C25 Barracuda - how to lower the compression ratio

[KCF21]

In rebuilding my C25 Barracuda I want to increase my chances that the rebuild will outlive me. I know a little of how BSA managed to get a basic C15 engine to outperform what was expected for this engine so I want to try to sensibly detune the rebuild; if I want speed I will ride something else. I have read that it would possible to fit a Fleetstar piston and that would bring the compression ratio down, but these are difficult to find at a reasonable cost. Does anyone know of a "off the shelf" (OK it might be an old and dusty shelf) piston that would fit a standard bore barrel that would achieve the same outcome? If you can help I would be very grateful for the advice.

[rhyatt]

On my b44 I put a thicker cylinder base gasket to lower the CR.
Worked well, from memory it was 30 thou thick.

[2LZ]

I've been looking for the thicker base gasket for my C25 Starfire, for the same reasons. Not ready for a rebuild yet (again, I'm sure) but I'd like to have one for when I do.

[rhyatt]

I did all my calculations (pie x r sq x h) and came to the conclusion it should be 31 thou.
So I got some alloy sheet and traced round the cylinder base and carefully cut it out.

[KCF21]

My internet search has found that Draganfly sell a 2mm thick head gasket. I'm not sure what maths to apply to determine by how much this would reduce the compression ratio.

Also, using a head gasket of this thickness will that mean modifying the push rods in some way?

https://draganfly.co.uk/shop/65816/head-gasket-2mm-thick-decompression-head-gasket/#40-0933D

[Spaceman]

Assuming the original head gasket was 1.2 mm (47 thou) thick, the 2 mm head gasket will lower the CR to from 10:1 to 9.2:1 according to my calculations.

The total valve clearance adjustment provided by the eccentric adjusters on the C25/B25 is about 3 mm. On my B25, the inlet adjuster has the flat at about 11 o'clock (I can't remember the exhaust adjuster position) showing the rocker is close to its highest position. This means, at least for my B25 inlet valve, it could not cope with a thicker head gasket.

However, the caps on the pushrods can be prised off and spacers inserted underneath to increase their overall length

[Spaceman]

Just to correct what I said. If a thicker head gasket was used on my B25, the inlet pushrod would then be too short increasing the valve clearance but my adjuster, starting at 11 o'clock, would have more than enough adjustment to compensate. Rotating the adjuster down towards 6 o'clock would lower the rocker by up to at least 2 mm.

[KCF21]

Spaceman - thank you - you clearly have far more knowledge about this than me. However, as a bear with very little brain, looking at your last two replies, are you saying that with the valve clearance adjustment that is possible on a C25 (assuming that this is the same as your B25 - which is a sound assumption I think) I should be OK to use the thicker gasket?
Thank you again, this is very helpful,
Kevin

[Spaceman]

Have a look at where the flats are on the RH end of the two valve spindles. If they are close to 12 o'clock then you should have more than enough adjustment left to cope with a 1 mm thicker head gasket. In fact, if the flats are higher than 9 o'clock for up to 2051 C25 engine numbers or higher than 3 o'clock for the later C25 and B25 engines, you should be ok.

What the adjusters actually do is to adjust the height of the rocker spindles so that, if you raise the head using a 1mm thicker head gasket, all you need to do is to lower the rocker spindles by 1 mm. Starting with the spindle flats at 12 o'clock, rotating the spindles down to 6 o'clock gives you up to 3 mm adjustment in the spindle and rocker heights.

But, if the worst comes to the worst, its not too difficult to pull off the push rod end caps and insert 1 mm spacers.