Author Topic: Compression Plate  (Read 1958 times)

Online olev

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Compression Plate
« on: 06.12. 2010 11:25 »
Gday,
I've managed to get a set of 8:1 pistons for the A7.
A set of 7.25:1 would be good but aren't available in 40 thou oversize.
Reading a couple of earlier posts, a compression plate sounds interesting.
My calcs (which are probably wrong) work out a 2mm plate under the barrels would drop the compression to about 7.4:1.
Are there any issues fitting a plate this thick?
The main concern is the extra distance between the followers and the cam.
cheers.

ps: who taught the poms to play cricket? its starting to look like a massacre!!

Offline MG

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Re: Compression Plate
« Reply #1 on: 06.12. 2010 11:58 »
olev,

according to my calculations, the compressed volume for a CR of 8:1 would equal 36.6ccm (calculated with the cylinders bored to +40). To lower this to 7.25:1 CR you'd have to increase this volume to 41ccm. For a bore of 67.02mm (= +40 oversize) this would mean fitting a reduction plate of 1.244mm thickness.

A 2mm plate will lower the CR to 6.9:1 btw.

The Shooting Star however has always been running on 8:1 or 8.25:1 (not sure), mine is running on almost 9:1 without any troubles and goes like s**t off a chrome shovel.
Peak and mean pressure are much lower than with a 650cc, so I feel the small crank and thin flange barrels are fairly strong enough to cope with a CR of 8:1.

Err, at second thought, we aren't talking about an early long-stroke engine, are we???
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

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Austria

Online olev

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Re: Compression Plate
« Reply #2 on: 06.12. 2010 12:28 »
Thanks Markus,
No, its a 1952 short stroke.
I'm building it for the wife and want it to be a gentle thing.
Ah well, my calc had less than 100% error (didn't it?)
not too bad.
A 1mm plate with a couple of gaskets shouldn't hurt anything.

cheers

Offline MG

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Re: Compression Plate
« Reply #3 on: 06.12. 2010 12:33 »
Quote
I'm building it for the wife and want it to be a gentle thing

Ah, I see. What about omitting one piston? Saves you 50% cash on pistons, rings, rods, bearing shells, etc. And if it is worn, you just switch to the other side. *lol*


A 1mm plate plus the additional gasket on top of it will be okay for sure, shouldn't cause any grief with the rocker arms as well.
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria

Offline Goldy

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Re: Compression Plate
« Reply #4 on: 06.12. 2010 12:39 »
olev. We invented cricket.
56 A10 Golden Flash - Restore, ride, relive.                                          
56 C12 BSA project ongoing

Offline muskrat

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Re: Compression Plate
« Reply #5 on: 06.12. 2010 17:42 »
G'day Olev,
                   a comp plate of 1.5mm + gaskets would be max. Any more and the ex followers won't reach the cam. The rocker geometry will be affected, at 1/2 lift the rocker should be 90 deg to the valve. Probably won't worry it if not racing/over reving. Easy enough to make longer push rods (same amount as comp plate) out of durallium rod.
Cheers

PS, they got rid of their best players 200 years ago.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline dpaddock

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Re: Compression Plate
« Reply #6 on: 06.12. 2010 20:58 »
poms? Is that like a farthing?
Merry Christmas, all!

David
David
'57 Spitfire


Online bsa-bill

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Re: Compression Plate
« Reply #7 on: 06.12. 2010 21:10 »
POM - an Australian term of endearment  *whistle* for  a Brit.
Many theories as to it's origin from - Pomegranate - due to the colour our skins turn when exposed to Aussie sun to - POM - Prisoner of Mother England and many many more
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 Brian

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Online olev

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Re: Compression Plate
« Reply #9 on: 07.12. 2010 11:16 »
Brian,
They look about 8:1 or more with those valve pockets.
I have a set of 8:1, 40thou O/S, OEMs which I'll use.
good price though.
Do you have an opinion on those circlips?
I've always been told not to use the ones with a tang (on my 2 strokes)

and with any luck the poms will be celebrating their win with your local booze.
cheers

Offline BSA_54A10

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Re: Compression Plate
« Reply #10 on: 08.12. 2010 10:06 »
That is because the tang can foul on the transfer ports with the expected results
Bike Beesa
Trevor

Online bsa-bill

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Re: Compression Plate
« Reply #11 on: 08.12. 2010 13:46 »
Quote
That is because the tang can foul on the transfer ports with the expected results

I'm way behind in 2stroke technology Trevor (NSU Quickly and Ariel Sports Arrow way back in the early sixties )
But are there 2strokes with transfer ports at the sides of the barrels?
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 iansoady

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Re: Compression Plate
« Reply #12 on: 09.12. 2010 12:06 »
Your Quickly did........
Ian.
1962 Golden Flash (arrived)
1955 Velo Viper/Venom (departed)
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Online bsa-bill

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Re: Compression Plate
« Reply #13 on: 09.12. 2010 13:07 »
Quote
Your Quickly did........

Really, that's interesting, must find some images (not much else to do this weather)
I never got into the quickly engine, didn't have the skills and spent too much time constantly persuading dad to repair the broken key in the transmission shaft, it eventually gave up after he fitted an oil tempered key made from the handle of a flat file.
Hopefully my gear-changing is a little gentler these days
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