JS Piston Deck Height

Seeley920 said:

Piston hitting the head... head's not bolted down.

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I missed the visual. Thank you.

concours said:

Well Dan, what's the odd snapping sound?
The suspense...

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Seeley920 said:

Piston hitting the head... head's not bolted down.

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The quench pocket on the drive side is mislocated slightly forward. The Piston crown is catching the edge that protrudes past the cylinder bore. It is very slight. I applied Prussian Blue to the Piston crown and I could barely make out the transfer onto the cylinder head.

If you read the post from edgefinder that I quoted it will shed some light on the problem.

Now you know why my JSM pistons all have a clearance ring machined around the squish area, just like in the JSM instructions!

OK I have a total 0.040" machined off the head/barrel surfaces, but I would still do the same without. Because, really, it's a better idea to go straight for that and dump your base shim, which will both leak and let the barrels scuff around, and then leak some more.

You will still be back here later doing the same thing as in the video with plasticine (Play Doh) to measure piston to valve clearance.

I am sure your reluctance to clamp down is that you don't want to 'waste' a compressible gasket, but if you don't, you will never get accurate indications.

I suspect you are only seeing a light touch because the head is lifting. Also note that with the typical skirt clearance the forged piston is free to move a little (well quite a lot in real terms) within the rings in the bore when cold. Just rock them with your thumbs and watch.

All of the posts regarding rod length and deck height related specifications are moot because, as I have said repeatedly, there is adequate deck clearance. It is a misalignment issue with the quench pocket.

Here is a shot showing the slight Prussian Blue transfer from the piston crown to the cylinder head. It is just right of center at the top.





Here is s shot of the relationship of the quench pocket to the head gasket that came off of the engine when torn down.

Note that while both quench pockets are shifted forward, the drive side is slightly more forward.




I can remove some studs and shift the head counterclockwise ever so slightly and there is no interference. Even without a head gasket present the piston does not contact the head when the pocket is shifted slightly.

This isn't my first rodeo and I am well versed at clearances of the internal components of an engine with altered geometry.

As expected, the new gasket shows a similar misalignment.




My son is coming over this evening to help and learn. After blackening the perimeter of the quench pocket with a felt tip marker, I'll bolt the head to the cylinder so I can reach up inside and scribe a line around the perimeter of the bore. A little work with my Dremel and a sanding drum, I should be good to go. After addressing this minor problem, I'll use some modeling clay to check piston clearances and a round 2mm wire gauge to check valve head interference at overlap. If the 2mm gauge clears the valve heads at overlap I'm going to go 1 1/2mm oversize on the intake valve since the seats are a bit pitted and need to be re-cut.

I'll post updates as they become available.

Are you talking about qunnch our squish band clearance. Closer squish bamd clearance probably stops quench from happening. I think the quench effect is the effect where poor mixing within the cylinder adversely affects performance. Squish band clearance must be enough that the piston does not touch the head when the engine is rinning, or cause detonation. From memory, in a Manx 30 thou was common, but with plain big end bearings ecen closer might be used. If you put kid's play dough between the piston and head, you can section it with a razor blade to see how much clearance, you have.
With a dial micrometer on top of the piston, measurement might be more difficult.

acotrel said:

Are you talking about qunnch our squish band clearance. Closer squish bamd clearance probably stops quench from happening. I think the quench effect is the effect where poor mixing within the cylinder adversely affects performance. Squish band clearance must be enough that the piston does not touch the head when the engine is rinning, or cause detonation. From memory, in a Manx 30 thou was common, but with plain big end bearings ecen closer might be used. If you put kid's play dough between the piston and head, you can section it with a razor blade to see how much clearance, you have.
With a dial micrometer on top of the piston, measurement might be more difficult.

