Author Topic: a7 gudgeon to crown distance?  (Read 438 times)

Offline Rocket Racer

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a7 gudgeon to crown distance?
« on: 13.12. 2020 01:06 »
Can someone with a A7 piston to hand possibly tell me the distance from the top of the gudgeon pin to the outside edge of the piston crown.
I believe the A7 pistons run a low pin height relative to the crown and interested in the difference with the A10 piston. 
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Online RichardL

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Re: a7 gudgeon to crown distance?
« Reply #1 on: 13.12. 2020 01:26 »
The crown is right around 0.250" high. Can measure it better later if it helps.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at https://www.gentlemansride.com for details.


Offline Rocket Racer

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Re: a7 gudgeon to crown distance?
« Reply #2 on: 13.12. 2020 03:29 »
The crown is right around 0.250" high. Can measure it better later if it helps.

Richard L.

Thanks Richard, ignoring the crown, or assuming a flat top piston what would the measurement be? (effectively to the top of the bore)
cheers
Tim
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Online RichardL

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Re: a7 gudgeon to crown distance?
« Reply #3 on: 13.12. 2020 04:18 »
Tim,
I've only disassembled an A7, never assembled one (yet). Didn't pay close attention to where the break from vertical wall to crown slope occured with respect to top of bore. I assume it occurs right at the top of the cylinder. Others might comment on this. In this picture it looks like the caliper point is not aligned with the crown break. It was actually better than it looks (since I was holding things in place while holding the phone, etc.) I would guess measurement within 0.005". Short of a special measurement setup, that may be as close as I can get.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at https://www.gentlemansride.com for details.


Offline Rocket Racer

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Re: a7 gudgeon to crown distance?
« Reply #4 on: 13.12. 2020 05:32 »
Richard thanks again, so approx 28.4mm, which compared to an A10 piston which is around 23mm, so close to 5.4mm diff.

I have a 5/8" flange small fin a7 barrel currently at a worn 72mm+ so thinking perhaps I take it back to 70mm and put a custom 64.5mm stroke crank in it.
So I'd reduce the stroke 8.6mm (4.3mm deck height), then also bring the piston down a further 5.4 mm, so to keep the heights fairly std would be looking at 9.7mm additional rod length.
Interestingly the equivent notrun to the a7; the eighty eight ess ess is a popular race tool when fitted with a 6.4" rod.  Looks doable...

Just preliminary measures at this point but would like to keep the barrel and cases unmolested. 
At this point I just have a few bits of an engine and some ideas. But I think I have a strawman plan.
Scheming in progress  *whistle*
Tim
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Online Swarfcut

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Re: a7 gudgeon to crown distance?
« Reply #5 on: 13.12. 2020 09:34 »
   A7 has a smaller diameter pin, and in my estimation the con rod eyes look a bit slim to bore out to take an A10 bush. But I am sure folks do this to produce a big bore A7, as the barrel here has already been well opened up from the original 66mm bore. Is it an original A7 barrel or a shaved down A10?   A7 has no cutaways at the base of the cylinder spigots.

 A flat top A10 piston measures 25.5mm crown to top of pin hole, (AE piston casting 11062). An ex A7 Shooting Star piston measures 30mm from the point where the raised crown starts to the top of the pin hole as shown above. This datum lies 5.5mm above the top ring groove.
 
  Not saying these figures are set in stone, but that's what I found on used pistons to hand and may be of interest.

 Swarfy.

Offline Rocket Racer

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Re: a7 gudgeon to crown distance?
« Reply #6 on: 14.12. 2020 04:06 »
   A7 has a smaller diameter pin, and in my estimation the con rod eyes look a bit slim to bore out to take an A10 bush. But I am sure folks do this to produce a big bore A7, as the barrel here has already been well opened up from the original 66mm bore. Is it an original A7 barrel or a shaved down A10?   A7 has no cutaways at the base of the cylinder spigots.

