Removing Weight From Pistons

[Vincent]

I've been instructed to remove 4-5 gramms from this piston as it's a bit lardy compered to the other two I'm using. First place to look to shave some weight off are the gudgeon pin bosses but they look a bit on the thin side to me. Any 2nd opinions?

https://i77.photobucket.com/albums/j71/Drewskovitch/Jap%2070s%20bike%20toys/Bike%20toys/P9213265_zpskfukjibu.jpg
https://i77.photobucket.com/albums/j71/Drewskovitch/Jap%2070s%20bike%20toys/Bike%20toys/P9213266_zpsoh9yq1ph.jpg

To get a (very) rough idea how much needs to come off, this disc weighs 5g.

https://i77.photobucket.com/albums/j71/Drewskovitch/Jap%2070s%20bike%20toys/Bike%20toys/P9223268_zpsfqbum4eq.jpg
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[mentalboy]

Start by cleaning up moulding spurs, then I'd probably run a bit of coarse paper on the inside.
4-5 grams is naff all!

EDIT: I'm surprised it'd make much difference to something like a 70's machine

[mentalboy]

That washer looks a bit large -what are you using to weigh these with? Drug paraphernalia is probably the best bet for those types of weight!

[Vincent]

[Vincent]

[Vincent]

[MCN]

a 5p coin is a baw hair under 4g.
Nickel plated steel.

And \ drugist scale would be bestest. A smaller scale has a better sweet area. Scales are always more accurate in the mid range due to charachteristics of springs

Maplin do a nice wee druggy scales I have this one (but I say 'No' to Drugs )

Not for Drug Use

I can confirm it's accuracy as I used a calibrated 100g weight to test mine

There are heat fins under the piston you can clean off the sprue on them.
A carbide de-burr tool in a drilling machine to 'scrape' the inside of the skirt.
Cut Weigh
Cut Weigh
etc.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[bikenut]

interesting...what 2t engine are these for, staggered transfer ports......a side exhaust port??

https://i77.photobucket.com/albums/j71/Drewskovitch/Jap%2070s%20bike%20toys/Bike%20toys/M%20Engine%2014_zpsgmapudos.jpg

is this the engines crank??
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[Vincent]

[MCN.]

I hope you are having other work done because balancing stuff on it's own won't help with power on the standard pipes and cylinder head.

[MCN]

[orac]

aren't the GTs piston ported, which mean that the pistons need to be in the correct barrels which is the reasoning for the staggered skirts.

while on the subject of piston skirts, you maybe able to shave a bit of the bottom of them. not sure how much as never played with piston ported machine but it maybe worth a look.

although the gudgeon pin bosses may look on the thin side, I would be tempted to removed some material from the internal surface - ie the faces that would be closest to the con rod.

FYI I buddy of mine had a GT750 motor dropped in some random kwak frame. had the head (iirc) skimmed the thickness of a gasket which from what I am told gave a nice bit of a performance boost. never got to ride the thing before something went badly wrong and it was decided to part it out instead of fix it. I was sad when I was given that news.
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[pepperami]

Stupid question ? : reduce the length of the skirt?



EDIT : someone mentioned that already
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[stevo as b4]

You can't take a few mm off the skirt though as this will change/extend the inlet port timing. It's more important to keep all the port timings the same between cylinders than it is piston weights.

Removing casting flash and just a light sand around the gudgeon pin boss is probably the best way to take off a few grams.

[MCN]

[Vincent]

[almostthere]

why not add some weight to make them weigh the same?
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[orac]

[Kickstart]

Hi

Can you save weight with a different gudgeon pin? Any with the same outside diameter and length but with a tapered bore?

All the best

K
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[Ariel Badger]

Knife edge the skirt, gives better cooling. Grind out all ID marks, Ventilate the skirt.
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[MCN]

[Vincent]

I suggested the skirt.......but got told the pin bosses. The people doing the crank do a lot of work for tuned motors.....powerboat engines, race bikes etc where all these things matter........ they're used to GT750 cranks and know their stuff which is why I went there. I'm not sure they're used to the make of pistons I have though

Keith...I can balance out 1.8g by just swapping the gudgeon pins I have and the rings around.....that's nearly 50% less to file off
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[MCN]

[Vincent]

[Vincent]