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Powermax
#806417 04/23/20 6:54 pm
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I'm going to try a set of old stock powermax pistons in the 650 race bike. They look like the TT pistons that Triumph used. 11-1?
Anyway, I've heard that they have a habit of growing a good bit when at operating temps. Like they want to seize when set up at 5.5 thou clearance. Anyone who might have used these back in the day have anything to say in this regard?
Thanks

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Re: Powermax
Mike Baker #806421 04/23/20 7:49 pm
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In the day we used a lot of them. This isn't "in the day!".
If you are going to use them racing, use a cam with a lot of overlap, harden the combustion chamber against detonation, etc. you might have some luck. Otherwise they are good bases for a lamp or a novelty for a trophy.
John (who stopped selling these retail 40 years ago).

Re: Powermax
Mike Baker #806431 04/23/20 8:36 pm
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Power max are low expansion pistons, so they should grow less than standard pistons.

John, what do you mean by harden the combustion chamber? Only thing I can think of is removing detonation like setting correct squish and radius the pistons


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Re: Powermax
Mike Baker #806432 04/23/20 8:40 pm
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Thanks for the response, John. With a dual plug head I'd hope to get past some of the shortcomings of these pistons. But to the question I asked, Can anyone speak about what is a reasonable clearance to set these up at?

Re: Powermax
Mike Baker #806445 04/23/20 9:58 pm
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We posted at the same time, Allan. I have not been able to find any manufacturer's literature for these. I do know one experienced motor builder that has seized them at 5.5 thou. And another equally experienced builder that has said no less than 7 thou. From where do you know these are low expansion? Have you or someone you know used them? Was just hoping someone would share some knowledge.
Perhaps the best thing to do is put them in the oven and see what happens before building.

Thanks

Re: Powermax
Mike Baker #806452 04/23/20 11:21 pm
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Okay Mike, you are making me work... Its been a long time since I talked about this. Hepolite (and their performance brand Powermax) always had you bore the cylinder to "the nominal size as shown on the label plus the oversize." They calculated the desired clearance when the machined the piston.

As you might expect that that didn't go over in the US and people wanted a figure.

So I dug out a set of 750 11:1 Powermax pistons and measured them and figured the clearance. The 750 is 76mm 2.9911". This piston I am working with is .060" oversize. So following their instructions the bore would be 3.0511" (-.000 +.001"). The piston measured 3.0475. This would give you .0035" clearance (.0045" if you bored the cylinder .001" over their recommended 3.0511"). This isn't bad for a 750 twin.

For a 650 twin the clearance calculated by the bore plus the oversize would be more. Take your your standard cylinder bore 2.7948 (70.988mm) and any oversize and measure you piston. The difference is their recommended clearance. My guess it will be close to .0045".

Now what did we run, and why. For the street we would always add at least .001" to their clearance. If we were REALLY racing them we would old it closer to their recommended clearance (that is bore + any oversize less actual piston skirt diameter measured about a 1mm above the bottom of the skirt.

This will give you an idea what Powermax recommended. The reality varies a lot with the application, who put the motor together and how it is ridden (raced).

I also would like to address the other experts who have had trouble seizing with these pistons but an running out of time.

Allen I will try to address Hardening the Combustion chamber when I get time - I should be posting a invoice with 100 line items to do.

Re: Powermax
Mike Baker #806508 04/24/20 12:39 pm
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[Linked Image from tioc.org]

We were Hepolite distributors in the US and had many conversations with them. They would NOT provide you with a clearance specification. They also insisted that you NOT remove the rings from the pistons before installing. (Improper ring removal, and reinstalling lead to a lot of seizure problems) For break-in of the top chrome ring, used in most Powermax pistons, they are coated it with "Cargraph" (a mild abrasive - probably similar to what Total-Seal (quickseat) rings supply). While they recommend the piston be cleaned in solvent before installing they insist that the coating not be removed from the rings. If I was to use a set of these I would coat the rings with Total-Seal "quickseat".

Re: Powermax
Mike Baker #806540 04/24/20 3:55 pm
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The Quick Seal appears to be popular with auto drag racers that don't have time to seat rings in the usual manner.They generally use coated steel or nodular rings. However ,I know nothing about strange brews to seat rings...I also don't think high dome pistons are the best idea..But as said, Mike's engine had dual plugs so it may be just fine...


