Pistons

Here is the new Venolia "tractor pistons" for my bike. I expect another 100.000 miles from them.



A little longer but 20 grams lighter than a stock 850 piston.



A whole lot different than the ultra -short pistons. But look at where the short piston has had contact with the cylinder wall above the top ring.



Here is a well used JE Dreer full skirt piston . Note there has been no cylinder wall contact above the top ring.



Here is the cylinders from the short piston bike. Note the cylinder wear above the top ring. The barrels were worn .004 out of round. They had 3 partial seasons of racing so they were used hard.

Jim,,,,,,,,,,are the pistons coated? And how do compensate for th longer rod?.......


,,,,,Tim

Tim_S said:

Jim,,,,,,,,,,are the pistons coated? And how do compensate for th longer rod?.......


,,,,,Tim

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Tim, I don't see longer rods mentioned above? or have you picked this up from some previous knowledge?

Steve

I think it is the new piston is longer at the skirt but still lighter than a stock piston.
Is the Venolia piston available off the shelf or special order.

Now, now Jim, they're not tractor pistons, those would look equally space age next to my NOS 1976 Omegas I put in my pictures!

They look very nice, and, as you allude to, very durable. Why are they stepped, I would have thought that only necessary if going for a CR above 10:1? As already asked, are they stock items?

I assume you reason that the contact issues above the top ring on the short pistons are due to 'rocking' ? Is it just the area around the valve pocket that has 'gone' ? If so, would you suggest it advisable to remove this prior to assembly? I thought the sharpness of the edges looked like a potential 'hot spot' and would benefit from removing or at least smoothing?

It is hard to tell from the photos if there is more height above the pin or not. Sounds like you better get the class seated.

Russ

Do a search on short skirt race type pistons endurance to see its similar to soft compound race tires, meant to be used up for competition advantage not long going legal public economic useage. The short skirt piston have knocked the socks off young folks on factory MX hot rods who can't afford most a new engine after a season or so of use off road. Jim has not revealed the rod lengths he's installing yet. So far since JSM kits in street use bikes we've not heard of this excessive wear so may not be that big a deal in engines not keep in red zone much as possible. Comnoz is aiming for hi torque engine not hi rpm. If and when my Peel engine with JMS piston and rods finishes her picking fights with elites in pecking order shoot outs next few years, i'll likely revert back to 40+ yr old factory rods and similar long in tooth pistons, set rev limiter down some and get my kix more from stunt play and general traveling, with a surprised sport biker now and then. Peel's flywheel has treadled plug holes to diddle balance factory with the bit heavier pistons.

I am using a rod that is .100 longer than stock plus a stroke that is .080 longer than stock.

The compression height has been reduced by .140 to compensate.

The color of the pistons is a hard anodizing that is impregnated with Teflon. They refer to it as Hard-tuff anodizing. It's something that was made for the military and used a lot on drag race pistons.

The pistons are made to my drawings. I wanted something that would give long service life and possibly live with the addition of a little boost so I kept the top .250 thick and have fit 20 mm thin-walled pins .

The small popup gives me 10-1 with a head that has been opened up a little with the addition of a bigger valve. I will add a spacer to drop the compression if I do eventually add a blower.

The wear over the top ring and wear to the bore is caused by piston rocking. I ran into the same thing when I was using ultra-short pistons in my racebike 20 some years ago. It is not a big issue on a racebike or hot rod that is going to see limited mileage but, it is not something I wanted to deal with on a bike I intend to put a lot of miles on.

I also do not need to worry about weak cases and crank so weight is not a big deal.

I would not want to cut back the ultra-short piston over the top ring. With a high pin and short skirt the whole side of the piston is used for thrust. If you cut it back it will just rock more. Jim

Intersting that Molnar turns the H the other way on the rods. Carillo does this as well. The fellow who supplied my 102mm stroke crank and rods feels this is the wrong way round, likes Carillo pistons but not their rods, so I had to wait about a year to get custom shortened rods made with the H turned the same way as on a stock Norton or Vincent rod.
It seems to be one of those things that can be done either way, a simple matter of opinion, like the short rod/long rod debate.

I'm using some stuff from Molnar right now (cases). Quality is very good, although I have found a couple of "Oops" items, all fixable.

Glen

Hi Jim

Nice piston. Nothing like the Venolia race pistons I had in the 80s.
Keep building them & posting why you do things like this.
I heard Venolia still do small batch runs but that is a big investment.

all the best Chris

worntorn said:

Intersting that Molnar turns the H the other way on the rods. Carillo does this as well. The fellow who supplied my 102mm stroke crank and rods feels this is the wrong way round, likes Carillo pistons but not their rods, so I had to wait about a year to get custom shortened rods made with the H turned the same way as on a stock Norton or Vincent rod.
It seems to be one of those things that can be done either way, a simple matter of opinion, like the short rod/long rod debate.

