Piston Deck Height

" Take off a few more points for the extra deep valve seats. "

the Hexhaust would run otter . Which could do anything .

If the seats were clean further out Id find nice oversize valves to suit .

Valve recession is a power loss prodcer . Tho a BA10 with nastilly recessed valves but with 10:1 pistons still manadge 16.9 it was pretty much a barge .
Valves pround in the chamber , long as they clear the pistons & wont tangle , wont slow you down at all .

Unless your planning fitting a good ead shortly , all the high C.R. will do is make it run hotter , and likely further recess the seats / valves .
Thus if youd be so kind as to produce a picture of your chambers , thus confirmindg. You would be advised to go with a approp[riate C:R .
8.5 to one would be fine and broaden the powerbande no end . To high a C.R. can get the engine a bit off or on. Also.

Most realise if you want quick you first minimilise ALL LOSSES . Woulda thought youd have a dozen heads , or 1/2 dozen. Anyway.

Fast Eddie and myself went through this with the Omega pistons. Even with the Omega's in his much modified set up I don't think we got even close to 12:1 in calculation, so not sure what work would be needed to get there. If I remember correctly he was looking for around 10.4:1.
A radical cam fitted does not always make the bike hard to start or ride, with over 10K miles on the PW4, and FA head and standard everything else, the bike puts the power down nicely, even in traffic, torque is fed in nicely no matter from any throttle position. The only problem with it is that those that like 'delivered in one big lump' torque delivery will find it strange.
When I am at work and the bike is there then feel free to come and try it - it is not hard to ride, just very different.

Is your engine set up with a working squish band of 40-50 thou?

Glen

Matt Spencer said:

Not to mention the pushrod lengths after such meanderings .

Click to expand...

Good point. Rocker geometry changes with gasket thickness. My engine needed 1/16" shorter pushrods after valve grinding and decking the cylinder mating surface.

maylar said:

Good point. Rocker geometry changes with gasket thickness. My engine needed 1/16" shorter pushrods after valve grinding and decking the cylinder mating surface.

Click to expand...

We’re going to find out after the rockers get installed and the head on.

worntorn said:

On the subject of piston height/ deck height- in order to get the squish band to work at 50 thou on a standard 850 with RH 10 head I calculate the CR will be 12.2 to one.
I get 13 to one for doing same with a 920?

Are my numbers correct?

RH10 combustion chamber, head upside down and levelled on work bench then filled with fluid to 50 thou over top the squish area.
= 37 cc
828/2 = 414 cc +37 cc = 451 cc

451÷37 = 12.2 to 1
Not gonna work.

For the 920 it's
37.5 CCS in the combustion chamber as the squish area gets widened out to 82 mm or so.
My pistons are 80.5 mm, so it's actually a 910 cc motor, not a 920.

455 + 37.5 = 492.5

492.5 ÷ 37.5 = 13.1 to one CR.

Really not going to work.

Nigel must have gone through all of this already with his 920.
I recall discussion of dished pistons in order to get a close squish while keeping a workable CR.




What is the cubic capacity of the 750 combustion chamber? It might be good to do the calculations.

Glen

Click to expand...

I think your 37.5 must be wrong Glen. Where did you get that figure from?

My 920, with the .040” discs in the crowns, is 46cc.

Mine comes out thus:

917cc / 2 = 458.5.

(458.5 + 46) / 46 = 10.96

I used a syringe to fill an upside down RH 10 combustion chamber.
31 cc brought the fluid level to just breaking over onto the flat ( potential squish band area)
From there I calculated the volume of 50 thousands or 1.27 mm of squish column at 77 mm dia.( 850 sized recess) to get 6 cc for a total of 37 CCS.

I will recheck this today, my wife came into shop and was giving me detailed gardening instructions as I was measuring....

Glen

swooshdave said:

So don’t anneal to resoften the copper?

Where do you get the copper wire?

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No you don't need to soften the copper head gasket - just use the pliobond as described in my earlier post. You can get the .005" copper wire at McMaster Carr.

A .062" thick head gasket with a combat head would only be .020" lower than stock and should be OK with premium fuel (I run this compression on the street).

No gardening instructions today, but same result-

Starting with 40ccs in Syringe



Filled to just breaking over



9ccs remaining = 31 CCS in Hemi dome.
50 thou high squish column adds 6ccs for 37 CCS total.

Glen, how about measuring the CCV of your stock 850 as a cross check. It will be very easy to calculate the effects of greater / lesser piston height as it’s a straight forward cylindrical effect.

And, it may be easier to fill the head and squish area completely, using the glass plate method, and subtract from there?

Maybe your head has been skimmed / changed? Valves protruding more than stock?

Perhaps someone who has dealt with many heads could chime in and say what a ‘normal’ figure would be ...?

