Author Topic: Old codger needs lower compression pistons  (Read 1522 times)

Offline BSA Biker

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Old codger needs lower compression pistons
« on: 06.11. 2017 13:36 »
Hi all,
I am a new member and this is my first post in the forum. I have owned my 1960 A7 Shooting Star for over 50 years and had many memorable trips etc on it, originally my only means of transport then as a "toy" used on high days and holidays. It took a year to get it re-registered here in Spain where I now live, Traffico caused a lot of hurdles for me to get past before they finally relented and gave me a new registration number last year. In between that time my late wife was diagnosed with cancer so that took priority, but with any Spanish bureaucracy they lull you into a false sense of security where you think all is sorted then hit you with the immortal words "just one more bit of paper" this happened at least three times during the process, sadistic or what. It makes the DVLA seem like saints in comparison and I've had enough problems with them in the past.

Anyway my reason for this post is :-


Due to an accident many years ago to my right knee, old age has caught up with me and now its impossible to kick over my 1960 A7 Shooting Star using my right leg, yes it is possible to do it on the centre stand with my left leg, but it feels very strange.
At the moment it is on Taiwan 8.0:1 ratio (standard bore 66mm) these were all I could buy when one of the old ones developed a crack, I have been told by different suppliers that they have these in a lower compression but this is not true, as I found out after buying the same ones from Austria. There is now a request for the correct ones on the Wanted and For Sale site so if any member has a set in good condition , please help a poor old codger in his last years of bike enjoyment.    i e,   7.25:1 on standard bore 66mm

Online Greybeard

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Re: Old codger needs lower compression pistons
« Reply #1 on: 06.11. 2017 13:57 »
Would an electric starter be out of the question?
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Offline BSA Biker

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Re: Old codger needs lower compression pistons
« Reply #2 on: 06.11. 2017 18:35 »
I had thought about that but the price is rather high, plus SRM (the chap not the company he sold on) reckons that you need to change to 12 volts and a very large battery like the ones used on Harley Davidson bikes. Also it has to be accurately drilled into the back of the primary chaincase. So at the moment as I'm living in Spain and not able to visit him in South Wales, its the reduced compression piston route.

Regards,

Morgan

Offline JulianS

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Re: Old codger needs lower compression pistons
« Reply #3 on: 06.11. 2017 18:59 »
Have you seen these;

https://www.draganfly.co.uk/index.php/accessories-a-misc/product/1179-

I am not sure the drop in compression 8 to 7.25 would sort the problem. You could consider a compression plate under cylinder as well;

https://eborbikes.com/index.php?route=product/product&path=62&product_id=60

Offline muskrat

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Re: Old codger needs lower compression pistons
« Reply #4 on: 06.11. 2017 19:25 »
Yep, I'd go with the comp plate.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Online morris

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Re: Old codger needs lower compression pistons
« Reply #5 on: 06.11. 2017 19:27 »
De Groot has some low compression (6.6) pistons for A7;
https://www.degroot-bsa.nl/index.php?route=product/category&path=47_80&page=8
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Online KiwiGF

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Re: Old codger needs lower compression pistons
« Reply #6 on: 06.11. 2017 19:31 »
Yep, I'd go with the comp plate.
Cheers

I have a spare compression plate, 2 or 3mm thick from memory, free to a deserving home, but the cost of posting from nz might not be practical!

It's off a 56 A10, will it fit an a7?

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Offline coater87

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Re: Old codger needs lower compression pistons
« Reply #7 on: 07.11. 2017 01:02 »
 A compression plate is a spacer that fits between the cylinder head and barrels.

 Its like using a super thick head gasket. Its a bit of a guess how much you are lowering the compression without doing all the math (which I dont know) but it will absolutely work. Its cheaper than new pistons, and not as permanent (meaning you can go thicker or thinner on the plate depending on what works best).

 The only expense (besides the plate) is an additional head gasket, you need one on eat side to seal it.
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Offline JulianS

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Re: Old codger needs lower compression pistons
« Reply #8 on: 07.11. 2017 09:36 »
The compression plate I referred to actually fits between cylinder base and the crankcase.

You can also buy a 1.5 mm A7 head gasket. Normally about 1 mm thick.

https://www.ebay.co.uk/itm/BSA-A7-SOLID-COPPER-HEAD-GASKET-NOT-A10/142563593039?hash=item213173cb4f:g:nkEAAOSwNm5ZqCPa

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Re: Old codger needs lower compression pistons
« Reply #9 on: 07.11. 2017 10:17 »
The compression plate I referred to actually fits between cylinder base and the crankcase.

You can also buy a 1.5 mm A7 head gasket. Normally about 1 mm thick.

https://www.ebay.co.uk/itm/BSA-A7-SOLID-COPPER-HEAD-GASKET-NOT-A10/142563593039?hash=item213173cb4f:g:nkEAAOSwNm5ZqCPa

The compression plate I have spare fits under the barrel.
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

1949 C11 rigid, but why!!! (cos it was cheap)

1937 B21, project missing parts, mission impossible?

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife

Offline coater87

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Re: Old codger needs lower compression pistons
« Reply #10 on: 07.11. 2017 11:28 »
 I wonder what advantage you get putting it under the barrels as compared to under the head? *conf*
 
 Lee

 
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Online bsa-bill

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Re: Old codger needs lower compression pistons
« Reply #11 on: 07.11. 2017 13:36 »
Quote
I wonder what advantage you get putting it under the barrels as compared to under the head?

I'm assuming it would require two cylinder head gaskets and greater risk of leaks there
All the best - Bill
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Offline muskrat

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Re: Old codger needs lower compression pistons
« Reply #12 on: 07.11. 2017 18:18 »
If adding a comp plate under the barrels (as they should be) use a plate no more than 60 thou" (1.5mm). At 80 thou" (2mm) the cam followers will be lifted off the cam greatly reducing performance and create a lot of clatter.
A quick fix often used is two head gaskets. The problem here is it alters the geometry of the pushrod/rocker/valve and if extreme will wear the guides quickly.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Re: Old codger needs lower compression pistons
« Reply #13 on: 07.11. 2017 18:39 »
...the cam followers will be lifted off the cam greatly reducing performance and create a lot of clatter.
When my bike is fully warmed up, the engine is embarrasingly clattery at tickover. I put a camshaft in that has been reprofiled and the cams have presumably been reduced in height. Am I suffering from the same problem as fitting a thick compression lowering plate?

Offline BSA Biker

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Re: Old codger needs lower compression pistons
« Reply #14 on: 07.11. 2017 21:55 »
Have you seen these;

https://www.draganfly.co.uk/index.php/accessories-a-misc/product/1179-

I am not sure the drop in compression 8 to 7.25 would sort the problem. You could consider a compression plate under cylinder as well;

https://eborbikes.com/index.php?route=product/product&path=62&product_id=60
Hi Julian,
Yes I fell for Draganfly's stock list as well, unfortunately they don't have any of that type, only the Taiwan 8.0:1 ones. I'm now waiting for a reply from the Australian "Mikes" Shop about the old new BSA pistons, keeping my fingers crossed that they will be okay.  Morgan