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Squish Clearance / Deck Height

Brian Thompson

Well Known and Active Forum Website User
VOC Member
Hello Again,
Bit of an update.
Cases are assembled and the gearbox installed.Crank and gearbox spin nicely.
Oil pump is in. Deck heights next and then onto cam chest assembly. I know I'm a bit slow , but have to get it right.
cheers
brt650
 

vibrac

Well Known and Active Forum Website User
VOC Member
Hello Again,
Bit of an update.
Cases are assembled and the gearbox installed.Crank and gearbox spin nicely.
Oil pump is in. Deck heights next and then onto cam chest assembly. I know I'm a bit slow , but have to get it right.
cheers
brt650

I am not ashamed to say that although have put at least 50 Vincent engines together for Racing and Road and many other British and Jap bikes as well 'deck height' is unkown term to me,what ever it implies I expect I have done it, but exactly what it is I do not know. whenever I have heard it it was an American term closley associated with car engines. Only car engine I ever help build was a K100 :) only motorcycle engine with 4 wheels I built was a VW beetle :)
 

timetraveller

Well Known and Active Forum Website User
VOC Member
Just in case anyone out there does not know any better the article referred to by sidmadrid is for Porche car engines and should not be copied exactly for our engines. On standard Vins, with no squish bands, the solder or whatever should go for and aft so that it covers both valve cut outs in the top of this piston. That is the area which generally causes trouble for us. If you are using squish bands then you should know enough not to need my advice.
 

b'knighted

Well Known and Active Forum Website User
VOC Member
If you are using squish bands then you should know enough not to need my advice.

Too optimistic! I believe that my 636cc Comet has a Nikasil bore and squish band but as I didn't put it together I know less than I should.
While I think that I know what the squish band is I have no idea of the necessary clearance.
I would appreciate your advise for future reference.

Cheers,
 

Big Sid

Guest
On squish band clearance . This is the distance allowed to occur between that surface on the crown of the piston and the surface it approaches on the skull of the cylinder head s combustion chamber . This design helps to create intense turbulence used to spread the flame propagation . It does much to eliminate detonation , spark knock allowing higher compression to be used .
In commercial engines the distance is generally held to about a 40 thou. gap . In our 600 racer it is nearer 20 thou ., and is one real reason you stay aware of overrevving . It is measured using clay or play dough strips during top end build up .
This explanation is rather simplistic but useful I hope . Sid .
 

timetraveller

Well Known and Active Forum Website User
VOC Member
It is good to make the squish band clearance the smallest that one can BUT make it too small and any slop in the big or small ends or stretch of the conrod at high revs can lead the piston the hit the cyclinder head and too large and it does not work. Any figure on our engines larger than sixty thou would be useless and any figure much less than thirty thou would be potentially dangerous. However, if Big Sid has managed to regularly use twenty thou with no problems then I would alway accept someone elses experimental results. Twenty thou is not going to let you get away with much stretch in the conrod or clearance on the big and small ends but if BS can get away with it then maybe that is possible.
It is worth while to consider what the squish band is meant to do. It is intended to create a lot of turbulence in the cylinder just prior to the start of the ignition and combustion. This in turn means less chance of small pockets of mixture prematurely igniting or burning too rich or too weak. It also helps the flame front to move rapidly across the combustion chamber and to ensure as complete a combustion of the whole charge as is possible.
Perhaps I should also have added that the squish band is normally positioned so that the 'squished' charge is aimed towards the spark plug so that the ignition process starts where the maximum turbulence is.
 
Last edited:

Big Sid

Guest
More on squish . If your big end play allows 60 thou fling in the rods length then you got awful noises and lots of problems . Thought that was a given .
My Rattler spun up over 7,000 with no contact after I clearanced it's pistons , the eyelids fully removed on those Specialloid 13 to 1 s to clear the 2 Inch inlets . This also done in Lola . Like to see some of my trophies ? Sid .
 

Big Sid

Guest
Our 600 uses a very modern racing CP short skirt piston , and has a rather short pin , all very light . It has a circular squish band
Fully around its 90 mm bore , this in a 600 Prince barrel , this beneath an original Vincent head that has been reshaped around the chamber to create the squish band . I liked the idea of the big bore topped by the earlier compact 84mm chamber volumn this gives thinking it would give good results . We feel it does just that . Sid .
 
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