Glens figures sound about right.
Better accuracy would be had by using a cover plate.
Valve height will certainly affect that as will the cuts needed to re-angle the valves in a big valve conversion.

Am I correct in thinking that on a standard 850 the pistons are flush with the top of the barrel at TDC?

And what is the depth of the squish step on a stock head?

And finally, what was the thickness of a crushed head gasket on a stock engine?

My reason for asking the above is that we can calculate the theoretical CR of a stock 850 using that 37cc figure as a cross check...

850 pistons are generally flush or a little below.
A compressed original flame ring head gasket is ~.030
The squish band is going to be anywhere from .1 to .125 deep.

Somewhere between 7.5 and 8.5

These stocker 850 pistons were .5 mm below flush at TDC.

I'll recheck my recheck using a flat plate.

Thanks Jim.

Using those figures and assuming a flush piston we’d get:

.13 = 3.3mm = 15.34cc

15.34cc+37cc = 52.34

(414+52.34)/52.34= 8.9:1cr

Or...

.155 = 3.94mm = 18.32cc

18.32cc+37cc = 55.32

(414+55.32)/55.32= 8.48:1cr.


These CRs seem a tad high to me as Norton claimed 8.5:1, but many have claimed it was closer to 8:1, my own was 7.9:1 before I started fiddling. The squish step inc gasket on that was .145” if I recall correctly.

Glen, 414cc / 89mm = 4.65cc per mm. With this it’s easy to calculate the effect of different pistons heights. I’d therefore check the entire CCV inc squish area using the glass plate, then calculate from there.

With regards to the 920 it’s approx 5.15cc per mm so the CR may be different to what you estimated. Also, big valves have an impact as Comnoz has said. My Maney stage 3 head with 5mm & 3mm oversized / re angled valves is gonna have a different CCV to a stock head. Valve pockets will also change things, etc. Hence mine came out at 46cc (measured on assembled motor).

I've checked my other 850 a couple of times with head on, filling with liquid.
The Cr measured at 9 to one +- both times, thin 20 thou head gasket.
With the old copper .040"gasket it was at 8.5 to one.

The 37 CCS number was including a 6 cc or 50 thou squish space.
Dome only volume is 31ccs.

Going the other way, your calcs are with piston flush but neither of my 850s make it to flush.
I remember that my running mk3 was down a bit a TDC and this other parts bike is the same. As mentioned it's .5 mm down at TDC.
So total column height is 4.5 mm with a .040" head gasket.
That recess is 78 mm dia.



Glen

worntorn said:

The 37 CCS number was including a 6 cc or 50 thou squish space.
Dome only volume is 31ccs.

Click to expand...

Oops, quite right, in that case:

.13 = 3.3mm = 15.34cc

15.34cc+31cc = 46.34

(414+46.34)/46.34= 9.9:1

Or...

.155 = 3.94mm = 18.32cc

18.32cc+31cc = 49.32

(414+49.32)/49.32= 9.36:1cr.

So, that actually makes a big difference, and gives figures that I think we’d all agree are not close to stock, so sumthin’ ain’t right...

Fast Eddie said:

Oops, quite right, in that case:

.13 = 3.3mm = 15.34cc

15.34cc+31cc = 46.34

(414+46.34)/46.34= 9.9:1

Or...

.155 = 3.94mm = 18.32cc

18.32cc+31cc = 49.32

(414+49.32)/49.32= 9.36:1cr.

So, that actually makes a big difference, and gives figures that I think we’d all agree are not close to stock, so sumthin’ ain’t right...

Click to expand...

I'm not sure I'm following his numbers but if he only has .050 thou squish distance. That would mean the head has been milled so 9.36 would be believable.

The 50 thou squish clearance is hypothetical.
Given that's about the spacing that one should use for squish with stock crank and rods, I was curious as to what the Cr number would be if the pistons were made to come that close to the factory milled recess, by one method or another.
That hypothetical situation would give a Cr of 12.2 to one for an 850 with flat top pistons, according to my numbers

The stock setup is miles down from there with TDC of piston at 4.5 mm below the recessed flat.
For stock setup, when I add it all up as it sits on my workbench, I get a number very close to Nortons quoted 8.5 number with 40 thou head gasket.
I do not get the low numbers, ie in the 7s, that some have claimed for the stock 850.

Eddie's numbers are with pistons flush, these 2 850s aren't, they are down .5 mm at TDC.


OK I used a glass plate and did squeeze a bit more in. The earlier dome measurement was about 3 CCS low.
Total to fill up to the head joint is 48 CCS.
A 40 thou head gasket is 1mm thick.
Add to that piston down .5mm below cyl.deck

So in addition to the 48ccs we have 1.5mm x Nigel's number, 4.65ccs per mm
1.5 x 4.65=7
48+ 7= 55
414+55=469
469÷55= 8.52 , just like the sales brochure.
Why didn't I just look there?


Glen