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Re: Powermax
Mike Baker #806550 04/24/20 4:47 pm
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HB if you followed people going through the learning curve with Total-Seal rings you would understand why they came out with a method to insure the rings would seat. Back in the 1990's guys running ARHMA thought they were "the next best thing to sliced bread". I would watch them get black flagged where the bikes would smoking badly on the over run (deceleration). The web was full of people who were having break-in problems with them in all sorts of auto related forums. Once people got a handle on them and started to use "quickseat" people started to get some benefit from them. A few hard dyno runs usually settled them down, but who has a dyno in their workshop.

Re: Powermax
Mike Baker #806557 04/24/20 6:16 pm
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just about the worst thing you can do with new rings is take it easy. the guy who supplied my new GM 500 engine advised me to warm it up then wring its neck. Running on a dyno is much the same


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Re: Powermax
Mike Baker #806584 04/24/20 9:04 pm
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Originally Posted by Mike Baker
We posted at the same time, Allan. I have not been able to find any manufacturer's literature for these. I do know one experienced motor builder that has seized them at 5.5 thou. And another equally experienced builder that has said no less than 7 thou. From where do you know these are low expansion? Have you or someone you know used them? Was just hoping someone would share some knowledge.
Perhaps the best thing to do is put them in the oven and see what happens before building.

Thanks

There was a brief period where power max pistons appeared on eBay in quite large numbers. My dad bought a set for his 1970 lightning. I could have sworn the box stated lo-ex aluminium. I could be wrong, but the BSA pistons we’re lo-Ex I would expect hepolites gold seal pistons to have been the same. Sadly I now live nearly 100 miles away from my dad and I wouldn’t know where he had filed them to check.

If the rings aren’t run in properly you stand a very high chance of suffering a 4 corner seizure regardless of the piston.


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Re: Powermax
Mike Baker #806592 04/24/20 10:27 pm
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I have never seen Lo-Ex, or are there any printed on the boxes I have that indicate they are Lo-Ex.

Mike have you measured these Powermax pistons and compared the dimensions with what is shown on the box. What is the difference (clearance)? That gives you a good datum point to start with.

Now you have to take into consideration the fuel that will be available. "In the Day" we had real octane available and it made us think we were pretty clever. You could almost get away with anything. Especially the Triumph 650 that has the worst combustion chamber of any British bike. Twin plugging gives you some benefits
but it is not a cure all. You still don't have the squish band of the 750 twin or the early unit 500.

Spark plug selection is going to be important and I would start with a B9ES or B10ES.
Ring seating is going to be critical as most of the heat leaves the dome through the rings. With these pistons you will need at least .015" ring gap.

Then the combustion chamber needs work.
Any of the head's exposed threads around where the spark plug enters the combustion chamber need to be rounded off.
The valve contact width needs to be at least .060" wide or a tad more. Nearly all of the heat leaves the valve through the seat.
All of the sharp corners on the piston dome need to be rounded off.
The head gasket i.d. must be cut if the cylinder is over bored. You don't want any of the gasket hanging out over the bore.

As far as clearance it is going to depend upon how confident you will be able to seat the rings and how it is going to be ridden. It is easy to see where someone seized a set of these with .007" clearance. Poorly prepared with attention to preventing detonation, wrong grade of fuel with not enough octane, the wrong grade spark plug (too hot) and 30 seconds, at the most, of lugging the engine. We have forgotten how to shift.

When I look at what piston clearance I will bore a cylinder, and I don't know the customer, I add at least .001", maybe .002" extra from what is recommended if I am NOT going to assemble engine and break-in the motor. In cases where I know the rings will be fully seated by the time the rider exits pit-out I go with the low limit or a bit tighter. In today's world piston clearance is not a "one size fits all" deal. In street engines we have no idea what fuel is going to be used or how the bike will be ridden

From my experience if I was setting this up for the street, building the engine, and breaking it in, and didn't know the rider, I would go for .0065". If I was setting it up for road racing, half mile dirt, etc. I would be looking at .0045" to .005".

With the fuel available today, and the aging of the riders, there is no one fixed clearance. Also there is basic lack of understanding of why you cannot run your British bike along at 70mph at 1500/1800rpm like you can with your modern car. Then roll on the throttle to pass a car and have the engine fail to increase speed. Failing to remember you need to shift down these engines to lower dynamic cylinder pressure. Besides the longer stroke (more torque) there is this computer thingy that has made us forget how to manage these old engines.

Mike measure the piston and get back to us!
Hope this helps!