 A flat top A10 piston measures 25.5mm crown to top of pin hole, (AE piston casting 11062). An ex A7 Shooting Star piston measures 30mm from the point where the raised crown starts to the top of the pin hole as shown above. This datum lies 5.5mm above the top ring groove.
 
  Not saying these figures are set in stone, but that's what I found on used pistons to hand and may be of interest.

 Swarfy.
good point on the differing pin sizes, I suspect I may need some custom rods although 6.4 and 6.5" appear readily available. my barrel has the same deck height as a common big fin A7 but is an early american a7 one with the super thick 5/8" flange. currently is at a knocked around 72+mm . the early a10 small fin thick flange barrels have one more fin.
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Offline Radlan2

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Re: a7 gudgeon to crown distance?
« Reply #7 on: 14.12. 2020 10:22 »
Hi Tim
     This sounds interesting, the 64.5mm stroke was mentioned by Roland Pike as one of the sweetest engines and would rev freely, unfortunately it never got past the development stage, I dont know what the configuration was in his race bike. Do you have this crank or are you getting one made? A lot of work.
       I am in the process of using a thick flange A7 barrel as you have, mine was at 71mm, now bored +0.020 to accept Triumph T110 pistons, the pin size is 11/16", top of pin to piston measures 17.5mm. I am using A10 (6.5") rods and 74mm stroke A65 crank which has been metal sprayed on the drive side to accept the std (metric) main bearing.
       The project is taking a bit longer than I hoped but I am making progress, the through bolting/head is done, I am sticking with the main bearing bush but the larger A65 one.
                Best of luck
                      Chris

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Re: a7 gudgeon to crown distance?
« Reply #8 on: 14.12. 2020 19:12 »
G'day Tim.
I think I already mentioned with my A7SS racer I used std size 10.5:1 T160 pistons with shortened (about 2mm) A10 rods. This gave 14:1 comp for methanol. Over here we were allowed a 5% over size making a 500cc a 525cc.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline Rocket Racer

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Re: a7 gudgeon to crown distance?
« Reply #9 on: 14.12. 2020 19:30 »
G'day Tim.
I think I already mentioned with my A7SS racer I used std size 10.5:1 T160 pistons with shortened (about 2mm) A10 rods. This gave 14:1 comp for methanol. Over here we were allowed a 5% over size making a 500cc a 525cc.
Cheers

Thanks, you're not the first person to mention that option, triple pistons are common and at 67mm similar ish to std pistons. I think clubmans class allows for several oversizes. This would be the cheapest option with an A65 crank. At the moment I havent blown too much money but also not commited to a specific configuration. My A7 barrel could also still be potentially used at +90 or +100 before a resleeve and it would need custom sleeves (not a biggie) if I want to return it to an A7 bore range.
Perhaps a decision for the new year as funds permit...
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Offline Rocket Racer

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Re: a7 gudgeon to crown distance?
« Reply #10 on: 14.12. 2020 19:33 »
Hi Tim
     This sounds interesting, the 64.5mm stroke was mentioned by Roland Pike as one of the sweetest engines and would rev freely, unfortunately it never got past the development stage, I dont know what the configuration was in his race bike. Do you have this crank or are you getting one made? A lot of work.
       I am in the process of using a thick flange A7 barrel as you have, mine was at 71mm, now bored +0.020 to accept Triumph T110 pistons, the pin size is 11/16", top of pin to piston measures 17.5mm. I am using A10 (6.5") rods and 74mm stroke A65 crank which has been metal sprayed on the drive side to accept the std (metric) main bearing.
       The project is taking a bit longer than I hoped but I am making progress, the through bolting/head is done, I am sticking with the main bearing bush but the larger A65 one.
                Best of luck
                      Chris
Chris
I think a short stroke motor would be my best option excepting cost!  your build sounds like it'll be sweet and strong!
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
New Zealand