Cams road test PW3 vs JS2

I think that you will get some useful squish effect from clearances of .060 which are very safe. On a stock 850 (like mine) the clearance must be pretty close to .100. It must have some effect or the squish band would not have been provided. My bike will not detonate at all even with 87 octane gas when you lug it an it is hot. I even put 85 octane gas in it this summer and it was OK. Even though the squish effect might be a lot better if the clearance is less it does work with .060 clearance. I have Triumph with wide sqush bands on each side of the high dome pistons. It has measured (not estimated) compression ratio of 9.9 to 1 and it can hardly be made to detonate on 92 octane fuel. The squish bands must be working. Another simillar Triumph engine I have with no squish bands and only 8.4 compression can be made to detonate pretty easily even with 94 octane gas.

I used 12 to one 650cc pistons in my short stroke 500cc Triumph engine for about ten years. Whenever I lifted the head the piston always had black carbon on side of the crown away from the spark plug. With the Norton engine, I believe even the 12 to one pistons don't have such a high crown, and the squish effect must make better conditions in the chamber when compression is occurring. I believe a lot of fuel is wasted in high comp. Triumph engines. My 500cc Triumph performed at it's best when I centre plugged the head setting the plug back on a welded pad, and firing through a slot between the valve seats - plug fouling became a problem so I used to get the motor hot by starting it with two plugs in the normal position, then swapped the leads to the centre plugs. My friend recently increased the size of his 650cc Triumph to 750cc using Morgo Barrels and pistons. It has about a 6mm squish band around the pistons. The more I see of the 850 commando engine, the more I believe in it's design features. I believe the stroke, head configuration are better than the 650 Triumph. The valve train is adequate for the rev limit imposed by the standard crank. The camshaft is it's only difficulty, separate inlet and exhaust cams would have been better.

Hey Guys.
I have built a short stroke engine with;
Noruish crank (80,4), rods and piston (77,5) from JS Motorsports.
95 octan and maximum RPM 8500 - 9000.

Should I go for .040 squish band and compression ratio 9.8?
or
Should I go for .059 squish band and compression ratio 9.33?

Jan

Kvinnhering said:

Hey Guys.
I have built a short stroke engine with;
Noruish crank (80,4), rods and piston (77,5) from JS Motorsports.
95 octan and maximum RPM 8500 - 9000.

Should I go for .040 squish band and compression ratio 9.8?
or
Should I go for .059 squish band and compression ratio 9.33?

Jan

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Hi jan, well, personally, i'd say neither! I'd want more than .040, .045 would be my minimum. And I'd want more than 9.33 CR!

What is it that gives you the difference between your two options? If its deciding whether or not to fit the base gasket for example, then why not fit the base gasket, and them skim some off the barrel to get you somewhere closer to .045?

I agree with you Fast Eddie, but I was hoping to avoid having to remove the piston again and machined them down. :wink:

The difference is the head gasket with .040 and .020 thickness.

Kvinnhering said:

I agree with you Fast Eddie, but I was hoping to avoid having to remove the piston again and machined them down. :wink:

The difference is the head gasket with .040 and .020 thickness.

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Well, you could try it Jan, after all you have steel rods, which will expand and stretch less than alloy and a billet crank which will have much, much less flex.
I had the pistons touching the head at .040 or less, not hitting hard, but just 'kissing' and leaving witness marks, and that was with a norton crank with stock alloy rods. At .045 it did not happen.
Your set up may well flex / stretch much less than .005 less than mine.
The circumstantial evidence would appear to suggest it is worth a try!
Either way, you won't have to machine the pistons, in fact, thinking about it, all you've got to do is skim the head isn't it? Fit the thick gasket and take .015 off the head?
Do I assume correctly that this is a race bike?

No, there is a street bike Eddie, wolf in sheep's clothing (I hope). See this thread; building-new-short-stroke-engine-t12655.html

The head is allready machined down to .040 on a previous occasion, so I should probably take a bit of the height of pistons.

