Lightweight forged piston clearances measured hot

I wonder if we really need forged pistons in our relatively low HP Nortons, even those that are bumped up a bit in power?
There are some nice advantages in with running low expansion type (diecast) pistons, the main one being the ability to run at closer tolerances and cut oil consumption. When not running extremely hot, ie general touring duty many of us do,the low e pistons also should provide a little extra oomph as they can run at a tighter midheat range tolerance than a forged piston can. As long as tolerance is sufficient, then the tighter the better for power, oil consumption and cleanliness of oil.
Here I am referring to street/touring type riding where the most riding is done with a warm but not scorching hot motor. Since any piston/bore combo has to have sufficient tolerance to work at the maximum temps it will encounter, then the low expansion type can run much tighter than forged at the lower average temps roadbikes run at maybe 95 % of the time and still be OK for that occasional really hot period.

Glen

worntorn said:

I wonder if we really need forged pistons in our relatively low HP Nortons, even those that are bumped up a bit in power?
There are some nice advantages in with running low expansion type (diecast) pistons, the main one being the ability to run at closer tolerances and cut oil consumption. When not running extremely hot, ie general touring duty many of us do,the low e pistons also should provide a little extra oomph as they can run at a tighter midheat range tolerance than a forged piston can. As long as tolerance is sufficient, then the tighter the better for power, oil consumption and cleanliness of oil.
Here I am referring to street/touring type riding where the most riding is done with a warm but not scorching hot motor. Since any piston/bore combo has to have sufficient tolerance to work at the maximum temps it will encounter, then the low expansion type can run much tighter than forged at the lower average temps roadbikes run at maybe 95 % of the time and still be OK for that occasional really hot period.

Glen

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I have raced some pretty hot Commando motors with cast pistons. Jim

I'd have to agree with Glen (and Jim) on this one. The stock Norton pistons are more than sufficient for the conditions most of us encounter in street/touring riding, as long as the engine is not too far from stock, and you haven't loaded the bike with two riders and 200 lbs of luggage, then took a tour of Death Valley in Summer, with the mixture leaned out for fuel economy. I raced a 750 PR using cast Hepolite pistons for years with no piston problems, other than eventually wearing out the ring lands. I also raced the same bike with 920 Commando engines running cast pistons, and again never had a piston problem. Broken crankshafts, crankcases, and rods, but no pistons. The only piston I ever melted was my fault, for trying to run a Commando engine set up for an alcohol/gas mix on straight race gas, without loosening up the clearance to suit. That doesn't mean I haven't seen cast pistons break in other race bikes. I'm just saying they generally seemed to work fine for me. My 750 engines were only making low 60s rear wheel HP, so maybe the cast pistons wouldn't have stood up so well if I'd been making the sort of power that the Ron Wood/Axtell engines developed. I also had a strict 7200 rpm rev limit for the long stroke 750s.

On a race bike, the forged pistons give you a larger safety margin, because they are stronger and more ductile than cast. That lets them flex more under stress, with less risk of cracking than cast pistons, so they are more forgiving of abuse. But they do require more clearance, and will be noisier. I'm only familiar with the two alloys available from JE for forged pistons, low silicon 2618 and higher silicon 4032, but I think those are the same alloys the other US aftermarket manufacturers use. All the Commando pistons I know of that have been made for Commandos by JE are 2618, including those made for Kenny Dreer, Jim Schmidt, myself, and Steve Maney. It's generally accepted as the best alloy for serious racing use, but is softer than 4032 and most cast alloys, and will wear faster. 4032 pistons are somewhere between 2618 pistons and quality cast pistons in characteristics. It has a higher silicon content than 2618, but less than cast hypereutectic alloys. It's fairly common in high performance OEM car pistons, and also in some forms of racing, but they are not generally considered to be as good as 2618 for the most demanding racing applications, and I've not run into anyone using them on Commandos, although that might change soon. On a street bike that is seriously hopped up, and then run pretty hard for long distances (comnoz comes to mind!), using forged pistons seems like a reasonable bit of insurance.

