Balancing an 850 crank with emgo pistons

Hey guys,

Just doing a rebuild on my 850 and looked around for some weight listings for dynamic balancing just to see how far off my new pistons are from my old ones. Figured since there wasn't much there listed for what to expect weight wise other than this thread (crank-balancing-calculation-correct-t12957.html), thought I'd post up here to give some info for people looking to go with the new emgo pistons.

Weights:
Original Pistons: 393.5g (Left) 394.5g (Right) ['74 850, original pistons, rings, pin, clips, all cleaned].
Emgo Pistons: 398.5g (Left), 399g (Right) [+0.020" overbore with hastings rings and emgo pin/clips].
Conrod Small Ends: 76g (Right), 76.5g (Left) [originals were 76g and 78.5g, took off 2g off one and matched with lighter piston].
Big end shells: 43g (Right or Left) [Current production from AN/OldBritts]
Conrod Big Ends: 364.5g (Left), 364.5g (Right) [originals were 364.5 and 370.0, weight taken off heavier cap, measured with bolts, nuts, no bearings]
Oil Weight: 102g [measured after cleaning, CC'ing and measuring 5W40 oil. Oil feed blocked off for measurement].

Bobweight Calculation:
Rotating Weight = 364.5g + 43g + (102g/2) = 458.5g
Recip Weight = 398.5g + 76.5g = 475g
Balance Factor: 52%
Bob Weight (Per Journal): 458.5g + (0.52*475g) = 705.5g

Unsure why my numbers ended up different form the link, but definitely confident in the measurements (checked on two scales, both BE/SE weights add up to total rod weight, both scales precise to < 0.5g)

Either way, I imagine based on the numbers, one could take a few grams off the emgos and get them to be close enough to the originals to not need a rebalance. Figure I might as well dynamically balance the thing while I'm here :mrgreen:

Cheers!

Those numbers are all so close to original it doesn't need a rebalance. ?
Wait and see what others say.

Good idea on the dynamic balance.

So dynamic balancing is a more accurate method than static balancing but both are addressing balance factor ?

Yes/No.
Dynamic balancing ensures that the (balance) weight is distributed evenly across the crank,
so that one side of the crank or the other is not significantly different to the other.
Saves them trying to rotate in different planes, like multiple skipping ropes.

Imagine a V10 engine, where ALL the balance weight is located in one spot.
And the same V10 crank, where the weights are distributed along the length of the crank, to where it is needed...

Note though that dynamic balance uses (calculated weight) bob weights to substitute for the rods and pistons,
so you are not balancing an actual spinning engine on the wing, so to speak.
The skill of the operator/machine is the black art part of it...

If all the components attached to each journal weigh the same as they have done with static blueprint balancing forever, then what ?
Is static and dynamic along the lines of a wheel balancing on a stand manually and the balance machine, same thing but different ?
The reason I ask is having received a reliable recommendation of a dynamic balance shop in Sydney and I have the Commando crankshafts sitting there it seems like a good idea to have them balance checked.

(I see you added a bit while I was posting )

The dynamic balancer machine analyses the spinning crank, and can advise if the weight needs to be redistributed along the length of the crank anywhere.

So a dynamically balanced crank and a static balanced crank may be the exact same weights, but exactly WHERE the weight is has altered a bit.

There was a thread here a while back where Comnoz posted a you tube video about this, quite informative since a video is worth a thousand words, so to speak

I remember a video posted but can't find it, there was a great post here though.

engine-balance-t5775.html

Can someone remind me, 750 and 850 have different balance factors ?
Against better judgement I am doing both the 750 and 850 at the same time but working on the assumption that doing it right the first time can only lead to normal maintenance with drama free riding.
Its only as good as the foundation.

Time Warp said:

Can someone remind me, 750 and 850 have different balance factors ?

Click to expand...

Same balance factor, but different weight pistons.
So different weights of drillings in the cranks.

Learning from 1st time mistakes to not make the same ones again is how most folks learn Nortons ??

Exactly rohan!

The idea with the dynamic balance is primarily to look at balance left to right on the crank (from drive to timing side). Imagine the simplest case of a single cylinder cylinder crank. If there's only a flywheel on one side of the connecting rod the motion of the connecting rod can be balanced by the flywheel, but whenever the piston is at BDC and the centrifugal force is pushing down on the crankpin, you'll have a balanced force pushing up at the top of the crank weight. Flip the situation, and the force directions switch. Leads to a nasty rocking couple where the crank wants to rotate (in a see-saw motion) about a point between the crank weight and the crankpin. Add another counterweight and the problem is solved. The issue is that if the counter weights don't have weight distribution, the same thing happens again.