What worries me is the rocking pistons, Will they touch the head, that is the question.

Jan

The rock is greater with short pistons. I was measuring squish clearance yesterday and taking note of piston rock. It seemed the rock was more than I am used to seeing even tho the piston to bore fit is. 0035", quite close. Then it dawned that these are slipper type pistons, of course they can rock a bit more than those with longer skirts.
I used playdough to measure squish, ended up with. 045", steel rods, al.barrel.

Glen

Kvinnhering said:

No, there is a street bike Eddie, wolf in sheep's clothing (I hope). See this thread; building-new-short-stroke-engine-t12655.html

The head is allready machined down to .040 on a previous occasion, so I should probably take a bit of the height of pistons.

What worries me is the rocking pistons, Will they touch the head, that is the question.

Jan

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If its a street bike, and you don't want to take the head off to check (perhaps a couple of times if necessary) and you want to be 'safe' then perhaps you should go with the 0.059 squish / 9.33:1 CR option.
The squish will still work (just), the CR should be fine on pump fuel, and the piston to head clearance will be plenty.

I've been able to slip a piece of soder into the gap and hold to turn over then extract and measure. Only checks one spot or two though. Thank you for the alert of short piston rock factor to keep in mind. Putting the scope of Norton squish together implies there is more to loose than gain pressing luck on less than .045-050" or more gap. Those quotes about .060" gap as common limit for effect may not even hold on the tiny narrow rim of our heads even over bored wider
rims like a 750 head mated to 920. Implies to me the squish is never really effective in our heads unless made a bath tub shroding vlaves which then stifles flame fronts some. This is actually good news to me as means I can keep opening up Peel till 87 octane allows normal commuting operation.

So which helps anti-detonation more in our cases, tightening squish or lowering CR?

It is true that JS pistons will rock in the bore when cold. But when warmed and running they change their shape and the rocking motion is reduced. This is because they are stronger forged pistons - not weaker cast aluminum like Hepolite and forged pistons expand more when hot than cast pistons so they need more clearance. Once warmed up that extra clearance is reduced and you have practically no clearance (if you set em up too tight they will just scuff). There are pistons that are extremely short with practically no skirt at all – but those are another breed entirely. Photo at bottom is a used JS piston from a race bike with .050" squish band clearance. Note that there is no evidence of piston rocking enough to hit the head and this motor revved to 8000RPM. Other brands of pistons hit the head at .038" cleanarce and I figure .050 is a safe mimimum margine. But I have had piston to head interference with stock pistons & rods in my racing days at .050" clearence.

There is also confusion about the squish band. No evidence that I know of has been shown that squish band improved HP in a Norton. To get enough squish band to make a difference (as in an auto engine) you need a bathtub combustion chamber in a Norton and then you lose HP due to valve shrouding. If you raise the periphery of the piston and dish the center then you create more crown surface area and that is a negative that you have to account for and it cuts into your squish band advantage. There are motors that do gain advantage with adequate squish band but they don’t need the piston to head clearance that a Norton needs and they usually have a lot more squish band to start with.



Photo at below is a used JS piston from a race bike with .050" squish band clearance - no evidence of piston to head interference after being revved to 8000RPM. Wear on pin side of the crown was due to a head gasket misalignment. Note piston to bore contact from top to bottom perpendicular to the pin axis, this height adds up to more than the height of a Hepolite skirt alone.

'There is also confusion about the squish band. No evidence that I know of has been shown that squish band improved HP in a Norton. '

Are you making the assumption that something which increases peak HP also increases midrange torque ?

acotrel said:

'There is also confusion about the squish band. No evidence that I know of has been shown that squish band improved HP in a Norton. '

Are you making the assumption that something which increases peak HP also increases midrange torque ?

