360 Magnum Piston Weights - Factory Balance

[Bruceynz]

Hello,

I have a set of 360 mag pistons, 7 from engine and 1 from another engine. One was damaged and I bought a used piston/rod on ebay.

So I have gone through and weighed them on the work scales. (counting scales) and these are the weights in grams.

1354
1352
1353
1359
1350 - This is the piston from ebay
1350
1350
1354

So we have a 9 gram weight difference, I have no idea what the factory weight tolerance are for a 360 mag, I would suspect that I should be able to bolt up with my 1350 replacement piston/rod as its not way different.

I do understand that everything should be the same, but in the factory real world its not. After market probably get to .5 gram difference. I know that balancing is quite an art. Weighing rods, pins, rod ends etc.

Thanks
Bruce

 

[SixBanger]

Great. You could use a grinder or put it in a lathe to reduce the weight to the lightest one.

 

[Bruceynz]

Where do you grind though, they rods are weighed end for end

 

[XfbodyX]

On the pistons, you can use a 3/8 bit on the bottom on the pad right and left of the pin, 2-3 on each side or you could do the same on the bottom of the crown, the think part, just no more then approx 1/16 or 3/32.

But really on a stock or near stock build, people just run them. You will find the rods will be off too.

Keep in mind if you do everything to the lightest weight of the sum of the parts you have there is still a oem target bobweight for the crank pre set so you will not hit dead on unless you have the whole assembly balanced at a shop and them they would weigh all parts and correct. I save money by getting all parts the exact same weight then just taking the crank, one rod, one piston, one ring set, one set of bearing. ect, so they dont have to do it. You could just figure your own bobweight and take them the crank but I like to have them at least weigh on set of all on there scale.

 

[XfbodyX]

You do the rods on the balance pad of the cap and the big old casting plug in the small end.

Do your research before you start drilling and grinding.

 

[Bruceynz]

So if used in a street car and just bolted it up and never went above 6000rpm I could consider myself to be safe enough with those piston/rod/pin weights?

 

[XfbodyX]

Yes, if you have another v8 apart there toss the whole piston/rod on the scale and see how much each oem one is off, it will shock you. 340-s ran well over 6k but id almost pay to have the assembly balanced if your gonna do it often, bearings will last longer.

 

[Bruceynz]

these are all OEM pistons weights on rods, see photo of my setup!

1354
1352
1353
1359
1350 - This is the piston from ebay
1350
1350
1354

Hmmmm looks like gravity has become one gram heavier since I last weighed

 

[XfbodyX]

run it, also those are just your total weights, not your bobweight, none the less run it.

 

[Oldiron440]

I'm surprised they are as close as they are....

 

[Bruceynz]

My feeling was this - If the replacement piston/rod/pin was with in a few grams run it, well the replacement was 1350 and the worst was 1359 so I think we are good to go! But not being an expert on balancing I wanted to consult the experts!

 

[kkritsilas]

Why is there a HP or Agilent Spectrum Analyzer behind your scale?

 

[Bruceynz]

Thats simple, my day job is radio technician and work has some pretty nice scales

Why is there a HP or Agilent Spectrum Analyzer behind your scale?

 

[kkritsilas]

Used to work on high frequency microwave and cell phone stuff for Nortel. Real familiar with HP SAs, Rhode & Schwartz generators and analyzers, and Anritsu generators. That HP Spectrum Analyzer looked familiar.

Funny story. I was setting up some high frequency microwave test benches (24GHz and up). Called up the HP sales rep., told him I needed 6 HP8565Es (50 GHz Spectrum Analyzer). I heard nothing from the other end, dead silence for about 3 minutes. I though the line had disconnected. I said hello, and sales rep said that he was still there. I asked if he heard what I wanted. He said he had, then said that I didn't have the money. I said here is the Purchase Order from Nortel corporate. He said that it would take a while, I said fine, you have 3 months. He says that there are only two of them in the world, it may take longer. I said if you can't deliver all 6 in 3 months, don't take the order, I'll talk to Rhode & Schwartz, or Tek, or somebody who can. I got them on time. I can only imagine what the commission was on a $US750K order.

 

[Bruceynz]

Yeah the gear is expensive, I use the 8920, its old but still works well, we can still get them calibrated in NZ and spare parts are still available, I have Used R&S CMS52s as well, but like the HP better. The company has a digital test set for DMR as well, a Freedom 8000. I have not used it much, its slow and everything is tiny on the screen, can plug a monitor into it.

Used to work on high frequency microwave and cell phone stuff for Nortel. Real familiar with HP SAs, Rhode & Schwartz generators and analyzers, and Anritsu generators. That HP Spectrum Analyzer looked familiar.

