Squish Clearance / Deck Height

[Brian Thompson]

I should have thought out my update a little more and I apologise if I have made anyone feel poorly for it.
I appreciate the response and the learning from the squish band talk, but what I should have said is-I need to check that both front and rear cylinders have the same piston heights. I believe that the deck heights front and back can be different(cylinder base mounting face). Hence I would need to adjust the cylinders with different size base gaskets.
But, now I will be checking the squish band and I appreciate the information gained from my wrong wording. Maybe I should do this a bit more.
Once again thank you for your thought's and expertise.I have 3 x 1000 engine's to do and then 2 x 500's. Any help and advice is greatly appreciated. I have "Know Thy Beast" and I have read most of it now.I understand it's not a workshop manual ,but it is a man's life long knowing that gives you an in-site of what happens with these bikes. And I don't want to have to learn the hard way.
Another thing is, how many of these bikes must still be out there? My mate has 5 bikes plus a multitude of parts. His brother has 2 Complete Comets that haven't seen the light of day for about 40 years too. So there must be a lot missing. If you look at club membership being 2500 plus and most of these members would have bikes I guess. Though I don't. 11000 plus bikes built. Where are they all? I guess not all owners are members of VOC.
Just food for thought.
Cheers
brt650

 

[Brian Thompson]

I have 2 engines that will be 1200cc . All TP top ends and TP 30mm cranks and MK5 race cams. TP fitted all the crank bearings and cam and rocker spindles. I guess that's the beauty of him only being 100km/60 miles away.But he has a business to run and he is always busy. So I don't like to annoy him any more than I need too.
Cheers
brt650

 

[Brian Thompson]

Sorry for getting off topic. Should I have started this as a new topic?
Cheers
brt650

(I have started a new thread titled: Squish Height / Deck Height and moved the relevant posts to that thread. Eddy)

 

[Big Sid]

Measure closely the height top to bottom of both cylinder muffs . If identical all is well and the decks will be also . This is sometimes different if one has been trued off top or bottom thus shorter . Sid .

 

[timetraveller]

Just to go back over some of this; a normal Vincent engine does not have squish bands and the only place one normally has to be careful is the clearance between the valve heads and the cut outs in the tops of the pistons. I have found a lot of pistons where the radius of the cut out is marginal and cutting the curves back on their upper edges by about an eigth of an inch ensures that there is plenty of clearance. If using Big Sid type two inch valve heads then even more clearance would be needed. If one increases the bore size to 90 mm from the original 84 mm then it might be thought that there would be a squish band produced but if one measures the diameter of the cut out in the cylinder head this is 86 mm, presumably to allow for oversized pistons after a rebore. So going up to 90 mm only gives a 2 mm wide band and unless the piston crown is machined to match this it is not really a squish band. The Terry Prince head design is much superior in this respect and it sound as though BS has had his heads welded up to do something similar to the TP design. John Emanuel also designed his own modifications to his cylinder heads having had the use of an expert and a gas flow rig. To the eye, John's final design and Terry Prince's look very similar. The difference in engine behaviour is quite remarkable. Several years ago I was in the paddock at Brands helping Roy Robertson. I offered to move his bike from the lower part of the paddock to the upper part for him and made a total prat of myself by stalling the engine, three times, while trying to pull away with 2,000 revs on the clock. There was essentially no torque on that engine at 2,000 rpm. When Roy converted to TP heads the engine became more like a traction engine with regards to its ability at low revs. John Emanuel also found this.

 

[vibrac]

On the big bore motors we raced (before we found that a 500cc place gave us more points than a 600cc win) I ran at 30 thou piston clearance on the anular space on all the motors including the big bore twin. when I built the Methanol motor I specified it on the crown of the Venolia piston I had made Bear in mind that was all done thinking that "deck height" was the distance above a boats plimsole line

 

[hadronuk]

The term “squish deck” makes sense in the context of in line engines. With a shared head and block and due to accumulated tolerances each piston is likely to have a different and overlarge squish clearance.
I “squish decked” my Lotus Elan by having the top of the cylinder block skimmed, (reducing the “deck” height”) then measuring and machining each piston to give the same clearance.
It’s a great mod, because unlike most other engine tuning, it has no performance or engine longevity downside that I am aware of.
I also did the popular mod of fitting a Viper head to Venom to create a squish band. I was very pleasantly surprised how much better the bike went. It certainly had much more low and mid range torque, but top end power also seemed increased in spite of the smaller inlet valve.
Thanks for all the Vincent specific information in this thread.

 

[Big Sid]

Steve Hamel built up our 600 motor , used a Prince crank assembly , modified the cylinder head s chamber and ports . The desired piston shape was carved into an old piston built up with epoxy , so serving as a buck . Once the contours and band were carved into this it was sent to CP pistons and copied . We took over from there .
Interestingly while most building a big bore motor will knock way back it's timing point Steve advised us to run 35 degrees BTDC .
This has run strong from the first start . Sid .

 

[Big Sid]

Should anyone be interested I have photos Steve took of the components laid out before assembly , lovely stuff , and how it looked on arrival here . This Record holder , Steve Hamel , does elegant work . Sid .

 

[Big Sid]

Not a single bit of interest shown in seeing Hamels lovely work on our 600s internals . Shouldn't suprise me really . Sad tho that this is the level of keen interest so often seen today in others craftsmanship . Even a proven Record holder s skills . Sid .