Lightweight forged piston clearances measured hot

Today I heated up some cast iron cylinders and a lightweight piston to check what happens to the clearances when things warm up.

TEMP measured with lazer temp reader:
JE Forged pistons grow .007” in diameter from 75 to 250 deg
They grow .008” from 75 to 290 deg

Cast iron cylinders grow .002” from 75 to 250 deg
They grow .003” from 75 to 280 deg

Yes pistons get hotter - but these are the specs I'm showing today (mind you this was a spur of the moment idea after seeing my wife pull breakfast out of the oven).

Oven heated piston below measured with lazer temp reader



This means that when you install lightweight pistons with a room temp clearance of .0055 to .006” you will end up with about .001” clearance or less in a running motor. The pistons will be tight in their bores and you don’t want them any tighter.

There have been a few Hi RPM racers who avoid cylinder wear by treating their cylinders with silicone carbide or switching to Castor (bean) oil. Most Notable is engine builder Herb Becker who solved his 10,000 RPM 500cc Nort twin bore wear problems with the Bore Tech Silicon Carbide treatment. There is also Dave Watson in the UK who’s remarkable 1000cc Norton Seeley finished and won all open classic races in the UK in 2012 – beating all tridents, Westlakes as well as all the Jap four cylinders in all races – taking the championship – he had severe bore wear problems using stock rods and older design heavy pistons with the low pin location which would effectively only rev to about 6000 RPM – when he switched to castor (bean oil) and lightweight JS pistons for the 2012 season his effective RPM range jumped up to 7300 RPM and there was no measurable bore wear at the end of the season (bean oil has a higher film strength than other oils).

A normally ridden and average performance Norton will have no bore wear problems with either stock type pistons or lightweight pistons. There are plenty of riders out there with Stock Nortons and Nortons with lightweight pistons who know this from experience. But if a road Racer exceeds the practical piston speed limits at high RPM and experiences bore wear problems then he knows how to fix it by following the examples mentioned above. Those who thrash their machines and put wear on their bores will begin to have blow by and piston rocking the same as any worn out motor. The harder you thrash it the sooner its going to wear out.

I’d like to see more solutions on this forum. I’d like to see more advancement of technology for our old classics to make them run smoother, faster and more reliable. The old soft cast iron bores work fine in normally ridden street bikes but soft grey cast iron begins to fall behind when performance increases to higher and higher levels. That’s why modern bikes and companies like LA sleeve have left soft gray cast iron behind and switched to harder materials.

Most racers have no bore wear problems with soft gray cast iron which can then lead to piston rocking problems. Its the extreme racers who have most of the problems. See photo below of the only raced lightweight piston(s) I have on hand at the moment. After a season of racing its in perfect condition with no indication of rocking or adverse wear. Part of the reason may be because this is a medium compression piston (stock level compression). It was replaced with lightweight HI compression flat top pistons that are still in the motor.

Re: JS lightweight forged piston clearances measured hot

Such theraputetic test insights for me Jimmy. [hope ok nick name amoung all the Jims and Kens with Norton DNA?] Do it again at bottom of cyclinders for me and Peel please. See that little piston bottom wittness area, needs an oil resivoir just above the metal to metal contack and maybe couple either side near rings to handle the scoring contact off to each side of main thrust face. What's your opinion of Peels old fashioned aircraft .0055" to .0065"Canaga tappered Maney 920 bores? I could die with 8000 rpm tolerance. Caster oil is best stuff to press tight tough interference fit items too.

Ever think of putting a reed in a flute?
https://www.youtube.com/watch?v=qc9Zc2g9D94

Re: JS lightweight forged piston clearances measured hot

If you have cam ground pistons - which all are these days ? - then as things heat up,
the clearances can theoretically go negative - but the piston squeezes itself into/out-of shape,
and things stay good...

Piston tops can get more like 500 degrees, if you use those heat sensitive paint dabs.

??

Re: JS lightweight forged piston clearances measured hot

Rohan said:

If you have cam ground pistons - which all are these days ? - then as things heat up,
the clearances can theoretically go negative - but the piston squeezes itself into/out-of shape,
and things stay good...

Piston tops can get more like 500 degrees, if you use those heat sensitive paint dabs.

??

Click to expand...

