How much to take of a 711m Block

[tdafforn]

Hi Everyone,
Currently getting a 711M block machined to use on a twink and have a question before the deck is machined.
I am using the 711M crank, rods and standard bore hepolite twink pistons.
I have done the calculations for the block height using the 711M stroke, piston bore, combustion chamber volume (37ccs) and gasket thickness.
The result is that for 10:1 compression ratio I need the block decking to 1mm above the pistons at TDC. This means taking 5.58 mm (0.220 in) of the block.
Does this sound correct?
will the tappings in the block for the head bolts need lengthening?
Cheers
tim

[john.p.clegg]

Tim
It's that long since I did mine,but I have no recall of tapping the block after decking,so,just screw the bolts in and measure the "free length" and a quick measure of your head thickness and Voila....

John

[types26/36]

A 711 block fitted with 1600 crank (77.62mm stroke) and standard Lotus pistons (82.55mm) will give a capacity of 1661cc
This gives a volume per cylinder of 415.25 cc
As you know C.R. is the swept volume (sv) plus the clearance volume (cv) divided by the c.v.
C.R. = S.V + C.V. divided by C.V.
I note you have a combustion chamber volume of 37cc
A crushed head gasket will give approx 3.4cc (using an approx figure of .6mm thickness.)
You also say you have a deck height of 1mm (piston to block height) and a 1mm deck height will give approx 5.3cc
all this adds up to a clearance volume of 37 + 3.4 + 5.3 = 45.7cc
So 415.25 + 45.7 = 460.95
460.95 divided by 45.7 = 10.08 : 1
So my calculations seem to be in line with yours
The 1mm deck height does sound a little high as the books I have read suggest a deck height of 0.25mm to 0.50mm
How did you arrive at the combustion chamber of 37cc ? are you allowing for the cut outs in the pistons?
When I have done it before I found its a very inaccurate way of calculating the C.R. adding all the figures separately,
I found it more accurate to do a dummy build with an old head gasket and with the piston at TDC measure the volume
of the combustion chamber with a mixture of oil and paraffin added through the plug hole by means of graduated
laboratory equipment.
I have done the conversion a few times and haven't had the bolts bottoming out, make sure the threads
in the block are clean and the bolts screw all the way down as corrosion can set in at the bottom of the threads.
Measure the bolt length screwed into the block and the head thickness, you can work it out if they will bottom.

[tdafforn]

I am glad my numbers check out..
I hadn't taken into account the cutouts on the pistons however.
I wonder what their volume is..?
I aggree with what you say about a dummy build being best, I am not sure though that I am going to manage it.
My aim was not to go crazyily high with compression ratio as, if anything, the octane rating of fuel seems to be reducing with time at the moment!
I have just looked at the section in "technical" on Lotuselan.net and it recons 0.5-1mm and states that that means removing about 0.2inches from the block which matches with what my machinest is telling me.
I guess I'll sleep on it and let him know tomorrow
Tim

[denicholls2]

I suppose you already know this, but since you're recommending a method without a specific procedure, it's worth mentioning. Sorry for being such a stickler:

Paraffin has a massive shrinkage rate in the transition from liquid to solid. Unless all of these components are kept at sufficient temperature to prevent freezing of the mix, your readings will be crap if the paraffin ratio of your mix is large.

On the other hand, a very few cc's of pure paraffin to start and seal the rings (possibly before putting the gasket on so you can avoid filling in above the lowest part of the piston crown), followed by pure oil at a temperature low enough to avoid disturbing the frozen paraffin would work nicely. Depending upon which side of your target C/R you want to err on, you would add or not add this to your calculation.

[types26/36]

[quote="denicholls2"] "Unless all of these components are kept at sufficient temperature to prevent freezing of the mix, your readings will be crap if the paraffin ratio of your mix is large. On the other hand, a very few cc's of pure paraffin to start and seal the rings (possibly before putting the gasket on so you can avoid filling in above the lowest part of the piston crown)"



Sorry Dennis but I'm not following you here why should the mixture freeze being at room temperature, I use a mixture of two stroke oil and paraffin to thin the oil down slightly so it flows, oil by its self is just too thick and neat paraffin is too thin. I seal the piston to bore (at TDC ) with a smear of grease, I also fit the head gasket coated with grease.
Pure paraffin would not seal the rings (being too thin) and would run straight past them before a final volume could be established.
Just had a thought paraffin is what you guys call "Kerosene" are we having a language problem here
http://en.wikipedia.org/wiki/Paraffin

[tdafforn]

I agree it is a nomenclature problem, Parafin can also be a wax
Tim