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I'm making the assumption that its real when its proven on a dyno as it has been for many motors with adequate squish band but the Norton squish band may be inadequate to show on a dyno. Same with the rocking - you can see it with a cold forged piston but I doubt that it is any problem with a hot running piston when the piston expands and the clearances tighten. I'm trying to distinguish between whats real and what is just conjecture. Its fun to talk about ideas but its much more interesting when those ideas are put to the test and witnessed in the real world.

Well in my real/world I will allow for those little devils to rock around a bit, too much is/at stake should they come in hard contact with the head, I cannot afford to build the engine twice.
These are CP Carillo forged slipper pistons, not much for skirt at all.

Yes -but there has been no reported hard contact except when a crank broke and if you look at the photo you will see that the Hepolite piston has no contact with the bore in the piston ring area - its only has the skirt area below the rings to keep things straight. The JS piston contacts the bore from top to bottom and that vertical distance is actually greater than the Hepolite vertical distance - its a better design. So I'll do some more research on this and see if things are as tight as can be allowed when the piston is hot and make adjustments if necessary. But If there is not enough clearance to tighten up any further without risking seizure - then if it ain't broke - why fix it? Be more concerned about crank flex and having that cause interference with the head.

jseng1 said:

Yes -but there has been no reported hard contact except when a crank broke and if you look at the photo you will see that the Hepolite piston has no contact with the bore in the piston ring area - its only has the skirt area below the rings to keep things straight. The JS piston contacts the bore from top to bottom and that vertical distance is actually greater than the Hepolite vertical distance - its a better design. So I'll do some more research on this and see if things are as tight as can be allowed when the piston is hot and make adjustments if necessary. But If there is not enough clearance to tighten up any further without risking seizure - then if it ain't broke - why fix it? Be more concerned about crank flex and having that cause interference with the head.

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I wasn't criticizing your pistons in any way, only pointing out the difference in rock with these Carillo Shorties I have vs longer types. At that they are not going into a Norton, however the principle is the same.
Would all of that difference disappear when hot? I don't think anyone can really be 100% sure on that, so best to play it safe. For sure they can rock around when cold and I might inadvertantly wick it up before the pistons have gotten truly hot.
My crank is like the Forth Bridge, no worries there. Conrods same. I set things up for 45 thou directly above the pin or center of rock, should be OK.

Glen

It would be interesting to take a Norton barrel and JS piston of known room temperature dimensions and heat soak them to equilibrium in a 250F oven, then while at temp, stick a bore gauge in the barrel and mic the piston to see what sort of clearance exists in that temperature regime.

WZ that's exactly what i'll be doing on Peel's deal but will aim more like 300'F.

hobot said:

WZ that's exactly what i'll be doing on Peel's deal but will aim more like 300'F.

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Please keep us posted as to your findings Steve.

Ms Peel is up in the air in soo many ways, mostly because I am trying to build a sports bike spanker that may only have 87octane available. I have my pure factory Combat for a baseline that needs resealing from past ring blow by so will buy proper rod feelers and heat it with a propane tank top heater or buy an old gas oven. Wife would freak out to use "her's". Peel has Maney 920 cylinder with JMS forged pistons which needs a bit wider gap for forged expansion then some more d/t boost heat. Searches say boosted engines need more gap to swell so may need like .006" cold. May turn out the cylinders and piston distort out of round which may reveal where to relieve pistons for that. I guess I'll need two rod feelers to work around the heated bores to find out. I expect to take Peel apart a lot working up state of tune and checking for detonation and binding and cam degreeing.
To find out real value of one cam or another requires miles and miles of heated constant WOT or full on brake or crossing up rear to bleed speed in one direction while converting thrust into new direction. Past Peel had vernier cam adjuster but its locating pin got hammered on 11,000 rpm event and found loose and almost backed out after that flair up. i think I can hammer pin to wedge in enough for degree fiddling then replace vernier cog with re-slotted safer standard cog set to the best i can. One thing I'm relieved to learn here is squish bands are not much a factor in our hemi heads.