I was under the impression that cam ground piston shapes complimented the piston design (radial variation in skirt thickness, skirt rigidity, gudgeon boss mass, etc) whereby at operating temperature the piston was round and of appropriate fit in the bore. Intuitively something just seems terribly wrong with clearance going negative. I'd call negative clearance a perfect formula for a seizure.

Re: JS lightweight forged piston clearances measured hot

WZ507 said:

Rohan said:

If you have cam ground pistons - which all are these days ? - then as things heat up,
the clearances can theoretically go negative - but the piston squeezes itself into/out-of shape,
and things stay good...

Piston tops can get more like 500 degrees, if you use those heat sensitive paint dabs.

??

Click to expand...

I was under the impression that cam ground piston shapes complimented the piston design (radial variation in skirt thickness, skirt rigidity, gudgeon boss mass, etc) whereby at operating temperature the piston was round and of appropriate fit in the bore. Intuitively something just seems terribly wrong with clearance going negative. I'd call negative clearance a perfect formula for a seizure.

Click to expand...

:mrgreen: :idea: :!:

If the piston can expand where it has big clearances, and thus not expand where the clearance is important (thrust faces only really),
that is an excellent idea and result.

And since such pistons have been in use for how much of the past century by now,
not exactly a new or startling concept.

Rohan said:

If the piston can expand where it has big clearances, and thus not expand where the clearance is important (thrust faces only really),
that is an excellent idea and result.

And since such pistons have been in use for how much of the past century by now,
not exactly a new or startling concept.

Click to expand...

Running with negative piston/bore clearance is definitely a new concept.

In light of comnoz recent oil temp video, where he had cylinder temperatures of 390F, thought it might be interesting to use coefficient of thermal expansion (CTE) values of cast iron and 2618 Al alloy to calculate piston/bore fit to gain further insight as to when "fit" trouble may arise at a given piston clearance and temperature.

The model system employed to calculate piston bore fit at various temperatures consisted of the following. An 850 gray cast iron CDO cylinder with 77 mm bore and a forged aluminum piston from 2618 alloy (JE). The values used for CTE were 11.4 for 2618 alloy and 5.8 for cast iron (u inch/inch-F). Various RT (72F, 22C) piston fits were employed to observe clearance from ambient to 450F. The results are presented in the plot below and dovetail reasonably well (at least in my mind) with jseng1 suggested clearance of 0.006".



However, a word of caution is in order when interpreting the data because it is assumed for the calculations that all components are fully equilibrated at the specified temperature, whereas in the real world we know that both barrel and piston temp will vary from top to bottom, and that each component could also vary in temperature.

The final thought on my mind regards the cylinder temperatures comnoz observed (390F) when things equilibrated at ~ 65 mph after nearly an hr of cruising. What sort of cylinder/piston temps might be observed if the pilot were so ruthless and cold-hearted that with the engine at full operating temp he/she headed up a long gruelling rocky mountain grade while maintaining 2/3 throttle for a minute or two? In my mind 2 minutes at 2/3 throttle with the poor engine bellering for its life amounts to several eternities of time.

You misquote me / don't understand.

When the piston gets hot enough that this could be a problem,
the piston squeezes/expands into the cam ground territory.
No problemo.

No one says anywhere that pistons have to be ROUND ?
Its only the rings that have to seal, and the thrust faces take the thrust.
How the pistons behave behind that is in the realm of the designers/metallurgists/engineers.

Re: JS lightweight forged piston clearances measured hot

Rohan said:

If you have cam ground pistons - which all are these days ? - then as things heat up,
the clearances can theoretically go negative - but the piston squeezes itself into/out-of shape,
and things stay good...

Piston tops can get more like 500 degrees, if you use those heat sensitive paint dabs.

??

Click to expand...

That's right.

And one of the things I spent a lot of time on is getting the cam grind right - going through several different cam grinds till I found what works best. Some people don't go through this much trouble and just use whatever cam grind the manufacture decides to send them. Racing pistons use a more out of round cam grind because that way there is less contact on the pin sides (with most of the contact at the long sides of the skirt) and this means less friction and more HP. Using those racing pistons on the street can result in premature wear or leakage. You don't want them too out of round or the rings won't be embedded far enough into the ring lands on the pin sides of the pistons. Hepolites on the other hand are almost round with very little cam grind. Compromises have to be made.

Your note about pistons getting to 500 deg indicates that they may very well reach 0 or negative clearance. This is tricky business and it takes a lot of experimentation until you finally get a piston that seals as good as you can get it to seal without being so tight that it overheats and seizes. If you've seen my video then you know that I've done a lot of full throttle blasting up and down country roads. Hundreds of tests have been made so far and there is no end in sight.

WZ507 said:

What sort of cylinder/piston temps might be observed if the pilot were so ruthless and cold-hearted that with the engine at full operating temp he/she headed up a long gruelling rocky mountain grade while maintaining 2/3 throttle for a minute or two?

Click to expand...

Why only 2/3 throttle. ?
All the mountain passes I can think of locally I've had more like full throttle,
on the steeper bits.

And sometimes wished for more.
Especially after a Suzook with pilion passed me somewhere (not Commando, but wouldn't have made any difference).

WOT accelerates too fast in public Mt climbs so normal people not racing use rather less than 2/3 throttle to still accelerates brisky but that ain't as cooling a rich mixture as WOT to leap over crests. Btw bores don't stay round at full heat either so piston asymmetry should try to mimic that too.

Rohan said:

WZ507 said:

What sort of cylinder/piston temps might be observed if the pilot were so ruthless and cold-hearted that with the engine at full operating temp he/she headed up a long gruelling rocky mountain grade while maintaining 2/3 throttle for a minute or two?

Click to expand...

Why only 2/3 throttle. ?
All the mountain passes I can think of locally I've had more like full throttle,
on the steeper bits.

And sometimes wished for more.
Especially after a Suzook with pilion passed me somewhere (not Commando, but wouldn't have made any difference).

Click to expand...

My suggested example of 2/3 throttle was just that, a hypothetical example. Two full minutes at full throttle, under full load, starting at full operating temperature is a royal thrashing for sure that would melt down many machines. If you've got one that can do it then I applaud you as it is an exceptional piece of machinery tuned perfectly for WOT operation. Operating our dinosaurs under such regimes is often a one-way road to ever higher temperatures, and in turn failures.

Come along on a ride with me up comnoz' favorite steep long killer pass. Ready, Set, GO! OK, I'm at full throttle now and the bikes on it's knees at 85 mph so count with me slowly, 1 thousand, 2 thousand, 3 thousand…… 60 thousand - well I'm already half way done (honestly it feels like it's already been an hour), 61 thousand, 62 thousand, 63 thousand - oh oh, I can tell the engine is way to hot and the power and speed are sagging, 80 thousand, 81 thousand….. oh oh, it's tightening up and this is the end, better clutch it before it locks up. :cry: Well, I obviously did a poor job of tuning for WOT operation, or I fit the pistons wrong or something. I swear next time I'll get it right.

All my fav-o-rite mountain climbs are all like 20 or 30 miles long, but with a lot of zig zags,
so its like 20 or 30 minutes or 40 minutes at full throttle and then back to about zero speed every few hundred yards,
but about every thing aircooled I have owned has done these, and never blinked.

My old Enfield seized an exhaust valve once (not even hillclimbing), but that was newish bronze valve guide, when it should have been iron.
Fitted before my time, so I wasn't expecting that. But it just suddenly lost compression, no bad consequences...

Commandos are a fairly well proven motor, competition even, don't baby them too much,
stock they are fairly tough. 2 minutes of hillclimbing they should just laugh at.
Hours and hours may be different....

I suggest that with bore wear what you might expect is not necessarily what you get. I believe the chromium plated bores in Yamaha two strokes of the 70s might have been reverse etched in the chrome bath to bring up the grain structure so that the inter-grain areas can hold oil. Another thing - soft rings and hard cylinders can act the same way as cutting something very hard with a soft wheel - you lose a lot off the wheel however the cutting is faster. I suggest the wear characteristic is a combination of ring and bore material and lubricant properties. I believe the main rev limiting factor in a commando engine is ring flutter due to excessive piston speeds (apart from the whole bottom end). If I was going to alter the ring or bore material, I'd be researching what directions the MotoGP engines are going, to get somewhere near a good combination of materials. I don't believe bore clearances have as big effect on ability of a motor to rev out as some might believe, unless you are getting a lot of rock.
In the olden days the Yamaha development papers were available through the Society of Automotive Engineers.
Jim, with your longer rods the rock-over time at the top of the stroke is longer (accelerations are less) and that might beneficially affect the ring sealing ?

acotrel said:

I believe the main rev limiting factor in a commando engine is ring flutter due to excessive piston speeds (apart from the whole bottom end).

