E: Engine Squish

[vibrac]

Tim, your posting #25; I measure the internal volume of a standard post war cylinder head several years ago and made it 103.5ccs. This was done with a thin perspex cover, to avoid the meniscus effect, with a slight bleed hole and a pipette.

That seems mighty low thats 499+103=602 and 602/103= 5.84:1 but of course thinking about it the piston intrudes by 30 or 40 cc into that volume. Its a long time since I did a 499 engine so i guess 70cc would give you just over 8:1
I just have a spread sheet with all the volumes from 60 to 100cc against a swept volume of 572 (big Bore) and that gives me a CR range of 10.5 to just over 6.1
The play dough just sidesteps all that oil mess and leakage and of course when the dough ball is dropped into the measuring beaker it drops to the bottom and the meniscus on the scale is still the meniscus it was before - dont leave the ball in there long it soon gets very slimey

 

[vibrac]

Well I have a ball of play dough after I removed the two snakes from the plug holes and I am left with 81cc which gets me almost smack on to 8:1 CR on a 90 bore with a squish of 66 thou but thats with 3 standard gaskets and two barrel packers one thick one (with a fin) and one thin one about 20 thou I clamped it down hard on the head nuts but not 32ft lbs so I guess the 66 will go down to about 40.
Now at 8:1 I am going to set ignition at 32BTDC as a start point in about the middle of the BTH slot then sit it on Bens new rolling road and see how she goes hoping less advance is not necessary.
Hopefully I will get a road bike with an engine to match the fantastic road holding, heck I may even clean it!

 

[Cyborg]

Tim, your posting #25; I measure the internal volume of a standard post war cylinder head several years ago and made it 103.5ccs. This was done with a thin perspex cover, to avoid the meniscus effect, with a slight bleed hole and a pipette.

This is a NOS cylinder head. The number 103 stamped into the intake flange had me stumped until davidd clued me in. The port measures 32mm across.

 

[timetraveller]

Could someone explain how 103 indicates a 32 mm inlet? I am probaby being dense but I cannot think of a connection.

 

[Cyborg]

The 103 refers to the combustion chamber size.
The 32mm is unrelated. I just thought it was an interesting side note.

Sometimes I find my posts are a little cryptic, so you aren’t alone.

 

[Monkeypants]

Ken, has the port been opened out to 32mm ?
Standard inlet ports are 1 1/8" which is very close to 28 mm.

Glen

 

[Cyborg]

Yes the port has been opened out to 32, although it’s not perfectly round. It’s an interesting head. Never been mounted. The combustion chamber has been polished along with both the intake and exhaust ports. The porting, polishing and oil drainage ditch was all done before the guides were installed. It has what appear to be the original factory guide lock rings with the original stake marks, which suggests to me that all of the work was done at the factory before the head was shipped out. You are welcome to inspect it if you are ever out this way.

I’m probably going to regret mentioning the stake marks.

 

[Bill Thomas]

I stake them still , In fact I did a head that had been done by somebody else,
Without staking, And I was not happy,
Having seen guides that have come loose and the mess that is made.
Cheers Bill.

 

[vibrac]

This is a NOS cylinder head. The number 103 stamped into the intake flange had me stumped until davidd clued me in. The port measures 32mm across.

View attachment 46871

AS I said total head volume has little to do with the volume in the head when the piston dome intrudes into it. In fact you can calculate the pistons intrusion (assuming the plain head is 103) in my case its 103-80 =23cc and then its (572+80)/80=8.15 CR ( if I used 103 if would be 6.5CR ! I don't think so)I think on compression of the gaskets etc my 66 squish measurement is down to 44 thou when measured on the open cylinder so I guess that's nearing the 30 thou minimum under clamp pressure so I need another 10-15 thou base shim however I still need check and if required to do a bit of flat emery board rubbing to ensure I have 3 thou recommended clearance head to alloy barrel as there is a chance its touching somewhere on that flange and its not sitting perfectly on the liner top.

 

[Cyborg]

I don’t think anyone was disputing the fact that the piston dome is factored into compression ratio.
If I was concerned about squish, I wouldn’t be using an emery board to establish the clearance between the muff and head. The mating surface of the head where it meets the muff needs to be absolutely parallel with the recess in the head that seals onto the top of the liner y/n?. Wet/dry paper on a sheet of glass really isn’t the best way to do a street bike either. No guarantee the two surfaces will be parallel. At least when lapping the head onto the barrel with the two grades of compound there is some hope. Best cut them in a lathe or mill.