What is the compression ratio?

[rowan.bradley]

Can anyone tell me, by looking at the attached photos, what the compression ratio of this 1958 Super Rocket is likely to be? The pistons are new (or were 50 years ago) and have never been used.

Many thanks - Rowan

[JulianS]

They are 9:1.

There will either be a BSA part number on the crown or a Hepolite number on the inside skirt.

[bsa-bill]

They are 9:1.

Agreed

[rowan.bradley]

Thanks for a nice unambiguous answer to this question, it's good to know. Is 9:1 too high to run smoothly on Shell V-Power unleaded?

The pistons are Hepolite. They are still in their original box, which says they are part number 11412. This number is also cast into the inside of the piston skirt, and there is another number on the other side: 3568 AM 413.

Rowan

[JulianS]

The page from 1981 piston catalogue confirms.

[bsa-bill]

Is 9:1 too high to run smoothly on Shell V-Power unleaded?

You'll need to start it first so Magneto will need to be in very good condition, mine had been refurbished but would not provide a reliable spark at that compression that coupled with today's plugs that have no glaze on the inner insulator proved to be a no go, so installed electronic ignition - problem solved, also solved pinking that occurred on the few occasions I managed to get it to run on the magneto.
There are additives that provide a lead replacement and octane boost (Castrol Plus one is what I use)
Best of luck with it I like to know how you get on

[ellis]

Hi Rowan,

I run a 1960 gold flash built in the café racer style. I have 9:1 Hepolite pistons with new stepped liners and run on Shell v power fuel with no pinking at all and starts 1st or second kick. I would say 9:1 is a maximum for a road bike to run nicely.

ELLIS

[rowan.bradley]

The page from 1981 piston catalogue confirms.

Brilliant. Thanks for the scan. It's good to have solid evidence!

Rowan

[mikeb]

it will be an easier bike to ride if you had a lower CR. my 9:1s pinked like mad.
if pistons are new, what state are the bores in?

[rowan.bradley]

So should I consider buying lower CR pistons for my rebuild?

I'm feeling that since I have the 9:1 pistons, I should just try them, and only consider changing them if there are problems. Risks another engine strip-down, of course.

Thanks - Rowan

[muskrat]

G'day RB.
I'd go with what you've got. Tuned right it'll be sweet.
Cheers

[mikeb]

its would be just the top end to come off so you can do that in the frame. the problem with potentially later replacing your 9:1 with a lower CR piston is a rebore is done with respect to the actual piston diameter, so a replacement piston may end up with different clearances. you don't want that.

an alternative plan b to lower compression could a a decompresison plate under the barrels. that changes the pushrod angles a little but many/most regard it a fine. that's what i'd probably do. i bought a plate for that purpose a couple of years back when i had planned to fit some 9:1 hepolites tho ended up needing a rebore so sourced new lower CR ones. so those plates are still available online ebay i think.

[worntorn]

I have a 1963 Super Rocket with 9 to 1s, which is factory CR for 61 and on West Coast Super Rockets plus RGS bikes.
It starts easily, doesn't ping and is a very tractable motor.

Glen

[rowan.bradley]

Thanks Glen, that is reassuring to know.

Rowan