Crankshaft balance

The need for a balanced engine

Thanks to Charley Cole of Zydeco Racing for providing the following information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Charley's own words:

You absolutely have to do a crank balance if switching from the stock 83 mm Mondial pistons to 88 mm Gilardoni pistons. My 88 mm Gilardoni pistons were 70 grams heavier than the stock 83s. The Moto Guzzi connecting rods are reasonably well matched for gross weight...not so much for end to end weight which is another way of expressing the location of the rods balance point...which does matter. I went through seven sets of matched con rods before I found a pair that could be corrected for good end to end weight and have the same gross weight. The problem is, if you have a pair of rods that are the same weight, but have different end to end weights, by the time you correct one rod's end to end to match the other, you now have rods of different gross weights. To get the second rod down to the one you just corrected the end to end on, without changing it's balance point, can be very difficult. Considering all of that and the high weights of the MG con rods and heavy Gilardoni pistons, which complicates balancing the crank, my balance guy thought it would have made sense to go with Carillo con rods right from the start, since they are perfectly balanced and so much lighter. For 88 mm Gilardonis, you will have to open up the cases or turn down the cylinder spigots if your cases were not from a stock 950.

If you are switching from an 83 mm stock piston to an 83 mm Gilardoni, that shouldn't be too hard if you are happy with the stock balance of your motor. The Gilardoni pistons are just a medium quality cast piston and I'll bet you could get 5-7 grams out of them. I believe a safe minimum for crown thickness is 250 thou and I'll bet the Gilardonis are thicker than that. Often, material can be taken out of the wrist pin bosses. There are thinner wall and tapered wall wrist pins available in MG's length and diameter. IIRC, one cc of aluminum is 1.7 grams.

Be sure to tell your balance guy to add 27 grams to the crank pin weight during static balancing. That is how much oil by weight is in your crank pin while your engine is running, but it is not there during static balancing. That's a pretty big balance error, if that is not included.

A question from Dana Larson:

What would be the likely result of installing the 88 mm Gilardonis on an Eldo engine without rebalancing the crank?

Charley's reply:

At TDC, especially, on the compression and exhaust stroke for both pistons, you would not have enough weight in your crank shaft counterweights to counteract the force of your heavy pistons decelerating. Simply put, that would cause the con rod where it connects with your crank shaft...the big end bearings...to slam into the crank pin at that position of rotation. You will hammer the hell out of your bottom shell bearing and loose oil pressure.

Weight differences between original and replacement Gilardoni 850 cc pistons

Thanks to Kevin Kelly for providing the following information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Kevin's own words:

Yesterday I put new Gilardonis cylinders and pistons in my 75 Eldo with 34K on it. Notice that it seems that no one has weighed the difference between the stock pistons and the new Gilardonis.

Well I did and the new ones weighed 492 grams with the rings, clips and piston pin. The old stock ones weighed 474 grams for the same thing. The piston pins both weighed the same. So their is a difference of 18 grams per cylinder, and it does not seem like to much until you think about it. A penny weighs 2.5 grams so the difference between them is the weight of just over 7 pennies on each cylinder, sure to make some difference on the balance on the engine.

Follow-up from Kevin:

I did not notice any difference in the vibration of my 74 850 eldo with the new gilardoni pistons and cylinders from the stock ones.

Weight differences between original 83 mm pistons (850 cc) and replacement Gilardoni 88 mm pistons (1000 cc)

Thanks to Rob Anderson for providing the following information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Rob's own words:

I thought it best to start a new thread.

I just received my 88 mm Gilardoni kit. I should have had it about 10 days ago but moving house/workshop etc the addresses got mixed up but all OK.

The good news is that the current Gilardoni 88 mm barrels fit straight into the std eldo crankcase.

The bad news is that the piston assembly is substantially heavier.

STD guzzi piston assembly (piston rings wristpin clips): 482 grams

Gilardoni 83 mm piston assembly (piston rings wristpin clips): 492 grams (OK Whatever)

Gilardoni 88 mm piston assembly (piston rings wristpin clips): 544 grams (Hmm I gotta lose some weight)

The oil ring is a single piece ring and responsible for 4-5 grams extra weight (over 3 piece ring) I can get a tool steel wrist pin (drag technology hello mark) that saves 30 grams a side but I still want to relieve some of the piston

I will get a chance to check deck height when the crank/rods are in my hands in a few days.

Weight differences between original 750 cc pistons (side to side)

Thanks to Steve (chucktbt) for providing the following information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Steve's own words:

Spent a few hours out in the shop today polishing the valve covers and decided to weigh out and measure all the motor components.

Here are the weights on my stock 1969 Ambo reciprocating bits:

Right cylinder:

• Circlips 2 grams
• Wrist pin: 97.2 grams
• Piston and rings: 432.2 grams
• Rod with bearings: 613.9 grams
• Total: 1145.3 grams

Left cylinder:

• Circlips 2.2 grams
• Wrist pin: 98.3 grams
• Piston and rings: 431.2 grams
• Rod with bearings: 614.1 grams
• Total: 1145.8 grams

So, Guzzi had the overall balance nearly spot on. Half a gram side to side is nothing to sneeze at. I'll post the Cycle Garden big bore piston weights as soon as they arrive.

Weights of the Cycle Garden big bore kit

Thanks to Steve (chucktbt) for providing the following information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Steve's own words:

I finally reassembled my motor yesterday with the Cycle Garden Big bore kit.

First and foremost - what a NICE kit to work with. Everything went together like butter. I checked all of Moe's work, and everything was spot on - total waste of my time to doubt, but as they say, better to check than to be sorry.

Anyways, here are the new weights:

Right cylinder:

• Circlips: 2 grams
• Wrist pin: 132.5 grams
• Piston and rings: 389.8 grams
• Rod with bearings: 619.4 grams
• Total: 1143.7 grams

Left cylinder:

• Circlips: 2.1 grams
• Wrist pin: 132.1 grams
• Piston and rings: 390.9 grams
• Rod with bearings: 619.3 grams
• Total: 1144.4 grams

So, overall, I went from 83 mm to 88 mm, added a heavier wrist pin, but still wound up below my original weights and wound up only 0.7 grams off side to side.

Not bad at all :-)

In case anyone is wondering why I didn't use the old wrist pins and lighten the top end, there is a reason.

The Cycle Garden kit uses forged Mahle pistons. Generally, you want a finger/thumb push fit for forged pistons as they expand at a more consistently than cast units and tend to retain their dimensions better. Also, a stronger wrist pin is a really good idea when you are producing large amounts of torque with a motor that is this over-square :-)

With the original cast pistons you want a slight interference fit so that when the dimensions get wonky with the heat you don't have too much slop with the piston bouncing around.