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The closer the piston crown to head deck clearance, the more quench that occurs. However there is a minimum distance to allow for expansion and lengthening of the rod, especially at the EX/IN. Normally, .035" is considered adequate for automatic transmission vehicles and .040" for manual equipped vehicles as a bit of insurance if a shift is missed and the engine overrevs briefly. Anything over .055" diminishes quench dramatically and even though CR might be lower, preignition (detonation) can be exacerbated. I should be at about .044" so I have a bit of cushion. Closer quench distance helps prevent detonation.

This was just a quick, easy test for something I was anticipating.

AGAIN, I'm not new to this although this is my first Norton. My 1st altered geometry engine was a '64 Panhead with flathead 80 wheels offset .120" for a stroke of 4.520". This was not a "stroker kit" and I had to verify everything. The older Harleys are easy to work on because you can bolt the timing side case in the engine stand, bolt on the cylinders assemble the rotating assembly and check for bottom end as well as top end clearances. I had to take a file to the inner piston skirts for clearance at the bottom of the stroke. Intake valve clearance to the piston was .055". Plenty for the intake as it was on the opening but not adequate for an exhaust valve on the closing event IMO.

Kerplunken

Head must have been machined after lunch at a pub. lol

Barn yard engineering thoughts:

Did you try running any of the other bolts down into the barrels without fully tightening them to see if the head shifted enough to eliminate the interference?

Looks like CNW nuts and bolts are being used. If the 3/8" stud inserts are not 100% straight and perpendicular to the head surface, it would move the head around some. The other bolts might help realign it.

Your plan to use the Dremel will give you the space and a larger safety margin.

BTW, I installed JS high CR Wiseco long rod pistons in a 750 and had no interference to deal with. Different motor and irrelevant. Just bringing it up for the hell of it.

Fun with old Nortons.

Schwany said:

Kerplunken

Head must have been machined after lunch at a pub. lol

Barn yard engineering thoughts:

Did you try running any of the other bolts down into the barrels without fully tightening them to see if the head shifted enough to eliminate the interference?

Looks like CNW nuts and bolts are being used. If the 3/8" stud inserts are not 100% straight and perpendicular to the head surface, it would move the head around some. The other bolts might help realign it.

Your plan to use the Dremel will give you the space and a larger safety margin.

BTW, I installed JS high CR Wiseco long rod pistons in a 750 and had no interference to deal with. Different motor and irrelevant. Just bringing it up for the hell of it.

Fun with old Nortons.

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Jim Schmitt and I have been exchanging PMs and we have concluded that the pockets are misaligned. Yes, Friday afternoon after a barely pop lunch at the pub I suspect.

This boys and girls, is why you should always check things when you make changes!!

I am accustomed to quench being something else. Whwn you raise compression, you need more fuel to restore the balance between como.ratio, ignition timing and fuel / air ratio to maintain combustion condirtions. THeoretically, because you use more fuel you should gain more energy. Hou do not aalways gain as much as you might expect, due to quench. If you use a tight squish band you get better mixing within the combustion chamber which improves the efficiency of the process.
If you look at any Triupmh 650 cylinder head after it has been used for a while, there is always coke on the crown on the side which is away from the spark plug. It is probably the reason Nortons have better heads than Triumphs. Triumph 650s do not usually have squish bands in their heads.
In effect, you can probably go either of two was - get the mixture and timing right without changing the compression ratio - or raise the compression ratio and get the mixture and the timiing right. Either way might produce the same result. The problem is - unless you race, keeping the jetting right can be a pain. I always avoid moving my ignition timing. If it moves, it neans playing with jets.
The alternative is get the jetting right using an oxygen meter, then asdvance the timing on the the dyno to get maximum torque.