Ken

Just thought of another data point for cast pistons. The original Short Stroke 750 pistons were cast items supplied to Norton by Omega. I raced a short stroke engine with those pistons for several years, and the only problem I had was burning through the valve recess into the top ring groove. That one was my fault, because I'd pushed the piston so far up into the head, trying to get a decent CR, and I had to cut the recesses deeper to get enough valve to piston clearance with the oversize valves. Still, it worked really well for quite some time.

Ken

I would run cast pistons in my roadbike in a heartbeat if I could get them to fit my oddball motor without ordering 10,000 of them. Actually I have been eyballing some cast JCC pistons made for a Mazda. Jim

Yeah, the ones I ran in the 920 were made for a Ford.

Ken

lcrken said:

... I raced a 750 PR using cast Hepolite pistons for years with no piston problems, other than eventually wearing out the ring lands. I also raced the same bike with 920 Commando engines running [Ford] cast pistons, and again never had a piston problem. Broken crankshafts, crankcases, and rods, but no pistons....
Ken

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We all raced with cast pistons back in the day. But my high revving 750 road racer would not make it through a season with cast pistons without the ring lands wearing out and the rings breaking. Then the cases and cranks started cracking.

If there were lightweight cast pistons available and longer forged aluminum rods - similar to stock Nort rods (but didn't fatigue and break) then thats what I would be using. But neither are available and the stock setup with the comparatively heavy low pin pistons and the short rods just shake and break things too much for my taste.

To follow on to Glen, Jim and Ken's comments, regarding fit and durability of cast and/or lower CTE aluminum pistons, thought it might be interesting to redo the plot provided recently showing effects of temperature on fit of aluminum pistons in cast iron bores for both a 2618 and a 4032 aluminum alloy piston. A significant deviation was made in this experiment relative to the earlier experiment, and that was to employ different CTEs than those used earlier. This was done because although CTE is often employed as a fixed value, CTE actually changes with temperature (increases with increasing temperature), so one needs to bear this in mind and choose a CTE appropriate for the temperature regime being explored.

In this case, MatWeb (web site) gives a variety of CTEs for 2618 and 4032 alloys over different temperature ranges. For instance, from 68F to 392F CTEs of 12.9 and 11.2 (u-inch/in-F) are given for 2618 and 4032 alloys respectively. From 68F to 572F CTEs of 13.4 and 11.7 are given. In this case it was decided that these latter values, encompassing a broader temperature range, were most appropriate for this study. One could also argue that the lower CTEs might be appropriate, in which case it should be straight forward for the reader to examine the existing plot and estimate differences in fit based on the lower CTE values. Lower CTE values would decrease the slope of the lines, thereby extending the usable fit to higher temperatures. And one final thought on CTE might be that a cast Al piston would likely have a slightly lower CTE than those quoted here, depending on alloy, thus exhibiting an even lower slope line.

Although the plot below is a bit busy it clearly shows that, with all other variables held constant, the 4032 piston can be fit significantly tighter than the 2618 piston. As in the past, don't read too much into the plot and heed the cautions provided by all the contributors here when assessing the data. Hopefully as imperfect as the plotted data is, it still has some merit in that it allows the reader to visualize something he/she knew intuitively but perhaps had not previously seen quantified.

WZ507 said:

regarding fit and durability of cast and/or lower CTE aluminum pistons, thought it might be interesting to redo the plot provided recently

Click to expand...

Remind us again where these values come from, and how they were ascertained ?

Its unusual to see temp plots vs etc that are pure straight lines ?
Usually as temps climb, pistons shed heat in more and more ways, and you'd expect to see curves or log curves ??
Also the cylinder expands as temps get hotter, which also adds a curve component to the plot ?

Are these piston metals anything commonly found in Commando pistons ?
7 thou is a fairly hefty clearance to claim that is required to give sufficient clerarance for spirited (hot) running.
Or are these Harley values......

We run the Omega hypereutectics as low as 2 thou in the Vincents. Oil consumption on a 3,000 mile trip to California & back last year was almost nil on Dan Smith's much modified 10.5 to one Shadow. It runs 2 thou clearance Omegas and Honda Chrome rings, just 4 ounces added at home. My Omegas were set up at 3 thou, my Rapide used 8 ounces on the same trip.
Not sure how much one would need to add to those clearance figures for use in a 73 mm or 77 mm bore cast iron Commando cylinder rather than the 84 mm bore cast iron liner and al. Muff of the Vincent.

