I've just finished stripping the engine of my project Rossa (1275 A+ from an MG Metro) and I'm not surprised to find that it's a dog, just like the rest of it It will be getting a rebore, crank regrind and a skim off the top of the block and face of the head just to make sure they're flat. It occurred to me that the head could be skimmed a bit more to raise the compression ratio a bit from its present 9.4:1 but by how much, roughly speaking? I realise that I'll need to measure the available thickness first but I'm just looking for a guide. Many moons ago we used to take 90 thou off the 12G295 heads to raise the comp ratio when putting them on 998s but I haven't had any experience of the 1275 with its 12G940 head. And just for the record before somebody suggests a change - I want to stick with the A series for its simplicity (no electronics or pesky ECUs) and its nostalgia value

If it was me

I would measure the volume of the chamber with a burret [filling it with a known amount of liquid] then with some maths you can work out your compression ratio, it will tell you what the current compression is

WEST

If you're having a rebore you would be better off controlling the compression ratio with the pistons.

These pistons



give close to 10:1 with the standard chamber volume.

Raymond,
Invest in a copy of Vizards tuning the A series book, it'll tell you everything you need to know including what skim first. You can run 10-10.5 cr although you would probably need to use super unleaded especially at the higher ratio. The reason you had to skim so much off the 12g295 when fitting it to cooking 998 is because they usually had dished pistons where as Coopers, 1100 Spridgets and MG 1100s used flat top pistons

Mmmm...........pistons Should have thought of that earlier if I'm changing them anyway I've just investigated the cost of maching locally and got quite a shock - how about £100 to bore 4 cylinders, £70 to grind the crank, £52.50 to skim the block and £45 to skim the head? And that doesn't include shells or anything else. These prices are almost double what Minisport charge and they do a half engine unbuilt for around £430 exchange so that may be the way to go.

I have a copy of the Vizard book, Tim, that I bought new when it was first published back in the 60s (giving my age away now ) so it's a bit out of date now. Maybe I should invest in a later copy.

I'm off to the garage now to investigate the diff - the output shafts are flapping around so the bushes are probably worn. I've never stripped a diff before so should be fun I'm not looking forward to setting the preload on it though...

Raymond, i pay £30 locally for a head skim and if i want a particular combustion cc i take my burette and if more needs to come off he does it for no extra charge It's worth bartering or asking for a cash price in these difficult times especially if it's a small business.
The later Vizard book covers the A+, MG Metro and turbo variants too so it may be worth investing in a copy

Cheers Tim Problem is that there is only one machine shop locally and he has a monopoly. Also, he is very well known for the quality of his work and his name adds value if you're selling something that he has worked on - I've seen engines on ebay with his name mentioned in the blurb. Maybe I'll just have to bite the bullet and pay up I really want to recondition the engine I have because I can prove via the log book that it is the original from the 1990 donor therefore 1990 emissions apply and it will also be a big help in getting a 1990 reg rather than a Q. I could transfer the engine number plate to another engine but I can't replicate the fixing rivets and don't want to take any chances with testing and registration.

The rivets are available. Also don't forget even if you keep your original engine you'll have to remove the plate if your block needs skimming.

Ah! I didn't know that the rivets are available - thanks for letting me know. Presumably from MiniSport or MiniSpares - I'll have a look. I hoped that they could skim the block without touching the projecting bit that has the number on it.

The CR of my engine as standard is 9.4:1 and I don't want to go lower but if I use 9.75:1 CR pistons will I be ok with normal unleaded? I had enough of searching for Super when I had a Subaru

Should be fine Raymond, a visit to a rolling road to optimise fuel and ignition may be a good idea
If you don't have an unleaded head then some of the decent adaditives (spelling i'm finding it increasingly difficult these days ) also raise the octaine (i think the Castrol one does ) it's worth looking at Millers too

No, you'll definitly loose the plate if you skim the block I'm afraid.
If you don't mind about originality you can just stamp the number into the block - there's no legal reason you need the plate.

Tim at Minimail may be your best bet for the rivits: FIXING SCREW - ENGINE NUMBER PLATE Product Reference: CP343S 50p
http://www.minimail.co.uk/ 01989 720111
He can do reverse stamping for the plate as well.

Be aware the rivits aren't really rivits at all, they're more like threaded nails if that makes sense!
A bit like this:


They're also very hard - don't just let the machine shop grind the heads down, you'll never get the shanks out of the block.

To get them out you have to prise under the heads, once you've lifted them up a bit you should be able to pull them out....
.... Or you could just rivit the new plate slightly further along......
......or just stamp the number into the block!

Thanks for that I knew that my engine was a bit of an old nail but I never thought that I'd be nailing it together...

They are really difficult to drill out if the heads are gone. They are harder than the cast iron block so the drill tries to wander.