12 replies to this topic

[blackbelt1990]

Hi there

Whilst I was getting ridiculed at MED today here I was told that my 7cc pistons where not 7cc at all. I don't believe this as I bought them new, how do I check this?

Thanks Thanks

[Ouster]

Put some perspex with a hole in it over the dish, seal it with grease, get a burette, measure the amount of water it takes for water to fill the volume.

[blackbelt1990]

A "burette" like a pipette? How do I convert the ml to cc?

Thanks Ouster

[Guess-Works.com]

1ml = 1cc

[Cooperman]

Boots-the-Chemist sell 5 cc and 10 cc syringes for a couple of quid. In fact they are very accurate and I always use them. Go for the 5 c one for the piston.
As 'Ouster' said, get a piece of perspex, but put two holes in it, both over the dish area, and as you put the water in one hole, the air can get out of the other. it's easy to see when it's full and to calculate compression ratio you really only need an accuracy of +/- 0.5 cc for a road engine. You do rthe same to measure the capacity of the combustion chambers in the head.
Post the part number on the top of the piston and it's likely that someone here will be able to tell you the capacity of the dish.
If you want more help with the calculations come back on here or PM me. We are all so helpful on here you know and no-one will ever put you down if you don't know what to look for. Everyone has to learn sometime and it's a pleasure to pass what skills and knowledge we collectively have here on to other Mini enthusiasts.

[Ouster]

Out of interest, does anybody know the cc size for the chambers in a 12g940 head as it came out of the factory?

[SolarB]

Is there confusion here between the volume of the dish in the piston and the total volume at TDC (which is a sum of piston dish, volume between piston and deck and volume above top piston ring)?

[james962]

Out of interest, does anybody know the cc size for the chambers in a 12g940 head as it came out of the factory?

21.4cc IIRC

[blackbelt1990]

Is there confusion here between the volume of the dish in the piston and the total volume at TDC (which is a sum of piston dish, volume between piston and deck and volume above top piston ring)?

I don't think so, as far as i know the volume at TDC is quite a bit more than 7, like 30 odd?

[mini13]

another trick i have used if I dont need a masively accurate figure for piston cc is to pop a cd on top, drop the piston on some digital kitchen scales tare it, then fill the dish with water,

as 1 gram = 1cc you can gat a good idea of what the dish is.

[blackbelt1990]

Tried the syringe method today... turns out my 7cc Pistons are actually just over 8cc... great...

Worth keeping or will the 1cc make loads of difference when I try and get 10.5:1 compression?

Thanks

[Cooperman]

Your dish is probably 8.4 cc whih is the standard for several Hepolite pistons for Minis.
In terms of compression ratio, the more critical dimension is how far down from the block deck level the piston crowns are sitting at TDC. It works out at approximtely one extra cc of combustion volume for every 0.010" down from the top. I stripped a 1275 engine which had allegedly been re-built by a Mini-engine expert 3000 miles ago to find that the piston crowns were 0.066", that's about 6.5 cc down from the block deck level and this had lowered the compression ratio to about 8.8:1 instead of the desired figure. I had to have 0.060" skimmed off the block and a bit off the head to get it to 10.4:1 which I wanted.
Don't worry about that extra 1.4 cc as you can machine the head down a bit to get the correct total combustion volume. Just make sure you measure everything accurately and do your sums right.
If I can help further, just PM me,

Peter

[blackbelt1990]

Thanks Peter!