Topic: Piston lightening

[KJ790]

Has anyone tried to lighten stock CB750 pistons with success? Just wondering what has been done and if anything works well.

[turboguzzi]

the only piston lightening i ever performed (and worked too), was reducing the piston skirt height by cutting off the pointy tabs at the bottom on a lathe.

essentially i made the bottom edge of the piston skirt flat.

I think that any hand grinding of the piston internals is going to be risky, let alone hard to duplicate exactly on 4 pistons and you definitely want the weight matched.

[KJ790]

Thanks. If I did anything it would be with a lathe and an end mill. I was thinking about taking a little off of the skirt on the sides, more like a modern "slipper" style piston (but not nearly as small as a slipper). I've seen where people have machined a little off of the underside of the crown of pistons with an end mill before. I would have to measure how thick a stock piston is here before doing anything like that.

[MRieck]

Just kep in mind that you can have a pile of aluminum there (after cutting) but only lose a couple of grams. 

[turboguzzi]

Just kep in mind that you can have a pile of aluminum there (after cutting) but only lose a couple of grams. 

+1, and now its the time to ask you also why?
I did it to a 500/4 road race engine that lived between 8000-11000 rpm most of the time, for an edge in reliability, wasnt even expecting any hp gains

cant see a reason to go into this even in a hopped up street motor

[KJ790]

Basically I want to try to increase the reliability of a 750 motor at high RPM on a budget. I know that lightening a piston sounds like the opposite of increasing the reliability, but cast pistons are not all that well finished. Even if I can only take one or two grams off of the pistons reliably then it would still decrease the force on the bottom end quite a bit at high RPM's. I know that the bottom ends tend to be a weak point on these engines, but I don't want to buy Carrillo rods on this motor. I don't expect this engine to run 14,000 RPM all day like my race bikes, but I would like to not worry as much should I get it up around 9-10,000 RPM once in a while.

[MRieck]

Basically I want to try to increase the reliability of a 750 motor at high RPM on a budget. I know that lightening a piston sounds like the opposite of increasing the reliability, but cast pistons are not all that well finished. Even if I can only take one or two grams off of the pistons reliably then it would still decrease the force on the bottom end quite a bit at high RPM's. I know that the bottom ends tend to be a weak point on these engines, but I don't want to buy Carrillo rods on this motor. I don't expect this engine to run 14,000 RPM all day like my race bikes, but I would like to not worry as much should I get it up around 9-10,000 RPM once in a while.

I would find lighter wrist pins if you do not want to go with light rods. You could cut them down and run buttons to make them even lighter. That's what I would concentrate on for real weight.

[Big Jay]

It is very unlikely that you could get two grams off of a stock 750 piston, without rendering it useless.

[KJ790]

I don't think that shaving 1-2 grams off is that unlikely. I like MRieck's idea of saving weight with the wrist pin. I'm sure a tapered wrist pin would save about 4-5 grams right there, then if I could shave off another 1-2 grams from the piston itself it would be great. If I could save a total of 5-6 grams I would be very happy.

I have had great success with running only the top compression ring on race motors I have built, this saves around 4 grams as well as cuts down on friction. The difference is actually noticeable on the dyno, but longevity is not as good as with both compression rings, so I don't really want to go that route.

I was hoping if I could take a little off of the skirt, and then a little off of the bottom of the crown like this:

VW Drag Racing Piston Lightening

Obviously I wouldn't be able to take off 26 grams like in this video, but 1-2 grams doesn't seem that unreasonable.

[bwaller]

I wasn't able to find tapered pins in the length I wanted so a friend machined the taper in some CBX1000 pins which were the exact length I needed and saved 6 gms each, from 44.4 to 38.2. A 750 pin weighs 40+ so at most you'll maybe remove 4gms. It takes a lot of time.

[BLUE71TURBO]

 To bad yer messing with cast pistons.  Forged is the way to go, especially if yer wanting to lighten the hell out of them.  Check out the piston configuration of this Honda CR 450 piston........

             

   
        Then some of these will really lighten up the lower end.........

             

[KJ790]

Yeah, I'm very familiar with the CRF450 pistons, building motocross engines are actually what my background is in. That is a slipper piston that I was referring to in my first post. I would never take away that much of the skirt from a fully skirted piston, but there are cast slipper pistons being made in the car world.

[jaguar]

          Then some of these will really lighten up the lower end.........

             

how much lighter are they really?
And ive heard mixed things about running them