Gary at SRM has added a little more info to the mix.

"Our 750 kits are 6mm shorter than standard cylinders" "Approx from top of gudgeon pin to edge of piston 19mm" "crown shape and sizes are pretty much same as the BSA B44 piston"

Considering the above, I've used the B44 piston specs from Ed V. for the SRM Big Bore pistons, and also made the change to the Block Height spec for the SRM Big Bore cylinder.

In regards to full height Big Bore cylinders, "Long cylinders for the strokers not currently in production and we do not have stocks".

Long stroke billet cranks are also not currently available from SRM, "proving difficult to find suitable company that can make them".

The chart has expanded to include input information and formulas to help determine what the resulting compression ratio will be.

It's impressive how much the CR can be altered by varying the thickness of the head gasket, or having the deck height of the piston below the deck of the cylinder at TDC.

I've used 70cc as combustion chamber volume for a standard A65 head. Not terribly exact but in the ball park, and it allowed the formula to do some work.

Anybody have a reasonably close estimate for a standard B44's combustion chamber volume?

BSA Matchless Triumph

#414908 - 01/23/1210:21 pmRe: Unit Twin Bore and Stroke Combinations
[Re: ]

If you know the standard compression ratio, bore and stroke you can calculate the standard combustion chamber volume using TTs formula. You can then amend the chamber volume for changes such as head shaving, extra thick head gasket thicknesses etc and then recalculate the compression ratio again using TTs formula. No need to go into a cold workshop---you can do all the calculations in the warm! HTH

#414986 - 01/24/124:03 amRe: Unit Twin Bore and Stroke Combinations
[Re: Tridentman]

The easy and accurate way to measure chamber volume and compression ratio is to use an acrylic plate with a small hole, grease for sealing and a burrette. Grease the valve seats and head gasket area, put on the plate and fill the chamber from the burrette. For the piston, move the piston down a known amount, seal the rings with grease, put on the plate and fill the cylinder from the burrette. Subtract the cylinder area * piston distance below TDC from this measurement and then add this (negative number) to the chamber volume. The final volume plus the piston swept volume divided by the final volume is the compression ratio.

#415001 - 01/24/125:57 amRe: Unit Twin Bore and Stroke Combinations
[Re: DMadigan]

C = Bore E = Stroke P = Deck Height (Piston deck below cylinder deck is expressed as a positive number) S = Head gasket compressed thickness T = Head gasket bore U = Piston top volume (Dome is expressed as a negative number) V = Combustion chamber volume

BSA Matchless Triumph

#415032 - 01/24/1212:45 pmRe: Unit Twin Bore and Stroke Combinations
[Re: Tridentman]

Joined: Aug 2001 Posts: 6,903triton thrasher
BritBike Forum member

If you know the standard compression ratio, bore and stroke you can calculate the standard combustion chamber volume using TTs formula. You can then amend the chamber volume for changes such as head shaving, extra thick head gasket thicknesses etc and then recalculate the compression ratio again using TTs formula. No need to go into a cold workshop---you can do all the calculations in the warm! HTH

It's not really my formula. It's simple measurement. Compression ratio is the ratio of enclosed volume at BDC to that at TDC. Messing around measuring it with a disassembled engine will usually bring inaccuracies in.

Let's not lose sight of the fact that we are dealing with theoretical combinations here. Very few of us actually own any of these bored and stroked unit twins.

Without the actual engine, it's going to be pretty difficult to get an accurate measurement by pouring fluid in the spark plug hole!

The idea behind the chart was to bring the most common possibilities together in one place, making it easier for an interested person to sort out which variation would make most sense for them.

BSA Matchless Triumph

#415083 - 01/24/126:54 pmRe: Unit Twin Bore and Stroke Combinations
[Re: Two Alpha]

Joined: Aug 2001 Posts: 6,903triton thrasher
BritBike Forum member

Yet another update (see link in first post, page one).

Realized that I could work out approximately the BSA B44 piston top volume from information received in an email from Gary at SRM. Turns out that they are quite close to the A65 piston top volumes for a similar compression ratio.

This would put the B44 combustion chamber volume at about 81cc's, it would be great to have some confirmation on this.

So, since we're likely using custom rods for all of the variations over 800cc, it might be very beneficial to have a rod length that leaves the piston deck 1mm to 2mm below the cylinder deck at tdc, making the compression ratio a lot easier to live with.