Over enthusiast machine shop

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.

Re: Over enthusiast machine shop

Postby Wocha » Sun Jun 18, 2017 9:19 am

Another related question. I have just weighed the pistons and I have 2 x 486gm abd 2 x 446gm. I gues i should get the weights equal. Where would i remove the excess metal?
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Re: Over enthusiast machine shop

Postby dfast » Sun Jun 18, 2017 4:13 pm

Before you remove more material, check if it's maybe balanced together with connecting rods. Weigh them first together with connecting rods.
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Re: Over enthusiast machine shop

Postby rjbeamer » Sun Jun 18, 2017 5:55 pm

Is this the weight of the pistons before or after you had them machined. It is very hard to tell you where to remove piston material without seeing the pistons. If they have not been machined yet I would get input from the automotive machinist as to his recommendation. You might let him do it if he has the scales and tools. Based on the weight of your pistons
you want a max. variation of 10gm.

Do not include the weight of the rods when balancing pistons. (The big end of the rod is considered rotating weight and counted very differently)

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Re: Over enthusiast machine shop

Postby dfast » Mon Jun 19, 2017 1:22 pm

Not that long ago (end of 80'), I brought may car engine to machining shop. My wish was to get more power from that small engine. Very common engine, so the guys already knew what and where. So, flywheel, crankshaft, rods, and pistons are lighten an polished (rods and crankshaft). I didn't really pay attention to that work, but I remember that flywheel and crankshaft were balanced on some test bench (table) and pistons and rods are matched per weight. Engine producers aren't making all parts in same weight, there are weight classes, and parts are combined to proper tolerance. Connecting rods aren't rotating.
Anyway that small screamer was spinning gladly afterword's.
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Re: Over enthusiast machine shop

Postby rjbeamer » Mon Jun 19, 2017 1:43 pm

Dfast wrote:
Connecting rods aren't rotating.
__________________________________________________________________________________________________
Most Engine balancer's considers the small end of the rod to be reciprocating wt. and the large end to be rotating wt. They are weighed and equalized separately. Each weight is entered into the Bob weight makeup formula separately. The big end of the rod and bearing are given a different value in the formula than that of the piston and small end. Other things that are weighed and go into the formula are Pistons, Piston rings, Wrist pins,Rod bearings and an estimated weight of the engine oil that would be on the piston and rod assy. There is a different formula used for inline engines than say a V type Engine.



Check out the forum Post below.


http://speedtalk.com/forum/viewtopic.ph ... d&start=15

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Re: Over enthusiast machine shop

Postby artificer » Mon Jun 19, 2017 2:25 pm

FI:
Piston pins & lock bolts can be slightly different weights & are sometimes mixed & matched to get piston assy's [reciprocating weight] better balanced.
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Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.
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Re: Over enthusiast machine shop

Postby bazza46 » Mon Jun 19, 2017 3:22 pm

Of course, You mean 'gudgeon pins', don't you, John. :D
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Re: Over enthusiast machine shop

Postby rjbeamer » Mon Jun 19, 2017 3:35 pm

I think "Grudgingly" and with the flick of his "Wrist", he would agree with you Bazza.

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Re: Over enthusiast machine shop

Postby Marty, SoCal » Mon Jun 19, 2017 4:12 pm

When I rebuilt a couple GPW engines a couple years ago, one for a friend, one for me, I decided to weight mach the pistons and rods. I was able to balance the four piston assemblies to be equal weight.

Like Artificer said, the piston pins sometimes were different weights and the heavy ones can be moved to the lighter pistons. A bit can be removed safely from the pins by slightly tapering the inside of the piston pin bores with a Dremel and a sanding roll. I also was able to remove a little weight from the heavy pistons by smoothing off the casting numbers on the inside of the skirts and smoothing some of the sharp machined edges, etc..

Total with pins:
Image

Individual weights of just the pistons:
Image

I also matched the rod's big ends and small ends weight, the pin bolt and washer make a difference, as Artificer noted! A slight polish of the forging marks and sharp edges on the rods made them equal up, along with matching bolts and lock washers. Note that I have four GPW rod sets, rods from different engines weights were quite a bit different. Points out that if you replace a rod or two in a set, be sure to have them weight matched.:
Image

Weight matching connecting rods:
https://www.youtube.com/watch?v=QLpiF0E0EJU

Weight matching pistons:
https://www.youtube.com/watch?v=_YMMQnBdVW4

It seemed to make a big difference, the rebuilt GPW engine I did for my friend runs much smoother than the old Willys one I took out. He noticed it from the first time he drove it.

Click on the pic to stat a video:
Image

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