Author Topic: Compression Ratio  (Read 340 times)

Online Slymo

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Compression Ratio
« on: 17.11. 2018 10:13 »
My 58 SR took its first run since a total overhaul today and it appears to be a bit over advanced my having set the timing at 13/32" btdc which I have read since is too advanced. I also understand that it is dependant on what octane of fuel and what compression ratio the machine has. My motor came to me with new pistons in a rebored barrel but I've never actually been certain what compression they were. I've assumed that they are 9:1 based  not much but I would love to know so am posting a pic of them with head off looking for wise counsel
NZ

Online muskrat

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Re: Compression Ratio
« Reply #1 on: 17.11. 2018 10:53 »
G'day Slymo.
Look like 9:1 https://tinyurl.com/y6vjyhgy
Yes too advanced, 3/8" max but would probably be happier with 5/16" on 95 octane. Too advanced will cause pinging (detonation, knocking) and destroy the pistons very quickly DAMHIK.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, .
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Offline RDfella

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Re: Compression Ratio
« Reply #2 on: 17.11. 2018 10:56 »
That profile looks like 9:1. Any part nos? Difficult these days I know, as aftermarket suppliers rarely mark anything.
BTW - comforted to see my bench is not the only one with stuff all over it. All my good intentions to have a clean, tidy bench soon evaportate. Only time it was really clean was in the days (long ago) when I used to overhaul BW auto gearboxes.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Slymo

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Re: Compression Ratio
« Reply #3 on: 17.11. 2018 23:06 »
Cheers all. Yes I work in a rather squalid environment which I rarely seem to be able to get on top of. Fortunately now the bike is out of the shed I can do a tidy up which will last until the next thing I do in the workshop.
NZ

Online Slymo

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Re: Compression Ratio
« Reply #4 on: 19.11. 2018 04:05 »
Set the timing at 5/16" and it does seem to be a sweet spot. Idles like a stationary engine when retarded and seems very willing and responsive on advance. Just need a seat now and I can start road testing proper.
NZ