Glen

I installed some big bore pistons in little Ondas, and to stop the pistons rattling required a zero thou clearance. !
Can't comment on the piston material, alloy cylinders though, with thin steel sleeves.
All still going strongly when sold....

Not as big a piston as a Commando though.

You might find this article interesting :

https://www.highpowermedia.com/blog/376 ... ng-flutter

If I was going to build a high revving commando engine, I'd only ever use forged pistons. For the same strength a forged piston is usually lighter, so the inertia loadings at the ends of each stroke are lower and the bike accelerates faster and so top speeds are faster. In Japanese two strokes of the seventies, pistons were made in a machine where they were spun cast, then forged to get the grain flow better - minimise seizing. In their road versions, the piston rings were usually much thicker than in the racing engines. You will note that some pistons have been Teflon coated. Armacoat comes in water emulsion and can be applied in a 100 degree centigrade oven. The reality is probably that if you keep the revs below 7000, you don't need to accept the expense of playing with that stuff.

Rohan said:

WZ507 said:

regarding fit and durability of cast and/or lower CTE aluminum pistons, thought it might be interesting to redo the plot provided recently

Click to expand...

Remind us again where these values come from, and how they were ascertained ?

Its unusual to see temp plots vs etc that are pure straight lines ?
Usually as temps climb, pistons shed heat in more and more ways, and you'd expect to see curves or log curves ??
Also the cylinder expands as temps get hotter, which also adds a curve component to the plot ?

Are these piston metals anything commonly found in Commando pistons ?
7 thou is a fairly hefty clearance to claim that is required to give sufficient clerarance for spirited (hot) running.
Or are these Harley values......

Click to expand...

Remind us again where these values come from, and how they were ascertained ?

A bore dia (77 mm) is the datum from which the calculation begins. A variety of room temperature piston fits are chosen, then over a range of temperature piston and cylinder dimensions are determined using the specified CTE. The difference in size between the 2 components is the reported clearance.

Its unusual to see temp plots vs etc that are pure straight lines ?

No it is not the least bit unusual to see straight lines related to temperature vs whatever. The CTE specifies a fixed rate of dimensional change per deg of temperature change so there is no other option but a straight line.

Usually as temps climb, pistons shed heat in more and more ways, and you'd expect to see curves or log curves ??

It really makes no difference how piston heat is dissipated, it is the temperature of the cylinder and piston that are employed in this calculation. As I made clear in all cases this calculation assumes cylinder and piston are equilibrated at the same temperature. If this is not the case, the relationship still remains linear for the reasons given above, but a temperature offset would have to be applied.

Also the cylinder expands as temps get hotter, which also adds a curve component to the plot?

No, for all the same reasons stated above the cylinder expands or contracts as a function of temperature in a linear fashion given by the CTE. The bore size and piston size are straight line functions that are simply compared to each other to figure the clearance at a given temp.

Are these piston metals anything commonly found in Commando pistons ?

Did you not read lcrken's comments above about both 2618 and 4032 alloys? 2618 is the go-to workhorse employed in almost all forged Al performance pistons for severe service applications. 4032 is not as thermally durable an alloy, but allows tighter piston fit in lower stress applications.

7 thou is a fairly hefty clearance to claim that is required to give sufficient clearance for spirited (hot) running.

I never claimed anything about 0.007" fit being required for sufficient clearance, that's your interpretation of the plot. I specifically pointed out that the CTE chosen was to encompass thermal excursions to 572F whereas the plotted data range is 100 deg below that. Perhaps the lower CTEs covering thermal excursions to 392F would be more appropriate for the plot as these lower CTEs would reduce the slope of the lines thereby increasing the temperature range over which clearance exists.

Or are these Harley values......

Late model hog cylinders have a spiny lock steel sleeve in an Al muff, employ cast Al, CNC machined, barrel-shaped pistons fit at 0.001" and have piston squirters. If 2618 pistons are employed they are fit at ~ 0.002".

