12 replies to this topic

[Mart!]

Hiya!
i'm currently rebuilding a 1275 S engine and my pistons are sticking out of the block about .010 thou. is the copper head gasket thicker than the stock gray head gasket? (dont want to have the pistons machined down)

thanks

[Ethel]

I'd get the pistons decked, far better to do it right now than suffer the consequences and have to fix it later.

At high revs they'll stick even a bit further out too.

[minimender]

I'd get the pistons decked, far better to do it right now than suffer the consequences and have to fix it later.

At high revs they'll stick even a bit further out too.

I'll second that

[Sprocket]

Not sure what you mean by grey gasket as the standard gaskets used by Rover were the BK450 which is black and the AF460 which is copper faced. Both these gaskets are aproximately 35 compressed thicknes. The AF460 has a larger cc volume.

10 thou posative deck hight is not much, Id be tempted to measure everything up, calculate the compression ratio and then see where to go from there. Many of the Miglia racers run posative deck clearancies at 8500rpm, infact some engine builders take the clearance to the point that at 8500rpm, the piston touches the head without detremental effects. 10 thou posative deck hight with an 35 thou gasket isnt going to be a problem on a road engine, is it?? Compression ratio might be!!

[Mart!]

Not sure what you mean by grey gasket as the standard gaskets

erm, not sure either! i ordered a gasket kit and the head gasket was light gray, with some sort of aluminum powder on both faces that sticks to your fingers and might make you look like the lost member of KISS if you're not careful...

this engine is put together from parts so i'm not really sure what the compression ratio will be. pistons are 9cc dish +040 Hypatec, S block with S conrods and crank, SW5, head is modified 940 with windsor 1.5 roller rockers and isky springs. i've been wanting to mesure the chamber volume but i'm not sure exactly how that should be done.


pistons are already in so i'd rather not have to pull apart everything to get them decked!

[Ethel]

To measure combustion chamber volume you need the head upside down, level and with the spark plug in. All you do is fill the chamber to the brim using a calibrated pipette - paraffin is good as it has low surface tension. using a sheet of glass or thick perspex with a hole in is another way of eliminating the surface tension.

[Sprocket]

Not sure what you mean by grey gasket as the standard gaskets

erm, not sure either! i ordered a gasket kit and the head gasket was light gray, with some sort of aluminum powder on both faces that sticks to your fingers and might make you look like the lost member of KISS if you're not careful...

Bin it, it has no place on a performance engine of which this clearly is

[Mart!]

AF460 ordered!

cheers

[miniboo]

i was going to say use a BK450 but as you have already ordered the other one....

[Sprocket]

Compression ratio, assume that a BK450 gasket has 2.8cc volume and the AF460 3.2cc volume, Assume a ring land volume of 0.5cc. Head chamber volume is 21.4cc as standard, if the head has been modified and or skimmed, you will need to measure the cc using a method similar to what Ethel describes. cc of piston dish is known, posative piston/deck hight is known. the only thing you really need to measure is the head, then you can work out the compression ratio using a stroke of 81.33 and a bore of 71.63

Assuming the head has been skimmed to give a chamber volume of 19cc, using a AF460 gasket, assuming the piston clearance at the top of the bore is 0.5mm, this gives a -1cc for the posative piston/deck hight, and with all the information you supplied, i work out the compression ratio as 11.68:1 and pretty much requires high octane unleaded. Now re calculating the CR but this time using the standard volume for the head chamber and the same AF460 gasket, the CR works out at 10.9:1

From that, as long as you are sure that the cc in the piston dish is 9cc, the compression ratio should be somehwere around 10.9:1 to 11.68:1, dependant of the head chamber volume when using the AF460 head gasket.

[Mart!]

so i bought a digital measuring gauge, and finally got my specs together:

bore: 71.51mm
stroke: 81.27mm
piston dish: 7cc
AF460 gasket: 3.2cc

still not sure about the combustion chamber cc, but the total head thickness is 68.73mm so hopefully it tells you if it has been skimmed or not. (no idea of the stock thickness) head is a 940.



cheers

[minimender]

still not sure about the combustion chamber cc, but the total head thickness is 68.73mm so hopefully it tells you if it has been skimmed or not. (no idea of the stock thickness) head is a 940.



cheers

Standard head thickness is 69.9 so its had eerrrrrrr 1.17mm off it.

[Ethel]

If the head's been gas flowed the only way you'll know with any certainty what the chamber volume is, is to measure it.