I think squish produces more power, and quench is the reason you do not get as much power as you might expect.
Car information is usefuel, but car guys cannot race motorcycles. Motorcycles which are developed by car guys are often very strange. Many people work on specualtion - not experience.
One thing I would never do these days is help a racing car guy get into road racing motorcycles. They invariably stuff themselves.I knoe three who have done it big-time. To them road rascing must look easy. and you cannot tell them anything.

acotrel said:

I think squish produces more power, and quench is the reason you do not get as much power as you might expect.
Car information is usefuel, but car guys cannot race motorcycles. Motorcycles which are developed by car guys are often very strange. Many people work on specualtion - not experience.
One thing I would never do these days is help a racing car guy get into road racing motorcycles. They invariably stuff themselves.I knoe three who have done it big-time. To them road rascing must look easy. and you cannot tell them anything.

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Did you read the article? "Squish" is the layman's term for "quench" they are synonymous. The closer the quench height, the more quench you achieve as long as you don't run into the head with the piston. Quench increases efficiency and power. I learned about quench over 30 years ago. I was running. 035" quench in my third generation 5.7 hemi. I was cranking out over 235 lb pressure and yet we could crank more ignition timing into it without detonation. It was 10.5: 1 static compression ratio. Had a given compression ratio, the closer your quench height, the more ignition timing you can dial into it without spark knock.

This tuning was done with all the electronics, fuel injection, ignition timing and knock sensors.

As soon as I get things ironed out with my Trixie, we are going to Ottawa to put her on a chassis dyno solely for monitoring A/F ratios. Once I get that established I can play with the ignition timing.

Dan1950 said:

Jim Schmitt and I have been exchanging PMs and we have concluded that the pockets are misaligned. Yes, Friday afternoon after a barely pop lunch at the pub I suspect.

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Doubt it's a one off issue. Probably a whole gaggle of heads out there with the same issue.

"This isn't my first rodeo and I am well versed at clearances of the internal components of an engine with altered geometry."

We had a Ford Contour for awhile in the 90s. It's virtually the same car as Ford Mondeo sold in Europe and the UK.
It wasn't a fabulous car, although the DOHC Cosworth V6 it had was pretty nice.
Anyway I recently saw an actual Mondeo for sale locally. I wanted to buy it, still might. Why?
If people ask about the car, I could say.

" This isn't my first Mondeo"

Speaking of rodeos: I have not been to a rodeo in a long time. More than 60 years at least. Last time I went to a rodeo was in Scottsdale Arizona in the late 50's, early 60's. Next time I go to a rodeo I will be able to say with confidence this is not my first rodeo. I could say I have altered geometry right now since I think I've entered the shrinking height old man stage.

Since I'm not that in touch with my inner anal over thinking machine, I probably would have taken a Dremel to this 850 head already without all the techno talk. I'm funny that way though.

The kerplunken video certainly is a good lesson for people installing aftermarket performance parts in an old Norton.

Degree wheel? We don need no stinking degree wheel!


The red paint on the idler gear is hard to see but it's spot on.

Dan1950 said:

As expected, the new gasket shows a similar misalignment.

My son is coming over this evening to help and learn. After blackening the perimeter of the quench pocket with a felt tip marker, I'll bolt the head to the cylinder so I can reach up inside and scribe a line around the perimeter of the bore. A little work with my Dremel and a sanding drum, I should be good to go. After addressing this minor problem, I'll use some modeling clay to check piston clearances and a round 2mm wire gauge to check valve head interference at overlap. If the 2mm gauge clears the valve heads at overlap I'm going to go 1 1/2mm oversize on the intake valve since the seats are a bit pitted and need to be re-cut.

I'll post updates as they become available.

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I don't have this problem with my cylinder head, but, I did buy a Fullauto!

First thing I did was mark the perimeter of the quench pocket with black permanent marker.




Then I bolted the head to the cylinder and reached up into the bore and scribed a line around the bore perimeter..





Bear in mind that this line represents the amount of intrusion which is a bit more than it appears.


After that I placed the head gasket over the studs and scribed another line around the ID of the gasket.



I will use my Dremel with a sanding drum to relieve the pocket out to the line that represents the head gasket leaving the line.