WZ, your chart does indeed provide a nice tool to visualize the effects of different expansion rates on the fit vs. temp, in the same vein as Jim Schmidt's measurements of pistons baked in the oven. As you mentioned, you wouldn't use it directly to calculate what clearance to use, because there are so many other variables involved. But it does help to show why the higher silicon, lower expansion alloys can work with a tighter cold clearance. Your mention of Harley's tighter fit depending partly on the skirt profile brings up another subject. The skirt shape can have a significant effect on how tight the pistons can be fitted. I've been told that with modern design tools, pistons shapes can be refined so that the change in shape as they expand with heat can be controlled to allow a tighter fit than the traditional simple cam-ground shapes. I don't have any experience with this, but I'd like to find out more. I don't think it is something that can be simply applied to the full skirt designs I get from JE, but more likely something that can be done with their high end slipper style pistons, like their FSR (Forged Side Relief) series and their asymmetric NASCAR designs. JE uses some pretty sophisticated finite element analysis (FEA) in designing their higher end pistons. Some years ago I talked to the JE engineers about coming up with a forged slipper style piston for racing Nortons, based on the forgings they were using for modern off-road engines. The bore sizes available would have been limited, and the pistons would have been much more expensive, and the engineers didn't think they were really appropriate for the long stroke, low (relatively) rpm Norton engines, so I didn't pursue it any further. It's pretty tempting for us gearhead types to get carried away with the latest trick stuff technology, just because it's available.

Ken

jseng1 said:

We all raced with cast pistons back in the day. But my high revving 750 road racer would not make it through a season with cast pistons without the ring lands wearing out and the rings breaking. Then the cases and cranks started cracking.

If there were lightweight cast pistons available and longer forged aluminum rods - similar to stock Nort rods (but didn't fatigue and break) then thats what I would be using. But neither are available and the stock setup with the comparatively heavy low pin pistons and the short rods just shake and break things too much for my taste.

Click to expand...

Can't argue with that, Jim. I'm sure that one of the reasons I had such good luck with the cast pistons was the lower rpm I used. The standard stroke 750 and 920 engines I used hit their horsepower peak around 6800 - 7000 rpm, and I was just reluctant to spin them up much higher. If I was still road racing them now, with Maney cases and billet cranks available, I'd probably be comfortable with a higher rpm limit, maybe saving a shift in a corner here and there. In any case, any race engines I build now will have forged pistons. I just like the larger safety margin. The street bikes I'm working on now will also have forged pistons, but mostly just because that's what I already have.

Ken

For a 750 commando the 73mm Honda Fireblade racing pistons almost fit. You'd need to machine the crowns, and use long rods. They are very cheap and strong and very much lighter than the standard 750 pistons. The rings are very thin. - All good stuff.

acotrel said:

For a 750 commando the 73mm Honda Fireblade racing pistons almost fit. You'd need to machine the crowns, and use long rods. They are very cheap and strong and very much lighter than the standard 750 pistons. The rings are very thin. - All good stuff.

Click to expand...

It would be interesting to see how these go when applied to the longer stroke of the Commando. ?

Strong is a relative term - with some bigger G's , they may not be so robust.
There is a reason old big bangers used/needed lots of metal in the pistons...

WZ507 said:

7 thou is a fairly hefty clearance to claim that is required to give sufficient clearance for spirited (hot) running.

I never claimed anything about 0.007" fit being required for sufficient clearance, that's your interpretation of the plot. .

Click to expand...

Thats what the graph clearly seems to show.....

If this is all theoretical, with no real world basis and no Commando input,
we may as well all have had a session of navel gazing ?

Rohan said:

acotrel said:

For a 750 commando the 73mm Honda Fireblade racing pistons almost fit. You'd need to machine the crowns, and use long rods. They are very cheap and strong and very much lighter than the standard 750 pistons. The rings are very thin. - All good stuff.

Click to expand...

It would be interesting to see how these go when applied to the longer stroke of the Commando. ?

Strong is a relative term - with some bigger G's , they may not be so robust.
There is a reason old big bangers used/needed lots of metal in the pistons...

Click to expand...

Cast pistons and lack of decent materials and appropriate heat treatments ? Have a look at a piston out of a Jawa speedway motor . They are stressed to the max.