Machining piston

[ola_martin]

Hey.
I'm building a 69 Husky 400 engine.
I have a stock 82mm cylinder, but only a used 82,25 or ,5 (haven't measured) piston.
Do you think it's different castings for all dimensions, with just thinner machined walls, or is the ,5 piston ,5 mm bigger inside?
If not so, can it just be put in the lathe and machined down to fit?

Don't know if I bother to try, probably best to find one that fits, just thought about it.
Would think it has a little space in the ring groove to let the ring go ,12/25mm further in.
Anyone tried this?

Cheers

[tony27]

Pretty sure the casting is the same basic size then machined to size.

Don't bother turning it in a lathe though as they are tapered & the skirt especially is not round, haven't actually measured above the top ring to confirm if they are oval the full length

[bigk]

It would be heaps easier to bore the cylinder.
K

[sleepy]

Pistons can be ground down in a piston grinding machine that have cams or profiles that put the taper and ovality into them but the rings grooves are a problem due to the ring pegs. I've had a few sets of 4 stroke pistons ground undersize to get into capacity classes but the guy told me he wouldn't do 2 strokes. If the pistons you have are only slightly worn bore to suit but remember that most of the wear on a piston is at the bottom of the skirt and if the bore is undersized to suit the wear it will seize because the crown is most likely still close to the original size.

[ola_martin]

Ok, thanks, not a good idea then...

I know it would be easier to bore, but I think it's too bad to bore out a very very good original diameter cylinder to suit a worn oversize piston...
Will look for a new original instead...

[pancho]

Pistons can be ground down in a piston grinding machine that have cams or profiles that put the taper and ovality into them but the rings grooves are a problem due to the ring pegs. I've had a few sets of 4 stroke pistons ground undersize to get into capacity classes but the guy told me he wouldn't do 2 strokes. If the pistons you have are only slightly worn bore to suit but remember that most of the wear on a piston is at the bottom of the skirt and if the bore is undersized to suit the wear it will seize because the crown is most likely still close to the original size.

It surely I mean sorely will!

[Mick D]

Profiling center lathe. The rest of the job is a walk in the park too.

[Mick D]

Pistons are not round, and that is the reason why the correct tool/machine is indeed a profiling Lathe.
I am pretty sure it was the Honda Corp that realised the importance of an oval piston whilst R & Ding to produce this
slice of historyhttps://youtu.be/RhhWV2-stGs
Please let me know if you know of some one who did it before them.
There is a simple reason why oval pistons are more durable under sustained race duress.

A profiling lathe is also the best machine to produce the taper proportionally at the same time.

Years of study record that pistons produced  for air cooled engines have higher tappers.
Wossners are a good example. I have often thought they over emphasis the taper to protect themselves from the worst heat case scenario, definitely a strategy that seems to work to protect them.
Thermal expansion disasters usually happen toward the top,,

[Tomas]

How about geting an 82mm piston(Maico 440 or so) that is readily available with pin diameter matching Husqvarnas piston pin diameter and modify that. It would be a lot easier to machine/modify top(dome) of the piston, skirts and or windows(you can just drill a couple of holes with hand drill and file out the rest when worst come to worst. Plus you may need to use spacer under the cylinder,depends on dimensions of piston you will use. And also there is a good chance that ring peg/s will line up as they should. It may be hard or impossible to find someone who is able/have the right tooling to grind your old piston. Forget about piston tapers, ovality etc. science. Readily available suitable piston have all that already built in 

[Mick D]

How about geting an 82mm piston(Maico 440 or so) that is readily available with pin diameter matching Husqvarnas piston pin diameter and modify that. It would be a lot easier to machine/modify top(dome) of the piston, skirts and or windows(you can just drill a couple of holes with hand drill and file out the rest when worst come to worst. Plus you may need to use spacer under the cylinder,depends on dimensions of piston you will use. And also there is a good chance that ring peg/s will line up as they should. It may be hard or impossible to find someone who is able/have the right tooling to grind your old piston. Forget about piston tapers, ovality etc. science. Readily available suitable piston have all that already built in 

Exactly, why buy the correct expensive machine for a single piston reclamation, when its a simply case of a rebore instead, which you would have to perform anyhow(unless you had access to a mchine so you could set a piston up for an existing good bore, like an already good nick Nickasil cylinder that you cant get a slug for anymore, I guess I am voicing the obvious though).

Anti ring rotation pins are an easy, simple and VERY reliable mod.

[Daryl Jones]

Pistons are not round, and that is the reason why the correct tool/machine is indeed a profiling Lathe.
I am pretty sure it was the Honda Corp that realised the importance of an oval piston whilst R & Ding to produce this
slice of historyhttps://youtu.be/RhhWV2-stGs
Please let me know if you know of some one who did it before them.

Cheers, DJ

[Mick D]

Pistons are not round, and that is the reason why the correct tool/machine is indeed a profiling Lathe.
I am pretty sure it was the Honda Corp that realised the importance of an oval piston whilst R & Ding to produce this
slice of historyhttps://youtu.be/RhhWV2-stGs
Please let me know if you know of some one who did it before them.

Cheers, DJ

Hi DJ, that's good information, I see the date on that is 1947
I am a BHP fitter machinist and very proud of it.
Also two years of Certificate level Mech engineering.
It was a time of intense curiosity and interest for me, because of my love of bikes.
I remember the teacher using the example of the super six oval Honda pistons of an example of successful profile machining.

All "mechanical" profiling machines copy from a piece(cam)

I had been told and believed that Aluminium held grinding swarf when ground and had always believed Aluminium was "cut" for that reason,,,,,,
I have an open mind though, so I guess I will be looking that up(ACL website springs to mind) and tuning into some more of your posts in the meantime
love to learn, keep it coming DJ

[Lozza]

Mick the Honda RC 166 was not oval piston it was inline across the frame 6 cylinder. Oval piston didn't come about until the late 70's with the NR(Never Ready) 750. Which was first a GP bike then Endurance racer and finaly high end road bike.

You would be insane to machine a cast piston any more than it's machined already a forged piston maybe............on the crown or base of the skirt anywhere else would be asking for trouble. VHM have produced a range of full billet pistons that have next to no cam in them but run 0.01mm more clearence.

[HeavenVMX]

Mick the Honda RC 166 was not oval piston it was inline across the frame 6 cylinder. Oval piston didn't come about until the late 70's with the NR(Never Ready) 750. Which was first a GP bike then Endurance racer and finaly high end road bike.

You would be insane to machine a cast piston any more than it's machined already a forged piston maybe............on the crown or base of the skirt anywhere else would be asking for trouble. VHM have produced a range of full billet pistons that have next to no cam in them but run 0.01mm more clearence.

Lozza all commonly available pistons are oval by a small amount not round to allow for thermal expansion. You are referring to the Honda NR500 which had 4 seriously (as in twice as wide as they are across, although probably not perfect ovals) oval pistons, 8 rods, 8 plugs, 32 valves which was effectively designed to build a V8 engine while meeting the 4 cylinder restriction on the class .

[Lozza]

No not the new VHM pistons are nearly perfectly round. Yes the NR began as a 500 then went to 750