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Author Topic: How to tell a cast piston from a forged in an older overbore kit? (PICS now)  (Read 2866 times)

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Offline JLeather

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How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« on: January 22, 2009, 10:09:36 AM »
I've got three 836 kits with cylinders I've collected from parts engines over the years, and I was wondering the easiest way to tell the cast ones from the forged ones.  Some are obvious, like my ART 836 kit is cast, but others are not so easy.  I've got an RC 10.5:1 836 kit, the lightweight one with the shortened skirts, that I think is forged but I'm not certain.  The other one is really weird.  It's not no maker's marks on it anywhere, and the sides of the piston are completely smooth with no indentation at the wrist pin or anything.  You have to use teflon buttons because they weren't machined for a circlip either.  Anyone seen a set like that before and know the manufacturer?  I think they're forged, but I need to re-ring them and hone the block and I wanna make sure the tolerances are good before I do, so I need to figure out if they're definitely forged or not.
« Last Edit: January 24, 2009, 07:58:07 PM by JLeather »
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Offline Geeto67

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #1 on: January 22, 2009, 10:25:49 AM »
my RC kits came with teflon buttons but they are 1080cc pistons. KZ900/1000 pistons came (still come in some cases) with the teflon buttons as well from MTC, RC, APE, Yosh....etc....
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Offline NitroHunter

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #2 on: January 22, 2009, 10:43:12 AM »
Could you post some pics of these mystery pistons? Inquiring minds want to know...
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Offline turboguzzi

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #3 on: January 22, 2009, 03:15:57 PM »
in cast pistons there's more of a chance that you'll see tooling split marks inside the cavity for the small end. that's to be able to remove the casting core without having to make too steep draft angles. the central cores is often split in three.

In forged there's usually  a single core so you don't see those marks
just as well, many vertical surfaces will be milled/machined to remove weight.

That said, if all you are looking for is to figure out your clearance, you could heat them to say 120 (metric, dont know in F's...)  in your oven and measure their expansion with a micrometer.... A Kevin Cameron's tip.

TG
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #4 on: January 22, 2009, 10:11:45 PM »
In searching for the unknown manufacturer of my 73.75mm pistons Russ Collins told me he never made anything but forged pistons.
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Offline Big Jay

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #5 on: January 24, 2009, 12:19:51 AM »
Quote from: JLeather on January 22, 2009, 10:09:36 AM
I've got three 836 kits with cylinders I've collected from parts engines over the years, and I was wondering the easiest way to tell the cast ones from the forged ones.  Some are obvious, like my ART 836 kit is cast, but others are not so easy.  I've got an RC 10.5:1 836 kit, the lightweight one with the shortened skirts, that I think is forged but I'm not certain.  The other one is really weird.  It's not no maker's marks on it anywhere, and the sides of the piston are completely smooth with no indentation at the wrist pin or anything.  You have to use teflon buttons because they weren't machined for a circlip either.  Anyone seen a set like that before and know the manufacturer?  I think they're forged, but I need to re-ring them and hone the block and I wanna make sure the tolerances are good before I do, so I need to figure out if they're definitely forged or not.

Put up some photos and we will tell you what you have.
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Offline JLeather

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Re: How to tell a cast piston from a forged in an older overbore kit?
« Reply #6 on: January 24, 2009, 05:48:56 PM »
Allright, pics pics pics.  First three are the "mystery piston".

Side View
http://i86.photobucket.com/albums/k113/JLeatherman_CF/Bikes/2009-01-24-74147.jpg

Top View
http://i86.photobucket.com/albums/k113/JLeatherman_CF/Bikes/2009-01-24-74155.jpg

Bottom View
http://i86.photobucket.com/albums/k113/JLeatherman_CF/Bikes/2009-01-24-74134.jpg

Some odd things to note.  For starters, no indentations where the wrist pin goes.  I was incorrect, it does use circlips, but they are these weird flat-band circlips instead of the standard CB750 ones.  Also, the valve pockets are not offset like most CB750 overbore pistons are.  You can see it was stamped "K" on top (and it was removed from a running and heavily modified K motor).  No other marks.  I do believe it's a forged piston because there is no parting line inside it.  Any thoughts?

Oh, and while I've got the piston experts here I submit two other pics for your considerations.  I've got a used RC 836 kit (forged).  One of the pistons has a soft indentation in the top.  It is not from striking anything because the surface is undamaged.  If it were a cast piston I'd say it looked like it had been removed from the mold too hot and had contracted, but it's a forged piston.  You can see that it was once machined level and is now indented.  What could cause this?  Overheating?  Preignition?  Is it still safe to run?  The kit was recently re-ringed when i got it and it has a good set of jugs.  I was hoping to use it in a motor I'm working on, but I had failed to notice this previously.  If it's not safe to use, I suppose I'd ask if anyone's got a spare RC 836 piston?

http://i86.photobucket.com/albums/k113/JLeatherman_CF/Bikes/2009-01-24-74174.jpg
http://i86.photobucket.com/albums/k113/JLeatherman_CF/Bikes/2009-01-24-74185.jpg
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Offline turboguzzi

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #7 on: January 25, 2009, 09:27:42 AM »
yep, that's a typical internal configuration in a forged piston, smooth, no undercut junction between pin boss and and piston roof.

TG
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Offline JLeather

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #8 on: January 26, 2009, 03:22:44 PM »
So, anyone recognize the mystery kit?  Jay?  I need to replace one of the 3-piece oil rings since it was broken by the PO during installation. 
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Offline Big Jay

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #9 on: January 26, 2009, 03:35:33 PM »
It is an original Wiseco. The K on top is because that used to also make an F2 piston. I helped them to develops those kits, and the very early ones, they used Hastings rings with a big fat oil ring. I convinced them to leave the Hastings rings to the Harleys, and hooked them up with NPR.

What is the oil ring width?

Jay
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Offline JLeather

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #10 on: January 26, 2009, 04:13:55 PM »
Measured one of the good oil rings and it looks like it's 2.65mm?  So, if it's an early Wiseco then it's forged?  Also, what should the clearance be on the bore?  I need to check the cylinders before I run 'em again.  Also, it looks like all 4 pistons are the same, not 1/4 and 2/3 specific like a lot of other kits?  Oh, I also need to replace some of the missing circlips.  The weird flat ones.  Thanks Jay.
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Offline JLeather

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #11 on: January 27, 2009, 04:00:12 AM »
So my buddy Matt measured his new rings for a new Wiseco K836 kit and he came up with anywhere from 2.7mm to 2.88mm depending on where he took the measurement.  Does that mean a new Wiseco oil ring fits these pistons?
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Offline JLeather

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Re: How to tell a cast piston from a forged in an older overbore kit? (PICS now)
« Reply #12 on: January 29, 2009, 03:27:55 AM »
Bump for the ring measurements...
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