Funny story. I was setting up some high frequency microwave test benches (24GHz and up). Called up the HP sales rep., told him I needed 6 HP8565Es (50 GHz Spectrum Analyzer). I heard nothing from the other end, dead silence for about 3 minutes. I though the line had disconnected. I said hello, and sales rep said that he was still there. I asked if he heard what I wanted. He said he had, then said that I didn't have the money. I said here is the Purchase Order from Nortel corporate. He said that it would take a while, I said fine, you have 3 months. He says that there are only two of them in the world, it may take longer. I said if you can't deliver all 6 in 3 months, don't take the order, I'll talk to Rhode & Schwartz, or Tek, or somebody who can. I got them on time. I can only imagine what the commission was on a $US750K order.

 

[Bruceynz]

I use the 8920 for TX power, frequency, modulation/dev, RX sense, squelch levels, its easy to do sub tones on, its also easy to set up to test repeaters on the duplex port, I use the tracking gen to sweep the duplexers and align the notches in the duplexers. It can do a lot more than that, selcall as well and lots more! I use to fix radios down to board level but as technology has become more smaller and the radios have cans soldered directly to the PCBs its not so easy to get to the parts and if its a digital fault then well its in la la land for me!

As a side issue, my friend and I have been thinking to work on an EFI system that fires the injector off the dizzy at the time the inlet valve opens and use a map sensor to work out how much fuel to put in, as the voltage from the MAP changes just need to use something like a 555 time to change the mark space ratio. An analogue type carb that could be dialed in quite quickly with the adjustment of a pot. But this is all just a theory at this stage, look below, full throttle 4.9 volts increase mark space ratio to injector, 100 decrease mark space and anywhere in between Well thats the idea if it works is another story lol

You would leave the carb in place like a throttle body, could take the choke plate out.

 

[SixBanger]

Maybe you could weight both pistons and rods. And swap them to get a more equal weight of all cilinders. But, if i'm not wrong. Piston weight does nothing to bobweight. Only crankshaft and conrods?

 

[XfbodyX]

Piston weight has a huge part of it, remember its the furthermost distance from the crank center line. Just think if you had a 3 ft rope and had a one pound rock on it and spun it around, then you replaced the one pound rock with a three pound rock and spun it....get the idea.

I could type three ways to determine bobweight, all correct. People use different terms but in the end its all the same.

Here is a pretty simple link. When you set your tolerances for a bobweight for the type of built there is only ONE right bobweight in the end result. Its not something to guess or to F=up on. Its either dead right for the parameters or its dead wrong.

Bob weight = Rotating Weight + (Reciprocating Weight x .50)

Engine Balancing - Tech - How To - Terminology - Circle Track Magazine

Ive several here of my own, but this is quicker to post. Here is a typical card the balancer tech would fill out after weighing parts and calculating bobweight.

There are some good vids on you tube of the whole process and its a good set to watch.

Ok here is a good vid.

 

[kkritsilas]

I use the 8920 for TX power, frequency, modulation/dev, RX sense, squelch levels, its easy to do sub tones on, its also easy to set up to test repeaters on the duplex port, I use the tracking gen to sweep the duplexers and align the notches in the duplexers. It can do a lot more than that, selcall as well and lots more! I use to fix radios down to board level but as technology has become more smaller and the radios have cans soldered directly to the PCBs its not so easy to get to the parts and if its a digital fault then well its in la la land for me!

As a side issue, my friend and I have been thinking to work on an EFI system that fires the injector off the dizzy at the time the inlet valve opens and use a map sensor to work out how much fuel to put in, as the voltage from the MAP changes just need to use something like a 555 time to change the mark space ratio. An analogue type carb that could be dialed in quite quickly with the adjustment of a pot. But this is all just a theory at this stage, look below, full throttle 4.9 volts increase mark space ratio to injector, 100 decrease mark space and anywhere in between Well thats the idea if it works is another story lol

You would leave the carb in place like a throttle body, could take the choke plate out.

View attachment 38408

The problem with trying to trigger a fuel injection system off the distributor is that there is too much slop between the distributor tab (Mopars don't have a gear) the camshaft, and the cam chain (slop, and eventual stretching). Most third party fuel injection systems actually control the distributor with the fuel injection computer (for those that have ignition controls, I am only familiar with FAST EZ-EFI and Fi-Tech, so don't take it as gospel for all fuel injection systems). All lot of the factory fuel injection systems are crank triggered, and don't even use a distributor. All that is to try to get away from the slop in the cam driven distributor type ignitions.

Funny thing is , at high microwave frequencies, the test equipment is not the only major expense. We had cables (good to 50GHz) that cost $250 per foot, and the V and K type connectors were fairly expensive, too.

 

[Oldiron440]

Now on some balancing jobs I can think of one 500" 440 in particular heavy metal was needed to make everything come out correctly. In that case the hole was drilled from the side and the heavy metal put in. At the time 1991 heavy metal was expensive. It was on a very early mopar/scat 4.15 crank.