Hitchcocks Forum

I now have a new tube of ThreeBond for the Cases on my Woodsman, and have also sorted out the Crankshaft binding problem. Thanks to all for the advice on those 2 things. Now, on to the next question...Has anyone calculated the different compression ratios obtained when using each of the various combinations of Compression Plates & Head Gaskets on the 612 modification?

Not yet but I will be soon. I did post a thing with some of the measurements but then noticed some mathematical errors and kind of tied the thread in knots.



If it helps, the volume of the cylinder head with standard plug and decompressor fitted, measured to level with the gasket facing is 145.6ml.



The volume of the head to level with the top of the spigot recess is 110.37ml.



The domed section of the piston (rising above the flat part above the top ring) is 50.93ml.

Hi Guys----- Hi Woodsman... OK, just to refresh our collective memory}--- 1ml = 1cc and from Bullet Whisperer we have}--- If you divide the Swept Volume by the Stroke you will get a rating of cc's per mm...So, for a 500cc Bullet engine we get}--- 500cc divided by 90mm = 5.555 cc's per mm..............Now if you fit a de-compression gasket of 1mm thickness you are adding 5.555 cc's to the clearance volume ( Because you have raised the barrel by that amount )....Hence the term "De-compression" gasket..............As a Head gasket thickness also effects the clearance volume the same applies here as well.....

So by adding the two gasket "Volumes" as it were together, we can get the total effect that they have on the clearance volume above the piston at TDC................Remember that the Formula for Compression Ratio is}-------Swept Volume (SV) plus Clearance Volume (CV) divided by Clearance Volume (CV) so a small change in CV has a significant effect on the final figure, which is why we try to get an accurate reading as possible......"Hats-off" to Stinkwheel in the previous thread for using just such an accurate approach when measuring the various volumes involved.........OK, so the maths went a wee bit wonky but that can be rectified...

Real world figures.



When Paul (the Bullet Whisperer) built my 612 using our host's kit, he found that - even when using all of the spacers - the CR was around 11:1!



Machining the crown of the piston brought this down to a little more than 9:1.



He discusses this at about 0:55 in this video. https://www.youtube.com/watch?v=4p-5hgbwLYE