I'm using some stuff from Molnar right now (cases). Quality is very good, although I have found a couple of "Oops" items, all fixable.

Glen

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From what I have seen it is more a matter of taste. Both styles seem to hold up fine in Nascar engines.

Mr. Molnar is a gem to deal with. Jim

Jim,

Did you specify these pistons to Venolia and have them custom made?

Dennis

dennisgb said:

Jim,

Did you specify these pistons to Venolia and have them custom made?

Dennis

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Yes I did. Minimum order of four. Jim

comnoz said:

dennisgb said:

Jim,

Did you specify these pistons to Venolia and have them custom made?

Dennis

Click to expand...

Yes I did. Minimum order of four. Jim

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Guess that means we all need to buy another bike

dennisgb said:

comnoz said:

dennisgb said:

Jim,

Did you specify these pistons to Venolia and have them custom made?

Dennis

Click to expand...

Yes I did. Minimum order of four. Jim

Click to expand...

Guess that means we all need to buy another bike

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I had them make the second pair .010 larger so I can do a cylinder cleanup if I should have an "accident". Jim

comnoz said:

I had them make the second pair .010 larger so I can do a cylinder cleanup if I should have an "accident". Jim

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I was thinking the same thing after I posted, but with that coating we might wait a long time before they would be used.

When I was racing stock Hepolite pistons, they would not even finish a season. I was exceeding practical piston speed limits and breaking the top rings. I have heard of other racers having excessive cylinder wear with various pistons under hard racing conditions. Maybe the coating will help that. Hopefully the hard coating will reducewear on the piston.

I prefer to use the entire side of the piston as a thrust face (no dead weight). 5 years of cafe street riding & smooth freeway crusing so far with short pistons.

Ref coating of pistons, I've used these chaps before to Teflon coat skirts and thermal barrier coat crowns, with great success. They now do a process called 'DiamonDyze' which looks similar to the process used by Jim. They are UK based, check out: http://www.camcoat.com/

I think I'll try the DiamonDyze coating on my JS pistons.

Fast Eddie said:

Ref coating of pistons, I've used these chaps before to Teflon coat skirts and thermal barrier coat crowns, with great success. They now do a process called 'DiamonDyze' which looks similar to the process used by Jim. They are UK based, check out: http://www.camcoat.com/

I think I'll try the DiamonDyze coating on my JS pistons.

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Nigel,
I've used camcoat for exhausts and been happy, but never for piston coating. What's the script, do you get the pistons coated and then bore the cylinder barrel to suit the size or is the coating so thin as to make no difference to the piston/bore size ?
Ta
sam

Looks like your bike will be a cracker when you've done, I'll look forward to showing you a set of raygun tail pipes when it's up and running :twisted:

trident sam said:

Fast Eddie said:

Ref coating of pistons, I've used these chaps before to Teflon coat skirts and thermal barrier coat crowns, with great success. They now do a process called 'DiamonDyze' which looks similar to the process used by Jim. They are UK based, check out: http://www.camcoat.com/

I think I'll try the DiamonDyze coating on my JS pistons.

Click to expand...

Nigel,
I've used camcoat for exhausts and been happy, but never for piston coating. What's the script, do you get the pistons coated and then bore the cylinder barrel to suit the size or is the coating so thin as to make no difference to the piston/bore size ?
Ta
sam

Looks like your bike will be a cracker when you've done, I'll look forward to showing you a set of raygun tail pipes when it's up and running :twisted:

Click to expand...

They do different coatings Sam. The friction coating I've used on skirts in the past is so thin as to have no effect on tolerances, it is actually used by a lot of OEM automotive manufacturers. The Thermal barrier coating is thicker, but only applied to the crown, so is not relevant to tolerances. When I raced Triumph twins with Degans, we used to modify the heads and pistons heavily to get a good squish band and decent CR. Trouble is, only sock cast pistons were available then and under racing conditions they used to seize. Thermal barrier coating solved this completely and even allowed a hike in CR, I think I ended up at 11.25:1 using stock T140 pistons.

However, they also do a product that they claim can build up a piston skirt by up to 100 microns if it is sloppy in an otherwise good bore!

I'm going to try out the DiamonDyze anodizing I think, and this does not effect tolerances.

Thanks for the offer of showing me a set of Raygun tailpipes... I've never seen any before... its difficult to see them when looking over my shoulder... :mrgreen: