Hypereutectic pistons Jcc Hepolite

I am putting my commando back together and have bought a new set of Hepolite JCC pistons. Look fine but the gudgeon pins are a gentle push fit when cold and the ring gap is .030! This surprised me and I thought ah crap again but after doing a little digging, it seems this is absolutely correct for "Hypereutectic pistons" much lower expansion rate but top ring runs a lot hotter and needs big gap, at operating temp this then apparently closes up. Now if this the case, then we need less piston to bore clearance and a traditional 012. Ring gap could cause a lock up or bust the crown off the piston. Equally the bores can be tighter ie .003 clearance.

It seems nothing stays the same - not even alloys Does anyone know if this is correct?

Where does this info come from that JCC pistons have suddenly become "Hypereutectic".
What does the accompanying paperwork say about recommended installed clearances. ?

And why would the top ring suddenly run so extra hot that it needed those (huge) clearances ?
Just how hot do they expect it to get ???
30 thou is a huge (extra) clearance compared to 12 thou.

A quick google search reveals discussion that Jaguar investigated metal composition, and concluded that modern JCC pistons were about the same metal that Hepolite was making decades ago.

Follow the recommended clearances, anything else is an experiment.
And there are many that will tell that installing pistons as to water-cooled (car) clearances is not a good idea = too tight = seizure territory.

??

Ther are no instructions of any real use with the pistons and I total agree with your comments. I have always run about 4 to 5 thou piston clearance and 12 thou ring gap. However, the slide fit of the gudgeon pin and the large ring gap leads me to think they may be Hypereutectic - I have asked the dealer and will see what he says.

Sorry gents - I have just checked the rebored barrels and they are 005 too big hence the wrong ring gap ahhhhhh what a waste of time money and material. Still leaves the question with the gudgeon pin but somewhat irrelevant now

Nigeldtr said:

Sorry gents - I have just checked the rebored barrels and they are 005 too big hence the wrong ring gap ahhhhhh what a waste of time money and material. Still leaves the question with the gudgeon pin but somewhat irrelevant now

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The usual cast of characters, good machine work is hard to find.

If you are still curious about the alloy, JCC has a list of the alloys they use here. A couple of them (AS-19 and AS-23) are high enough silicon content that they are probably hypereutectic, but I haven't chased the alloys down to find out. I don't recognize the alloy designations, but they might be Japanese (JIS) standards. With enough google time, you could probably find out. Chances are the Hepolite copies are not in hypereutectic alloy.

http://www.jccpiston.com/style/content/ ... 0&id=29206

Ken

concours said:

Nigeldtr said:

Sorry gents - I have just checked the rebored barrels and they are 005 too big hence the wrong ring gap ahhhhhh what a waste of time money and material. Still leaves the question with the gudgeon pin but somewhat irrelevant now

Click to expand...

The usual cast of characters, good machine work is hard to find.

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Yep - a Damn Shame!
Any comeback from your machine shop?

Had me worried as I've just put a set of standard 750 Hepolites in a set of re-sleeved barrels. The ring gaps were fine on mine, except the oil control concertina spring was oversize, so needed grinding back before it would allow the oil control rings to seat correctly.
I understand the rings are Hastings, and was a little surprised by this.

I've still not heard any bad reports on Hepolite stuff yet...

lcrken said:

If you are still curious about the alloy, JCC has a list of the alloys they use here. A couple of them (AS-19 and AS-23) are high enough silicon content that they are probably hypereutectic, but I haven't chased the alloys down to find out. I don't recognize the alloy designations, but they might be Japanese (JIS) standards. With enough google time, you could probably find out. Chances are the Hepolite copies are not in hypereutectic alloy.

http://www.jccpiston.com/style/content/ ... 0&id=29206

Ken

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Thanks for that Ken.
Weren't many brit pistons in LoEx, or similar ?

And by all accounts, that hasn't changed, lately ?

I think you're probably right about that, Rohan. A lot of the early Hepolite pistons were supplied in Y alloy, which was pretty popular for high performance pistons in the pre-WWII era, but in the late '40s or early '50s they changed to a high silicon low expansion alloy, probably something like the British LM13 alloy. The AC8A alloy that JCC offers is pretty close to LM13 in composition and properties. Rolls Royce also developed the RR50 series of alloys that were popular for high performance pistons along with Y alloy. You can get a lot of interesting discussions going about the relative merits of cast vs. forged pistons. If you don't need the extra strength of the forged pistons, cast pistons are usually a better choice. The pistons I have JE make for Nortons are 2618 forgings, but they are meant for race engines, and it's really a bit of overkill to use them in street engines. Not that that's a particularly bad thing, but they do require more clearance and are a bit noisier when cold.

Ken

I believe that some fifties pistons were made from Rolls Royce RR56 Hiduminium alloys. The other elements give particular effects. High silicon alloys were used because of their flow characteristics when casting. Copper tends to indicate a stronger alloy, magnesium tends to be anti corrosive, and there are chemical spot tests which would show which type of alloy you have . They involve adding a drop of dilute sodium hydroxide solution to the alloy then a drop of dilute hydrochloric acid. If the drop of caustic goes black, you have a copper based alloy. If it goes white a magnesium based or zinc based alloy, if the white disappears when the acid is added, the alloy is zinc based. The Japanese two stroke pistons were made using a spin cast and forge machine to get appropriate expansion characteristics, so the alloy is probably substantially different from that used in British pistons. I used Hepolite pistons for years in many motors and never had a seizure, however I suspect the bore clearance which was specified by the maker, was sufficient to never give trouble. My feeling is that the bore clearance is better tight than loose, however a lot depends on how lean you run the motor to get maximum power. (Methanol is great in four strokes.) I don't know what hypereutectic alloy is, sounds like something with unusual melting characteristics when cast. It would be an interesting exercise to find someone with a direct reading emission spectrometer, and spark a range of pistons from various sources, and all would be revealed.
I wouldn't worry about gudgeon pins which slide in easily. They've been like that for years, in the old days we used to heat the piston with a rag dipped on very hot water. A loose gudgeon (within reason) never blew up a motor.

Silicon is of course alloyed with aluminium for its low expansion properties.
And is somewhat related to the subject of this thread...

acotrel said:

The Japanese two stroke pistons were made using a spin cast and forge machine to get appropriate expansion characteristics,

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Until this is shown to be documented somewhere, this sounds like an odd process ?

As I understand it, the process was fully automated. It is a reason that OEM pistons only, should be used in Japanese two strokes. The piston technology was critical to stopping seizures, and it seems to have arrived in Japan with Dieter Braun at Suzuki. If you remember the Greeves racers, they were accidents waiting to happen. The guy who was instrumental in starting historic racing in Australia had one, and never raced again after it destroyed him. The piston size was too big, and it is impossible to find a British piston which would not grab unless it was to loose. If you want to know about the Japanese piston forging, I suggest you get the Yamaha development papers from the Institute of Automotive Engineers. That is where a lot of our two stroke info came from in the 70s. Also, it would be worth asking Mahle how they make their pistons. They made forged pistons before the Japanese. You should note that Wiseco two stroke pistons are considerably heavier than those made by the Japanese . It is not as simple as pouring molten aluminium into a steel die. Years ago, if you ever wanted to race an old British single, the forged piston was the first thing that you searched for. The Mahle two valve Jawa piston is magic in an a 500cc Aerial motor.