Re: Powermax
Mike Baker #806656 04/25/20 11:54 am
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"but it is not a cure all. You still don't have the squish band of the 750 twin or the early unit 500. "

Squish,squish,squish, combustion chamber turbulence.....I am not an expert by any means....However, I have closed up the squish to the minimum on several modified car engines and two Triumphs....In all situations it increased power , sharpened throttle response and reduced the engine octane requirements....One of the Triumphs is my 650 land speed racer that makes competitive speed with less compression and milder cams than the others use....


79 T140D, 96 900M Ducati ,61 A10 .On a bike you can out run the demons..
“But I don't want to go among mad people,” Alice remarked. “Oh, you can't help that,” said the Cat: “we're all mad here. I'm mad. You're mad.
Re: Powermax
John Healy #806688 04/25/20 5:47 pm
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I
Originally Posted by John Healy
Now you have to take into consideration the fuel that will be available. "In the Day" we had real octane available and it made us think we were pretty clever. You could almost get away with anything. Especially the Triumph 650 that has the worst combustion chamber of any British bike. Twin plugging gives you some benefits, but it is not a cure all. You still don't have the squish band of the 750 twin or the early unit 500. !

If you have time, is there any chance you could elaborate on the benefits of twin plugging?

Re: Powermax
Mike Baker #806709 04/25/20 9:20 pm
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Locating the spark plug in the center of the cylinder is trying to do the same thing.

So SIMPLY stated:— Combustion should not be an explosion where all of the air/fuel mixture would ignite at the same time! Detonation is an explosion. Combustion should be a controlled and relatively slow burn of the incoming air/fuel mixture.

Incoming Air and fuel is heated as it enters the combustion chamber. This evaporates the incoming fuel droplets into a ignitable gas (gasoline has to mix with oxygen to become a fuel). Well in practice, not all of the incoming fuel vaporizes and collects in pockets around the combustion chamber.

Gasoline, our fuel of choice, without the presence of oxygen will not burn. It is the fumes, which are a mixture of air and fuel that burn. To burn the fuel must vaporize.

The vaporized fuel readily blends with air to form a combustible mixture. It is set alight at the source of ignition — the spark plug. Once ignited it starts to slowly burn across the combustion chamber. The rate of burn reflects the octane level of the fuel. The Higher the octane the slower the air/fuel mixture burns.

In a well designed combustion chamber, Triumph's hemispherical design is NOT well designed, this normally is an orderly process. The fuel ignites and combustion takes place evenly all across the combustion chamber. Triumph's combustion chambers are full of nooks and crannies (quote "The idiom originated in the 14th century and it combines 'nook', being used from mid-1300s which means - a distant corner, with 'cranny' in usage since 1440 which means - a crack or gap.")

Also the carburetor is pretty inefficient in producing the fuel in small enough droplets for all of it to vaporize into a gas. The front edge of the combustion flame, as it works its way around the combustion chamber, finds the "nooks and crannies" that have collected gasoline that has not yet vaporized. Engineers call this "end gas". At the same time the pressure is building as combustion continues. This creates heat.

The combination of increased pressure and heat causes the end gas to rapidly vaporize, mix with air and EXPLODE — Detonation.

So with a 2nd plug ignition starts in two places. On a Triumph this is typically where the nook and crannies exist. It is also where the most detonation problems develop.

It cuts the time for total combustion in half. You have two flame fronts that will meet in the middle of the combustion chamber. It speeds up combustion where you have to retard the engine. Typically a single plug Triumph engine requires around 39° before top dead center of spark lead. With dual plugs most people retard ignition to around 32° btdc.

Now add a proper squish band, where non vaporized fuel is no longer sitting around the edges of the combustion chamber, and forced to swirl into the middle improving vaporization, and you can further retard the ignition. I have built Triumph 750 wins where I set the timing in the low 20°'s btdc.

So with two ignition points what you get is a more controlled burn with less chances of detonation. This can mean your engine will be less susceptible to the ravages of detonation. And as Hill Billy said, "Squish, squish, squish...
John

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Re: Powermax
Mike Baker #806722 04/25/20 11:43 pm
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Ok, Measurements.
One piston measured 2.8070" and the other measured 2.8072" at the bottom of the skirt. Pretty close.
These are called .020" oversize.
Standard bore is said to be 2.795" so .020 over should be 2.815".
So if we look at the oversize bore at 2.815 and subtract the piston at 2.807, that leaves a clearance of .0080". If I'm doing this right.
Now what we found was there was about 7 thou taper on this piston. Thats from the bottom of the skirt to just below the oil ring.
The BSA motor that seized these pistons had the seizure occur just below the oil ring to the best of my knowledge. BSA-Triumph, could be apples and oranges so maybe best to not draw conclusions here.
Now, looking at the box, the label specs 2.7948" as standard bore. adding .020 to that would get us to 2.8148" or 2 ten thousands less on the bore. Not sure that there's anyone that can hold a bore to that kind of spec. Splitting hairs.
So what this means to me is that at .0080, there is one thou clearance under the oil ring. I'm finding all this hard to believe and maybe one reason why these pistons could have been troublesome. Again, if I'm doing this right.
One last thing- Honing for the chrome ring. Are we looking at the "coarse" finish that we use with cast iron tops?

