A7SS piston crowns for differing compression ratio's

[Rocket Racer]

I understand the  A7SS motors ran compression ratio's between 7.25, 8 and even 8.5:1 (sorry ignoring long stroke A7)
Does anyone have a piston that they know the compression of and can 1. post a picture of the crown of 2. ideally measure the crown height.
I'm looking at getting some made for a project and conscious I am unfamiliar with A7SS piston shapes.
I found one on Baxter cycles site claiming to be 8:1 and a Polsen.

The combustion chamber of the SS is quite small so not expecting significant height, but given smaller capacity to A10 expecting slightly higher crown than equivalent A10.
Not to concerned about gudgeon height at this stage. 

Are flat tops around 7.25?
Any help in clarifying a std BSA A7SS piston crown appreciated

[Swarfcut]

 I have a Shooting Star Piston to hand which looks like picture 2. Flat Top, casting number 11288, AE Brand. According to Drags' Piston Chart, this is 7.25:1

 Overall piston height (lower edge of skirt to piston flat top crown measured at front face) is 68mm. Top edge of gudgeon pin, measured on the circlip to the flat of piston crown is 33mm. Piston has curved relief to the skirt under the pin to clear the flywheel. Top edge of top ring groove lies 9mm below flat top. Bevel edge is 4mm wide, reducing at valve pockets. Crown diameter 62.5mm, all measured by eye and a crude caliper!.

 Given in good faith as being correct, but as always check from your own reference source.

 Swarfy

[rocker21]

i have a pair of new 8.5 :1 +20thou that i dont need if thats any help to you

[RDfella]

According to my 1973 Hepolite catalogue, A7 pistons were available in 6.6, 7.25 and 8:1 comps. All had profile '4C' - which basically looks like the 1st picture.
For the 8:1 piston, Hepolite give: comp height - 1.705, length - 2.767. The 7.25 CR is 1.625 comp height with length of 2.687.
It seems that 6.6 CR was std until 1961, whereafter 7.25 became the std CR.

[berger]

here is my two pennies worth. as i understand it the 54/55 shooting stars were 7.25 to 1 . 56/57 were 8.1 in 1960 they were 8.25 to 1 and now you can get 8.5 to 1s. the pistons on the right are BSA 8.25to1s  the piston on the left is of foreign make and is 8.5to 1. measured from the top of the top ring groove to the crown the ones on the right measure 10.5mm to the crown approx. the one on the left is 11.5 mm from top of ring groove to crown

[Rocket Racer]

thanks guys. unfortunately I dont have an A7SS reference piston of any compression, just a50 and a10 ones where the gudgeon heights are different.
Thanks for the offer of +20's but  I am trying to keep it either no more than 66mm and ideally around 10:1 although I could live with slightly less. those profiles certainly match the a10 common shape with a 30 degree profile and a similar crown height.   
Doing some back of an envelope calcs looks like I only need an extra 1.5 mm in height to make a 7.25 closer to 10:1.

Measurements I'm seeing for 7.25:1  are 1. top edge of gudgeon to top of crown 33mm
2. distance back to top of top  ring 9mm made up of 5mm to edge of crown, 4mm height of 30 deg bevel.

If someone with an 8 or 8.5 can clarify those dimensions I might be able to get making some underway

I couldnt find any off the shelf pistons in STD size that were'nt slotted around the oil ring. JP seemed to be the only current supplier and having had recurring nightmares with their pistons wont be tying them again, ever...

JE in UK are being very helpful. Initial bore likely to be 1/2mm undersize (65.5mm) or STD.  This is intended for a track only engine, albeit only mildly developed to give me another race bike to play with alongside "Bender" my 55 roadrocket outfit.

[JulianS]

From 1963 Hepolite catalogue. "Comp" column is compression height - measurement from centre of gudgeon pin hole to piston crown.

[Rocket Racer]

Julian, thanks mate, that's gold! admittedly from a reputable supplier rather than the factory but very useful  .  Nice to see 9.5's were available to the speed crazed "Rockers"

[Rocket Racer]

https://www.a7a10.net/forum/index.php?topic=15818.msg134683#msg134683

just trying to reconcile piston heights. scary to think I was circling this issue 14months ago and still trying to nail it down. However in the linked thread 1.354" was the height from top of gudgeon to top of crown, whereas the hepolite sheets has heights from ranging 1.565 to 1.954" - that difference range of approx 10mm suggests a tall dome much like the a10 10:1 in profile for the 9.5:1
does this mean the hepolite sheet is looking at the gudgeon center?

I note from the other thread that the gudgeon is also quoted as being smaller for the A7.  I'll be running 6" a50/a65 rods so 19mm gudgeon. What is the std A7 pin diameter?

looking at online images its very difficult to identify if the flat top is 6.6 or 7.25 and the 8's seem to mainly have a steeper taper of 60 degrees rather than the 30 degree (ish) often found on A10's. The sheet claims flat tops for all bar 9.5:1 but that seems suspect.

[Rocket Racer]

found pin diameter .6875mm

[Rocket Racer]

if the hepolite brochure refers to the centre of the gudgeon pin then we can take the 8:1 quoted length of 1.705" (43.307mm) and if we add half the pin diameter 8.73125mm (pin is 0.6825" or 17.4625mm) to the measured 1.354"or 39.3916mm  from the other thread we get 43.12285mm . that seems to be close enough to make sense and be consistent.

[Swarfcut]

 My measurements were given as a quick ready reference for any piston to hand. Yes, the Hepolite catalogue takes the centre of the pin as the datum, and in my 1989 catalogue at least, gives dimensions from this point to enable piston identity, along with over 120 piston crown formats. Needless to say the only BSA Piston included is the A75R Triple, so you need a copy of the earlier catalogue. Piston length/height can be defined by various parameters, so compare like with like.

 The A7 pin is certainly slimmer, a surprise to a few people.

 Swarfy.

[Rocket Racer]

https://mikesclassiccyclespares.com/product/67-466/ this link on someone elses thread also had reference to the 7.25 (67-467) and I've been lent what appears to be an 8:1 (67-476). Looking at both they like Muskrats image are very very similar. and doing a wee bit of maths that figures because the combustion chamber volume for a 7.25:1  is 34.48cc, whereas for an 8:1 is 31.25cc, only 3.23cc different which equates to just less than a mm of deck height difference.