Author Topic: Cheap Piston Kits  (Read 2345 times)

Offline UKlittleguns

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Re: Cheap Piston Kits
« Reply #15 on: 03.11. 2020 19:14 »
Just a comment.  It doesn't matter if a piston is cast or forged within common man road bike rpm's.  For the same geometries the engine performance will be the same.  The big difference is when it all goes wrong.  Cast pistons break up and stick a rod anywhere.  Forged pistons stay in one piece and leave something re-buildable.
 

Online RDfella

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Re: Cheap Piston Kits
« Reply #16 on: 03.11. 2020 19:50 »
Not sure about the last sentence. Pistons are usually the victims of engine damage - usually overheating but occasionally over-revving. Have seen rods that snapped at the little end (mainly), bent rods, lots of seized pistons but only ever one engine damage attributable to piston failure (and that wasn't too bad) so whether an engine is rebuildable or not really rests on other factors. Playing devil's advocate, one could say the fitting of forged pistons is more likely to result in engine damage (because of the reason they were fitted in the first place - ie the engine is going to be hammered).
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Jules

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Re: Cheap Piston Kits
« Reply #17 on: 05.11. 2020 01:51 »
Good logic RD, I think we sometimes get carried away with stuff from the good ol days vs modern hi tech manufacturing methods and materials. The problems come when you mix them up and dont understand the differences and why things were done a certain way in the past. Using forged pistons and crankshafts and billet conrods are all admirable advances in technology over what was originally used in these machines, but seriously, unless you plan to ride it hard and long, why?? even then, using those "upgrades" is pointless unless you also upgrade the parts that interface with them - I'm sure Musky and Kiwi Racer have lots of good stories to tell about doing such things and the resulting outcomes.... ::hh::

Offline Tomcat

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Re: Cheap Piston Kits
« Reply #18 on: 05.11. 2020 05:23 »
Good logic RD, I think we sometimes get carried away with stuff from the good ol days vs modern hi tech manufacturing methods and materials. The problems come when you mix them up and dont understand the differences and why things were done a certain way in the past. Using forged pistons and crankshafts and billet conrods are all admirable advances in technology over what was originally used in these machines, but seriously, unless you plan to ride it hard and long, why?? even then, using those "upgrades" is pointless unless you also upgrade the parts that interface with them - I'm sure Musky and Kiwi Racer have lots of good stories to tell about doing such things and the resulting outcomes.... ::hh::
Spot on Jules, horses for courses really.
'59 Super Rocket  TDM900

Offline BSA_54A10

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Re: Cheap Piston Kits
« Reply #19 on: 07.11. 2020 09:59 »
Good logic RD, I think we sometimes get carried away with stuff from the good ol days vs modern hi tech manufacturing methods and materials. The problems come when you mix them up and dont understand the differences and why things were done a certain way in the past. Using forged pistons and crankshafts and billet conrods are all admirable advances in technology over what was originally used in these machines, but seriously, unless you plan to ride it hard and long, why?? even then, using those "upgrades" is pointless unless you also upgrade the parts that interface with them - I'm sure Musky and Kiwi Racer have lots of good stories to tell about doing such things and the resulting outcomes.... ::hh::

Actually  I never fit any product that says "machined from billet " as it is totally meaningless .
If the vendor is trying to infer it is superiour to either a casting or forging then they are playing with themselves and you .
The first questions is
" machined from what kind of a billet ? "
Forged ?
Cast ?
Hot Rolled ?
Cold Rolled ?
Extruded ?

Even worse is most parts I see that are " machined from billet " are dimensionally identical to the original casting / forging which is so wrong in engineering principles I can not even think about where to start.

And the next question is Billet of what ? , butter, cheese pure aluminium .

There are a lot of automatic copying machines out there that will allow any old idiot to pot an object on a plinth, then scan it then machine an exact copy of it from whatever was  clamped into the work place, a block of wood a block of steel a block of cast iron.
However it does not have any mans of compensating for GRAIN SIZE & ORIENTATION and these are vital engineering parameters for engineered parts.

Now if they are just  gee gaws for a HD the fine cause the bike will never be ridden .
But if they are parts for your BSA have a real serious think about fitting one , or better still don't fit it .

A silly example of how a change can have drastic effects
Kohler introduced 2 new engines , the Command & the lower priced Courage 
The Command was designed, the courage was a modified Command
Some one decided to swap the expensive forged crank with a cheaper Cast malleable iron one on the Courages.
The result was over 20,000 warranty repairs because the extra weight & different vibrations charasterics caused the closure plate bolts to back out & the crank case to split ring down the barrel crankcase base.

Or to use a BSA example, the "Titanium Tragedy " when BSA copied the Victor frame used on the works bikes that was made from 531 high ensile steel tubing with titanium tubing to gain strength & loose weight.
Not a single one finished a race as the "stronger " frames cracked up & fell apart on the starting lines .

To get the same strength around the little end a "billet machined " con rod would need to be near 3 times the cross sectional area as a cast one and around 5 times the size of a forged one if it is cut from a billet with the grain running top to bottom ( which it should be ) because that orientation has planes of weakness in the grain direction so the little end is highly likely to let go a either edge of the little end bush hole .
To overcome this you have to increase the section size in this region so you have just added weight to the very place where you don't want it .

So don't take the description "machined from solid billet" to mean any sort of improvement in quality because alone it is nothing like it.
Bike Beesa
Trevor

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Re: Cheap Piston Kits
« Reply #20 on: 07.11. 2020 11:23 »
Totally agree. Never been able to understand how a machined item (eg con-rod) can be as strong and endure the stresses invoklved, as a forging. Likewise I could never see the advantage of machining rods by making then into an H section.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.