Thanks for everything!

Re: Powermax
Mike Baker #806727 04/25/20 11:58 pm
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Thanks for taking the time to write that John, it’s very much appreciated. I did a twin plug conversion recently and got into a discussion about the benefits. The general consensus seemed to be there wasn’t much benefit other than easier starting and some improvement in drivability. Some saying no gains according to the dyno, but not sure how well versed they were on the subject... I’m definitely not. I would have guessed with 2 flame fronts you would have to retard the ignition more than 7 degrees.
Interesting what Honda did with one of their duel plug engines. Ignition for the 2 plugs is controlled independently, but slightly different kettle of fish (speaking of British idioms).

My conversion. Not Triumph, but you will recognize it. Not much squish going on in there either.

And I’ll exit the Powermax thread now...

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Last edited by Cyborg; 04/26/20 12:02 am.
Re: Powermax
Mike Baker #806777 04/26/20 4:54 pm
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In general the Vincent combustion chamber (that's what is in the picture for those that don't travel in that crowd), and piston dome, don't make for a engine with a lot of detonation problems. The one thing the twin plug will do for you is protect you from that odd shot of low octane gasoline. I predict that those who continue to ride in the future, and still want the extra torque of high compression pistons, will be taking another look at twin plugging their engine.

Re: Powermax
Mike Baker #806781 04/26/20 5:15 pm
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Quote
If I'm doing this right.
Now what we found was there was about 7 thou taper on this piston. Thats from the bottom of the skirt to just below the oil ring.

First are you using a vernier caliper (often referred to as a guessing stick) or a micrometer?

Now with early (pre 1990ish) AE pistons a .007" taper would be normal. BUT it would be the largest at the bottom of the skirt getting smaller (by .007") just under the oil ring. So you reading where the diameter at the top of the skirt is larger than top the bottom (taper is upside down) confuses me.

So with your figures what we should be seeing .008" clearance at the bottom and .015" at the top. If it is the other way around, which is how I read what you are saying, these pistons should not be used!

In the 1990's AE changed the skirt taper of their pistons. While for nearly a 100 years pistons would have .010" ish, or more, under the oil ring, now they had only a little over .001" piston clearance under the oil ring. What they did was to radius the top of the skirt, that can only be seen using a optical comparator (https://en.wikipedia.org/wiki/Optical_comparator), just under the oil ring.

As you might imagine this drove the experts nuts. They could never come to grips with bore to the dimension, plus the oversize, listed on the box and they had taken care of the clearance and this created a lot of angry phone calls. Some where in a pile of binders I have letters from dealers and response from AE about the change.

If you measure JCC replacement pistons you will find they copied AE with only .001" taper, but instead of a micro radius at the top of the skirt JCC adopted a turned down area extending about .060" below the oil ring (this is the area where the skirt is thickest to support the oil ring) and will have the most expansion as the engine warms up.

Is the taper really larger than the bottom of the skirt by .007"? Are you using a micrometer?

Re: Powermax
Mike Baker #806854 04/27/20 4:30 am
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The factory manual states for the TT motor 11:1 comp.
Top of skirt 0.0146"/0.0125"
Bottom of skirt 0.0084"/0.0070"

Re: Powermax
Mike Baker #806883 04/27/20 12:11 pm
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WOW, Mike, sounds like you are going to be looking into another set (maybe modern) of pistons! PRT

Re: Powermax
Mike Baker #806949 04/27/20 10:11 pm
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I added the taper instead of subtracted it. The pistons are smaller under the ring than at bottom of skirt.
Measured with a micrometer by our friend Jake. Apologize for the confusion.
So getting back to skirt to wall clearance, by my arithmetic, it seems proper to size the cylinders to run at .008", especially in light of what GeoffL brought up, no?
These must really get big when they run......

Re: Powermax
Mike Baker #806950 04/27/20 10:12 pm
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Come on out to NJ in June, Tom, be good to see you.


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