Piston weight

I have been reading an article in an old magazine about a guy who is building an RC115 Honda racer from scratch. Before he died, Henk Van Veen donated a piece of Mahle alloy 124 as used in the Van Veen Kreidlers, to make the pistons. In the old days I used to play with 650cc Triumphs and pistons were always a problem. Standard Triumph pistons were made from Hiduminium alloys as used in Rolls-Royce Merlin engines, but in Australia that alloy was not available off the shelf. So our locally-made pistons were always heavier and prone to seizure. The Mahle pistons used in Jawa speedway bikes were relatively much lighter and stronger. With my Triumphs, apart from fitting race cams, the biggest improvement I ever achieved was through fitting lighter BSA 350 Gold Star pistons with the crowns modified to create a squish band in the cylinder head. I think Jim Schmidt's lighter pistons and long rods are a very big step in the right direction. I have a 73mm Honda Fireblade piston which is extremely light - the value analysis appears to have been carried to extremes. I know that with Japanese two-strokes the pistons were spun-cast then forged. So I have been wondering what is the best technology used in piston manufacture these days. The Mahle website does not give much information.

No argument there. The lighter the pistons the better, as long as they are still durable. Before the FIM banned them, one of the F1 car teams was trying pistons in an aluminum beryllium matrix metal alloy. But they were very expensive. And I've seen some titanium pistons that were run in one of the NASCAR teams. Still lots of possibilities left to try if you have the resources.

Ken

My friend is currently converting a 650cc Triumph to 750cc. He is having problems which seem to be related to the heavier pistons. One answer might be to find a Japanese piston, but it seems the only way to do that is look up the specs for Japanese singles and visit the wreckers and do some measuring. I thought that the smaller capacity Jawa speedway bikes might use Mahle pistons which are near the right size. But we don't do much speedway in Australia and usually only with the 500cc Jawas. When we race old Triumphs in Australia, we generally use methanol fuel, so the comp. ratio is higher. Then the weight of the crown becomes a factor, unless the piston is forged.

If you can find an oversize Japenese piston, that would enable it to be turned down to what you need, as the inner cup is standard across all oversizes. You may also need to remove the outboard edges of the skirt to clear the crank at BDC. You can now get batches of a dozen or so pistons made quite economically, these are not cast or forged, but with modern piston design software should give you something better than what was available in the past. CR this day and age is a red herring, just look at most modern bikes, these have CR's that surpass what an old bike would use even using methanol.

When converting 883 Sportsters to 1200, the use of Wiseco pistons rather than stock Harley parts enables the engine to be built without having to rebalance the crank. If Wiseco makes 750 Triumph Twin pistons, you could do worse.

MAP sell beautiful billet pistons, any size you like! But they sell 650-750 configuration off the shelf. Make the stock ones look like they belong on a tractor...

I am under the impression that the skirts are much thinner and stronger in forged pistons. If the MAP ones are billet, are they lightened inside the piston between the gudgeon pin bosses and the crown . Does anyone make slipper pistons for a Commando ?

Slipper pistons with existing stroke I have not seen, but have for a different stroke.

Were the slippers for the 850 bore size ? If the deck height is different, the barrels can be shortened or different length rods purchased.

You can have slipper pistons made for an 850 by at least some of the custom piston makers, if that's what you want. For street durability, I think long stroke engines like our Nortons are better off using conventional design, full skirt pistons, but that's just my opinion. For race bikes, the requirements are different, and it's sometimes worth saving weight at the expense of expected life. I talked to the JE engineers a few years ago about making some pistons using their FSR (Forged Side Relief) pistons, custom machined for Norton applications. The FSR pistons are a slipper design, and there were forgings suitable for 76 mm and larger bores, so you could make them in Norton 850 and larger sizes. At that time, they didn't have any forgings suitable for machining for the 750 bore sizes. That might have changed, but I kind of doubt it. The trend seems to be towards larger bores and shorter strokes, not the other way around. Using them would have required longer rods or shorter cylinders, and you would still have pretty short skirts. The JE guys didn't recommend going that direction, so I didn't pursue it any further. Not saying it won't work, just passing on the limited info I have on the subject.

Ken

Perhaps changing the stroke does not alter the problems ? When you go to larger bore and shorter stroke, the usable rev range usually rises, so piston inertia becomes more of a problem, but the piston speed becomes less because of the shorter stroke. You also tend to lose torque. Larger bore and shorter stroke gives more top end power at the expense of midrange, so the gearbox becomes more critical. However getting the best out of a long stroke motor also depends on the gearbox.

Remember that most pistons these days are not dependent on one pattern/die, even with billet, the machining will usually make crown weight the minimum it can be - the beauty of billet. Unlike some pistons of old like the WISECO's that were used in Nortons, they are actually 2 stroke forgings and cut to suit Nortons, so a compromise. If you are going to machine the crown of a modern piston, make sure you have the material to do, or get the piston manufacturer to adjust the height of the pin, or leave more material under the crown. They should also be able to tell you how much material is where on the crown to 0.10mm easily.

