Couple of engine build questions

[Spacekadett]

Just a couple of questions (to start with anyway ) before I start buying bits to build a new motor for the racer.
First one regards the block itself. it's a 1275 S one that's been bored +60. However I can't seem to find a measurement for the block height to check whether / how much its been decked in the past. I did look on here but could only find measurements for 1071 or 970 so wondered if someone could tell me how much taller mine should be?
The other question regards piston choice. The head I've got is an old miglia one with a 17.3cc chamber capacity and I'm wondering if I'll get away with using P21253's or not? An alleged expert reckoned Id need pistons with a slightly bigger (10cc from memory) dish in them, but as he couldn't be arsed to finish our conversation and hung up on me I'm disinclined to take his word for it
Cam choice is currently 649 with 1.5 ratio roller rockers though I'm open to suggestions, this is just what I happen to have in stock!

I'm awarethis is going to need measuring properly when I dry build it but if someone could point me in the right direction before I spend money on all the wrong parts I'd be grateful

[mk1]

I don't have a measurement for the 1275 block on me, but REALLY the only way to measure what you want to measure is to fit the crank rods & pistons & measure the piston to deck clearance. The height of the block is (comparitivley speaking) irrelevant. Having said the above, if you have found the "measurement" for the 170 / 970 on here, the 1275 will be about 1/4" taller (roughly speaking).

The details you give make a C/R of 12.7:1, this is in all fairness a bit high. With a 10cc dish this would come down to 11.28:1 which is a little more sensible. At this sort of C/R I would NOT be using 21253's

Your cam / rocker choice will work well with the CR & head & should give a decent amount of power.

Hope this helps.

Mark F

[wantafaster1]

You may not want to rebore but I managed to machine Keith Calvers +.072 pistons out to a 9.5cc dish. They will give you 1342cc. The pistons now have a couple of thousand miles on, including blyton and some rolling road abuse since to near 8000rpm. I'd certainly recommend them for the money, I also machined the ring lands for correct squish too.
I'm also on a 649 and my compression is nearer 12-1. I machined the pistons as my cylinder head is nice and I didn't want to mess with the chambers or use a special thicker gasket.

[Spacekadett]

Thanks for the advice. I mentioned the block height as the expert had said to check it as if the block had had a lot taken off the top the suggested 10cc might not work iirc. Unfortunately I've only got a set of bare rods and some odd Pistons that are too corroded to dare trying them in the bores think I'll have to see what I can scrounge!

I'm not averse to reboring it, though I hadn't given it a thought having had it honed and deemed usable as is. Like you I want to avoid messing with the chambers as the head is really nicely done as it is.

[wantafaster1]

One of the best tips I've seen was to use an old cam follower that has been lightly reduced in size, as a temporary gudgeon pin. It's how I sorted the deck height and squish anyway.

[coop12g295]

Wantafaster1 I had 4 turned in ally my engineer didn't want to play with followers
on his lathe