Author Topic: What is the compression ratio?  (Read 738 times)

Offline rowan.bradley

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What is the compression ratio?
« on: 03.11. 2017 14:05 »
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


Current bike: 1958 A10 Super Rocket (in bits), purchased in 1967.
Previous bikes: M21

Online JulianS

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Re: What is the compression ratio?
« Reply #1 on: 03.11. 2017 16:56 »
They are 9:1.

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

Online bsa-bill

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Re: What is the compression ratio?
« Reply #2 on: 03.11. 2017 18:22 »
Quote
They are 9:1.

Agreed
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline rowan.bradley

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Re: What is the compression ratio?
« Reply #3 on: 04.11. 2017 11:19 »
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


Current bike: 1958 A10 Super Rocket (in bits), purchased in 1967.
Previous bikes: M21

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Re: What is the compression ratio?
« Reply #4 on: 04.11. 2017 12:12 »
The page from 1981 piston catalogue confirms.

Online bsa-bill

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Re: What is the compression ratio?
« Reply #5 on: 04.11. 2017 12:40 »
Quote
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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Online ellis

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Re: What is the compression ratio?
« Reply #6 on: 04.11. 2017 13:05 »
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

Offline rowan.bradley

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Re: What is the compression ratio?
« Reply #7 on: 05.11. 2017 08:24 »
The page from 1981 piston catalogue confirms.
Brilliant. Thanks for the scan. It's good to have solid evidence!

Rowan


Current bike: 1958 A10 Super Rocket (in bits), purchased in 1967.
Previous bikes: M21

Online mikeb

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Re: What is the compression ratio?
« Reply #8 on: 05.11. 2017 10:17 »
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?
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline rowan.bradley

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Re: What is the compression ratio?
« Reply #9 on: 06.11. 2017 12:34 »
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


Current bike: 1958 A10 Super Rocket (in bits), purchased in 1967.
Previous bikes: M21

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Re: What is the compression ratio?
« Reply #10 on: 06.11. 2017 19:15 »
G'day RB.
I'd go with what you've got. Tuned right it'll be sweet.
Cheers
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Online mikeb

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Re: What is the compression ratio?
« Reply #11 on: 06.11. 2017 19:34 »
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.
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'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline worntorn

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Re: What is the compression ratio?
« Reply #12 on: 07.11. 2017 04:05 »
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

Offline rowan.bradley

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Re: What is the compression ratio?
« Reply #13 on: 07.11. 2017 13:57 »
Thanks Glen, that is reassuring to know.

Rowan


Current bike: 1958 A10 Super Rocket (in bits), purchased in 1967.
Previous bikes: M21