Re: Compression/spark plugs
Took a look at the BLMC Workshop Manual (AKD4021) last night just to see if it had any information that might help. It listed a cranking pressure for the High Compression HC engine (8.8/1) of 120 PSIG at 350 RPM. Also took a look at the compression gage we have in the shop and it gave me the following comparisons:
8.0/1 = 118 PSIG
9.0/1 = 132
10.0/1 = 146
11.0/1 = 151
12.0/1 = 175
13.0/1 = 190
The manual also listed HC engines as 8.8/1 and Low compression LC engines as 8.0/1.
If you were seeing 165 on your gage, then you are somewhere in the 11/1 range. My shop landlord's 1275 LPHP engine, built to be just a tick below the 11.0/1 SCCA limit pumped 165/160/170/165 front to rear. These pressures were obtained with the subject gage and probably a bit faster than 350 rpm. But, they are also from an engine using a cam with around 85 degrees of intake to exhaust valve overlap compared to around 26 for a stock cam.
Now with all this additional information, it still sounds like you have a small dish piston with about 0.030" worth of negative deck and you are in the upper 10's to low 11's on compression with the +20 overbore and whatever the head is giving you. Try the BP7ES plugs with a properly timed and carb'ed engine and see how the plugs and tail pipe color up.
Early stock pistons have a shade over 11cc's and the later ones have a shade over 15cc's. Just remember that these engines are over 35 years old and like Forrest Gump's box of chocolates, you never know what you will find in them when you open them up.
The AE21253 has a nominal 8.4cc dish. It is also taller in compression height by 0.020" from stock. The AE21253 were designed to be the Cooper 'S' replacement piston. Like Hap says, they make a great budget piston for SCCA HP racing and will handle rpms past 8K. If you are building a hot street motor, then these are the ticket. Look at the oil control ring oil drain back design and limit the rpm of the engine according to what you find. If you have big slots and side cuts to limit piston slap noise, then hold the rpms down to less than 6K or you might find you now have 2 piece pistons in a cylinder.
The most commonly listed number for combustion chamber volume on the 1275 is 21.40 cc's. But like anything BLMC, take it with a grain of salt. Depending upon valve margins, casting shifts and seat cuts, they could be anywhere from 21.1 to 22.0, coming from the factory. 35+ years later, you don't know what you will find.
Really hope all this new information helps with understanding what you might have. Just put 91-93 Octane fuel in it, drive it and enjoy it over the summer.
Mike Miller