6 replies to this topic

[govig]

Does anyone know what the standard deck height is on a MG Metro block? Unfortunately I 'forgot' to measure it before stripping the motor. It's all going to the machine shop next week and I've worked out that a 1380 with a 24 cm³ head, BK450 gasket and 7 cm³ dish on the pistons gives the following compression ratios with the various deck heights :

0 mm 10.9 : 1
0.25mm 10.6 : 1
0.5 mm 10.3 : 1
0.75mm 10.1 : 1

I'm surprised that a small amount makes such a difference so I want to get it right. I suppose it has to be measured in real life but given the compression height of the pistons is the same as standard, I'd like to know where we are starting from.

Edited by govig, 10 January 2012 - 09:07 AM.

[Notay]

Hi - i don't know the answer but would like to know how you worked this out - could you put your calcs up please? thanks

[Cooperman]

You need to do a 'trial build'. Assemble the bottom end into the block, then fit the pistons to the rods using a set of linished down old gudgeon pins. Remove the piston rings and fit the rods to the crank. Turn the engine to bring the pistons to TDC in turn and measure how far down the bores the piston tops are. Then machine off the deck top the amount they sit down the bores less 0.005".
That's the only way to do it properly.
Now you know why some engine builders are cheaper than others - they don't all do a 'trial build' (also sometimes known as a 'dummy build'. At 'trial build' you should also measure the crankshaft end float and make it 0.002" to 0.005".

[govig]

Thanks Cooperman, I thought that would be the case especially as the machining is being done in the UK and the real build overseas. Having thought about it some more, a trail build is the only way to go.

Notay - I'll scan my back of a fag packet calculations with drawings on here later. I'll soon find out if it's right :) but basically it all works on the volume of a cylinder being pi x r² x h. Pi is of course 3.1416, r half of the 73.5mm bore and h any of the below :

1) gasket "height" ie thickness
2) The cylinder stroke = 81.28mm
3) The deck height.

Add the head volume, the gasket volume, the piston dish, the cylinder volume at BDC to a total and divide this by the sum of the head volume, the gasket volume, the piston dish and the cylinder volume at TDC (given by the deck height) to give the ratio. A zero deck height where the top of the piston is flush with the top of the block at TDC is obviously 0 cm³. Be careful to convert the millimeters to cm if you want to do it all in cm³. Ie a deck height of 0.5mm = 0.05cm and r is 3.675cm. Obvious I know but it's easy to make a mistake.....

There was an excel spreadsheet on the net where you filled in the boxes and it gave you the answer but its disappeared.

Edited by govig, 10 January 2012 - 05:57 PM.

[govig]

For Notay and anyone else who is interested

I think this is about right but would welcome any peer review:

Edited by govig, 10 January 2012 - 06:35 PM.

[ANON]

There was an excel spreadsheet on the net where you filled in the boxes and it gave you the answer but its disappeared.

there are loads of them

http://www.johnmaher.../enginecalc.htm

Edited by ANON, 10 January 2012 - 07:18 PM.

[govig]

Cheers ANON. It seems to agree with my calculations (remembering the combustion chamber variable is of course the head + gasket + piston dish volumes)