As Madnorton points out, you can get almost anything you want in a piston today, thanks partly to CNC machines. Back when I first started having JE make pistons, they offered lightening options, including pocketing under the crown and above the pin, the same sort of work that anyone with a mill could do. Over the years they've added a lot more options..A few years back, they started offering precision CNC contouring of the underside to match the shape of the dome and valve notches, along with pinboss and window milling. If you want to go with all the lightening options they offer, you can get a lot of weight reduction. I've not had any made with the underside contouring option, but Jim Schmidt has offered that as an option on his JS Motorsports. Makers like JE offer so many options on their pistons now that you can make them as trick (and expensive) as your heart desires. Besides the lightening options, you can choose from all sorts of coating options, hard anodized ring grooves, gas ports, pin oilers, several ring options, accumulator grooves, oil squirter notches, any dome and valve notch shape you want, and in a choice of 2618 or 4032 alloys. And they have so many different forging blanks to start with, that you can almost always find one to suit your design. It's really tempting sometimes to have some made with all the bells and whistles, but it would be serious overkill for most of our bikes, and pretty expensive to stock in multiple sizes for sale. I've occasionally had some one-offs made in small quantities, but the price is much better when ordering in larger quantities. And so far the standard forged pistons seem to hold up quite well, even in a 920 on nitrous.

Ken

I have a 73mm Honda Fireblade piston which is about 100 gram lighter than a standard 850 piston. I don't know how it would compare with a standard 750 piston, but the Japanese don't seem to have any problems making extremely light, yet still strong pistons. It is not a matter of adding expense when you can buy something off the shelf and use it with minor modification. The Fireblade piston would probably work in a 750 commando if the crown was modified and longer rods used. The trouble with this stuff is it involves getting hold of old pistons from Japanese bikes until one if found which is OK, then buying new ones to modify. What bore size is a short stroke 350 Manx ? - Trying to buy pistons from Mahle might be very difficult.
A while back, I looked at the possibility of having barrels cast in aluminium for my 850 and I would have used the 750 commando bore size. If I can find some superlight, yet strong pistons, I would go that way. Every time the piston reaches the top or bottom of the stroke, it has to reverse and go the other way. When I have fitted lighter pistons to other motors, the performance has improved very noticeably.

Not sure you would save as much as you think by using the Fireblade piston, Alan. A 73 mm 750 piston is already significantly lighter than an 850 piston. The 73 mm Powermax pistons I used to use in the PR only weighed 235 grams, and the stock 750 keyhole style pistons only weighed 241 grams. That's pretty light already. The 73 mm forged JE pistons I sell are a little heavier at 245 grams. The JS Motorsports lightweight pistons are significantly lighter (around 190 grams, I think), but you also have to add the extra bit of reciprocating weight from the longer steel rod, but the combination still comes out significantly lighter than either stock pistons or standard JE pistons with stock rods, and even lighter when compared to either of the pistons with standard Carrillo steel rods. The problem with trying to use any of the modern Japanese four cylinder pistons is the 4 valve configuration, so none of the valve notches are anywhere near the right location for a two valve Norton. The forging blanks that JE uses for all my pistons are the same ones they use for their Kawasaki 2-valve pistons, but with Commando pin sizes and the valve notches cut to suit Commandos.

You can save some reciprocating weight by going to shorter pistons and longer rods, but don't forget to add in the extra reciprocating weight of the longer rods. You still come out ahead, as Jim has demonstrated, but it's not as much as you might think by just looking at piston weight. You could save even more by going to a shorter cylinder and using the lightweight pistons with standard length rods, but then we'd have to have the long rod vs. short rod discussion all over again.

Ken

+ 1 on what Ken said. There are already well proven ways of fitting light pistons in Cdo’s. For most of us, following the established route will always be better / safer / cheaper in the long run!

Jim’s pistons are light. My 920 pistons are lighter than stock 750 pistons!

We also have to remember that old Cdo’s and modern Honda’s et al are rather different beasts.

I while back, it was a popular mod to fit 73mm Suzuki GSXR pistons in 500cc NRE motors cos they were lighter.

But they weren’t up to it, they crack the crowns. I never got to the bottom of why, but I put it down the the fact that the heat and stresses in the old air cooled motors were higher than in the modern, liquid cooled motor with effective knock sensors, oil jets under the pistons, etc, etc.

I think commando engines usually run with more bore clearance that Japanese four cylinder motors, so there is probably more piston slap. Because of I use methanol fuel, I always let the motor warm-up on idle and avoid revving it until there is heat in it . The Japanese pistons might require a bit more care. Jim Schmidt's light pistons and longer rods sound very attractive. Also there would be an easier source for replacements.