Click to expand...

What happened to the cam limiting the breathing ???
And bouncing / floating the valves if you give it the big rev.
Not to mention G forces trying to pull the pistons apart.

The thread on bottom ends produced some long lived bottom end stories,
even in racing. As long as the bearing fix as for Combats had been applied.

A standard 850 motor will usually rev to 6,500 RPM reliably and spin up quickly. Better breathing, if used to increase torque will help the motor pull harder and spin up slightly quicker, however same constraints are still there - ring flutter and the strength of the bottom end, so you raise the gearing. Revving over 7,000 RPM to gain more power is not the way I would ever try to go with a long stroke commando motor. Cam timing in conjunction with the exhaust configuration determines where the power band will be. You should never get valve float when revving under 7,000 RPM unless your cam profile is absurd, or the springs have collapsed.
I cannot remember the commonly used figure for maximum permissible piston speed, however it is apparently fairly constant for most motors - where ring flutter occurs. I think the standard Norton rings are too thick. Honda Fireblade rings are about the same OD as a 750 commando - about half the thickness.

Realize that essentially every engine that led up to and uses the ash tray type pistons being adapted to Commandos, all have piston cooling oil jets pumping at least as much oil as whole engine oil pressure requires - on top of liquid or forced air cooling through ducting, including 20+ yr old cast iron lawn mowers. Ring flutter in astray pistons has been solved in adequately cooled engines so may always limit Cdo rpm if not some extra piston cooling provided.

Wedge shaped piston rings have been used in some engines to get better sealing. I don't know how that would work in high revving engine, however might be OK in a commando engine. As I understand it, ring flutter is a function of the piston speed and the weight of the ring, and a long stroke commando engine at full revs is near the practical limit.
I've only ever seen one of the short stroke (82mm ? ) commando racing engines, and talking to the owner, it sounds as though it is a better engine. However he is an aggressive young Australian/Italian and his successes might be more than partly due to that. Similar to myself, he has found that there is really no class which suits his production racer so he competes against the moderns.

Some pistons come with a sacrificial dry lubricant coating and when these pistons heat up they can enter into interference fit dimensions and survive because of the sacrificial coating. But the coating just wears off and you're back where you started with bare aluminum.

Rocking is not the cause of cylinder wear - its the other way around. Cylinder wear creates more clearance and then you have rocking - after that the pistons can rock so bad that cylinder wear accelerates. If you set up the bore with too much initial clearance - then you made a mistake and created unnecessary rocking when you should have given the proper clearance so the pistons heat up to near zero clearance - eliminating the possibility of rocking. If you are an extreme racer and have cylinder wear which results in rocking - then fix the cylinders with the carbide treatment or switch to castor bean oil.

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Here's what I have on clearance info for forged pistons in air cooled Nortons:

.004" clearance will result in partial seizure and scoring of the skirts and bore requiring rebore and new pistons. This is what happened to the long skirt forged pistons below.



.0055" clearance with iron cylinders can partially or completely seize if you take a brand new motor and run it continuously at WOT under heavy load.

.0055 to .006" clearance in a street or race motor should be fine but its always best to give it some break in time - the more the better.

Now here is what gets interesting.

.0055 to .006" clearance in a street or race motor that has several races or street hours on it will usually take plenty of punishment unless you seriously abuse and overheat it. I'm talking about running a much too lean fuel mixture at sustained WOT until the aluminum actually approaches or reaches the melting point. Or you can run way too much compression on gasoline (13.5 or 14 to one for example) in a race motor in hot weather on a fast track. The pistons may or may not seize but they can get so hot on top that thin spots near the valve pockets and ring lands begin to deform or even let loose. If you're lucky you will catch it in time and be able to brag about it. And here is whats really interesting - the tops of the pistons may show melting deformities without any scuff marks on the skirts.

These clearances and results can vary of course but these are the things I've seen through the years.

I used to race a very fast 850 Norton monoshock at willow springs raceway in air temperatures approaching 110 degrees. I never had any crank or crankcase cracking problems after I switched to lightweight forged pistons and I never had any problem with piston rocking or bore wear. I ran a blend of synthetic Amsoil and racing oil and that may have helped,