased my 650SS, one of the old time Westwood Racers here tried to convince me to hop it up by switching to 99 pistons. I didn't do this as it is already strong enough, it's the vibes I would like to do something about.
Of course increasing the CR only tends to make vibes worse.
Rohan, that is the first time I have seen CR listed at 10.2 for the 650. All of the 650 books I have here(quite a few) list either 8.9 or 9.0 as the stock ratio. Perhaps the Norton site is referring to the use of 99 pistons in a 650 ss, as it was apparently quite widely done.

Glen
 
Bill called me and gave be the straight scoop.

The 650ss 9:1 piston was flat top.

The 600 domni had a 9:1 C.R. domed piston option. Some people fitted the 600 domed piston (same dia) into the 650ss to achieve10:1 C.R. A little bit of skirt had to be cut away to clear the 650 flywheel and this has been done to these pistons.

Mystery solved

thanks Bill
 
worntorn said:
Jim, it could be a 99 piston.
<snip>
Rohan, that is the first time I have seen CR listed at 10.2 for the 650. All of the 650 books I have here(quite a few) list either 8.9 or 9.0 as the stock ratio. Perhaps the Norton site is referring to the use of 99 pistons in a 650 ss, as it was apparently quite widely done.
Click to expand...

Indeedy - a search on 18553 shows up as a lumpy topped 600 piston for a 99 and 77,
so fitting it to a 650SS seemingly gives that 10.x compression.

worntorn said:
Of course increasing the CR only tends to make vibes worse.
Click to expand...

Not necessarily ?
Its piston weight, balance factor and frame sympathetic vibes that make cruising vibes.
Higher compression can produce a rough feeling engine under acceleration, especially from low rpms.
Magazine tests of old used to quote a "minimum non-snatch speed", and higher compressions can raise this mph number, considerably.

I can vividly recall 1st time I ever a drove an EFI injected car (no motorcycle content ere),
and it pulled away smoothly from a standstill in top gear, when tested.
EFI (and new combustion chamber shape)really tamed some rough combustion characteristics,
which you don't altogether appreciate until its compared with something better.
No relation to balance factors.... ??
 
It was a generalization, bigger bang, more power, more vibration. Of course there are things that can be done to mitigate, such as lighter reciprocating components, but I think if all else is left alone, it is fair to say raising CR will tend to make vibes worse overall.
Like when you take a nice soft 7 to one Vinny and put ten to ones in it....

Glen
 
worntorn said:
Like when you take a nice soft 7 to one Vinny and put ten to ones in it....
Click to expand...

If the piston weights were still the same, would be a good test wouldn't it. ?
Vibes from reciprocating weights at cruise and combustion/roughness should be 2 different things....

But we diverge from 650SS pistons ere....
 
Hey Jim,
I sure am glad to see that you're making your great pistons for 650 Nortons. I've got 2 650 Nortons. One is a Manxman and the other a 650ss, so perhaps some day I'll have the bucks to get a set of your pistons and rods. Either way, it sure is nice to know that you're making them available to Norton owners.

As for info about 650 pistons and anything else Dominator related, I'd recommend this Facebook page. This fellow knows more about Norton Dominators than all of us combined. Very knowledgeable and a really great guy to talk to. Hope it helps:

https://www.facebook.com/pages/Norton-M ... 1501303123
 
I am holding a +20 BHB manxman piston. Pin (top) to top of flat top piston is 1.248".
 
dynodave said:
I am holding a +20 BHB manxman piston. Pin (top) to top of flat top piston is 1.248".
Click to expand...

This is what is refereed to as a 9:1 C.R. 650 SS piston correct?

It would have a compression height (middle of pin to top of piston) at 1.59"

But I calculate that as coming out to 8.5 to 1 (C.R.) Can anyone verify this.?

Also

I need the length of the 45 deg cut at the periphery of the piston from corner to corner. Somewhere around 3/32 or .1" I think. Can someone check this. I don't have one of these pistons.

thanks
Jim
 
jseng1 said:
Worntorn
The JS pistons use my longer rods which are avalable in an "ultralight version" which was designed for Dominators and solid frames and can handle 750s up to 80HP. The longer rod is what makes it possible to make a shorter lightweight piston. The original Hepolite brand of pistons as shown by the photo at the top of this thread weigh in at 230 grams compared to 170 or less for the JS lightweight jobs.

Rohan
The inside of the pistons casting has the raised lettels AE which would make it a Hepolite piston. I don't see anything resembling BHB. Do you think this is an aftermarket racing piston?

Jim
Click to expand...
If the dome top 600 pistons weigh 230 grams, I wonder what the weight is for the original flat top 650ss pistons?

Glen
 
jseng1 said:
I'm specing out a 650SS piston so I can make lightweight pistons for a customer (my 2nd set of lightweight 650ss pistons so far).
SNIP
I'm assuming that the commando has the same combustion chamber with the same depth and valves -
SNIP
Click to expand...

Jim
I'm learning all the time, especially about the small bolt pattern heads these days.
The very early heads are 1.3" intake, and the later ones are 1.4" intake including my 88SS build and my friends 650SS(Manxman) head. If the later 650 or mercury heads, w/no spigot changed valve size...???

The intake grew with the 750 heads to 1.5" as you must already know.
If interested, I can eventually measure the weight of the piston, CC the heads and measure the chamber height.
Just sold my house, closing wednesday. busy cleaning it out.
Dave
 
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