Triumph 5TA piston in a C15?

[shoal48]

good day all.
I'm just starting the rebuild of a '61 C15 which has lain in bits, in my brothers garage for the past 35 yrs. He bought it from the original owner in this state, who took it apart over 40yrs ago, so its definitely time it was back on the road again . My first bike was a ci5 back in the 60's which I tuned & rebuilt cafe racer style. I learned from this that if I wanted speed then start with something else , so the plan is for a pretty original resto but upgrade the engine a little for a bit more mid range power I have a new Alpha big end assembly and a scrambles camshaft and 376 carb & was thinking port the head, fit ss80 inlet valve.

This leads to my current question: I have a serviceable +40 std 8:1 piston/barrel but wonder whether the other mods will be worthwhile without raising the compression, as per ss80 with 8.75:1.

I have a new standard Triumph 5TA piston/rings/pin/clips which if barrel is bored to 69mm will fit but in the Triumph this gives 7:1 compression. I believe the combustion chamber of a c15 is a little smaller than the triumph engine and with the larger swept volume at 69mm would presumably increase the compression ration & the power very slightly but, looking at the piston crown, which is a little flatter than the std BSA piston, I suspect I would still end up with a lower compression ration than standard which probably defeats the objective. so, my question is, has anyone fitted a 5TA piston, do you know roughly what the compression ends up as and does it improve or degrade performance noticeably. The only comment about this conversion I have found is that it works well, but no specifics.
Any comments or advice would be very welcome

[scifi]

You should use a set of callipers to measure the distance from the gudgeon pin hole to the top face of the piston.   Then it is much of a guess to tell which sticks up the most.   I had the trials piston which was flat, and a road piston which was slightly curved, and a Methanol 12:1 piston which had about a half inch of dome.

Remember that the total volume above the piston is only about 25cc, so every 1cc will make a big difference, and maybe push the CR beyond modern pump petrol.

If you do the porting to match the carb and a bigger inlet valve, measure the weight of the valve, to make sure the springs can prevent valve-bounce at maximum revs.   Stronger springs can give the problem of pulling the head of the valve ( not sure if this is more for the inlet or the exhaust.)

Having messed about with these sort of things in my youth, and made many mistakes, I would settle for stock these days.  I hated having to put the push rod tube back on..!

[rhyatt]

I know you have an Alpha big end , is it plain bush or roller.
If former , I'd stay at lower comp

[shoal48]

I know you have an Alpha big end , is it plain bush or roller.
If former , I'd stay at lower comp

The alpha big end is a roller so raising compression a bit shouldn't be a problem although I learned a bit from my first c15...alpha big end and Timing side conversion, Bob Joyner sport cam, 10:1 piston, 376 carb, bigger inlet, ported. It went well for short periods, seized the big end twice they dont rev well, so looking for a modest increase in mid range rather than top end  which I think the scrambles cam should help with along with better breathing. Its the 5ta piston I am really unsure about, from gudgeon pin to crown its slightly lower than a standard c15 piston so I guess the compression ration will be lower than standard but wondered if anyone had bored out to 69mm & fitted one

[rhyatt]

I'm just thinking about the clearance from a new 69mm bore and the existing oil drain hole from the head down through the cylinder to the bottom end.
Will you have enough room ?

Cr not higher so shouldn't be a problem ,
What about any lift ratio with the new cam , will the valves clear the lower piston crown profile ?

[shoal48]

I believe a lot of folk have bored c15 barrels to 69mm & fitted triumph pistons so I would not have thought the oil drain would be affected, but I will have a good look.

With regard to valve/piston clearance, the cam would be a standard fitment on a scrambles or ss80 engine and in these cases the piston crown on an 8.75:1 or 10:1 piston would be higher but with deeper valve cutouts. What I have always done when building a non standard engine is to stick some bluetack on top of the piston and slowly turn the engine by hand to check clearances.

At present I am not sure which version of the S cam I have, upto 1961 the scrambles cam was an intermediate profile, between the standard and the SS80 profile giving more midrange power, after 61 it seems BSA used the SS80 profile for both C15S and SS80 engines. I will know more when I get a protractor & dial gauge on it

According to the bible (Mr Ratios unit singles manual) a rocket3/trident piston is an option having the same bore and combustion chamber size as a c15 so one of those should deliver the same known compression ratio as in a trident engine although getting a single piston may be an issue & I'd have to be sure the crown profile didnt interfere with the valves as you suggested.