Enfield interceptor cranks were also dynamically balanced from factory, hence the smoothness. Got used to that :mrgreen:

Also I definitely don't think the crank NEEDS to be rebalanced, in fact I'm not actually sure a 5-10% change in balance factor on an iso frame will do all that much other than changing the orbit shape.

What was tripping me up is the LARGE difference in small/big end weight between what I measured and what I've found.

What and how are you going to use the bike I would think is important here also.

You have gone to trouble of measuring the pistons twice an they are almost the same as old worn (so slightly lighter piston) so the engine should run as before not withstanding better bores etc.
If its a road bike then that should be ok.
If its a race bike then lighter piston are way to go.
If you want it smoother then lighter pistons and balancing.

But why buy cheaper pistons then spend on balancing. Am not trying to do you down but I would be more worried about the quality an service life of the piston and spend my money on best I could get not get cheaper an spend on balancing.

toppy said:

But why buy cheaper pistons then spend on balancing. Am not trying to do you down but I would be more worried about the quality an service life of the piston and spend my money on best I could get not get cheaper an spend on balancing.

Click to expand...

The pistons are only 5 gms heavier because of the overbore size (+ 0.020"),which makes the skirts thicker.The same casting is used to make standard size or + 0.040" pistons.
This would have been exactly the case if genuine old stock Hepolites were used.Overbore pistons are normally heavier.

If you pulled standard pistons out of 2 different engines,they could easily vary by 5 gms.If they're sold as a pair,the weights are matched (usually).

Since the Hepolite name was sold,any "Hepolite" pistons you buy now are made by JCC and packaged in a Hepolite box.Guess where Emgo pistons are made.The material is equivalent to old stock Hepolites,and the machining is even better.

1/2 the dynamic balancing has been done,if the rod and piston weights are equal.Only the crank counterweights need to be made equal to finish the job (they may already be perfect).Does anyone ever consider that if the engine is not perfectly centred in the frame,you still get a rocking couple after dynamic balancing?

X-file said:

[Does anyone ever consider that if the engine is not perfectly centred in the frame,you still get a rocking couple after dynamic balancing?

Click to expand...

The pistons rise and fall together.
So you get a torque-induced rocking couple only.
And it happens whether the engine is centred in the frame, or just running while sitting on the garage floor... !?

(Aside from any engine imbalance, which HAS to happen since a reciprocating weight cannot be perfectly counteracted by an out-of-balance rotating crank).

I would certainly appreciate a link to a standard compression,stock skirt and pin height dimension high quality piston set for either the 750 or 850.
Do C P Pistons do a Norton part ?

Your friendly Norton Dealer can supply piston sets which will work perfectly in your Norton.
Why not go with the flow, and use what everyone else is using.
They are at least as good as what was in them originally.
Correctly installed (i.e. not too tight, nor too loose, but just right. Etc etc).

My point was that Emgo pistons are at the lower end of the scale as far as cost goes an so I assume that there are budget issues an this is not a money is no object rebuild. So if the engine vibration was except able by the rider before an the difference in weight is within normal tolerance of a factory piston all should be ok.

Mick Hemmings has rebored my engine an supplied NOS AE pistons (he has still got some) and fully rebalanced the whole assembly. This was not cheap but I had the money so I spent it. He did point out that balancing was not essential due to the ISO mounting of the engine an the fact that the oversized pistons weight was within factory's spec for standard bore.

midnightlamp said:

Unsure why my numbers ended up different form the link, but definitely confident in the measurements (checked on two scales, both BE/SE weights add up to total rod weight, both scales precise to < 0.5g)

What was tripping me up is the LARGE difference in small/big end weight between what I measured and what I've found.

Click to expand...

Yes, there appears to be a significant discrepancy in the weights measured by you and Slim on both the large and small end of the rod. You and Slim got the following.

Large end - you = ~ 365 g , Slim ~ 318 g

Small end - you = ~ 77 g, Slim ~ 126 g

Total rod weight - you = 442 g, Slim = 444 g

I would think that someone either made an error in measuring or two different rod models/types were measured. Because you both come up with the same total rod weight (442 vs 444) it would seem you both measured the same rod, but apportioned the weight fractions quite differently between the rod ends.

http://www.ebay.com/itm/Norton-850-Comm ... a2&vxp=mtr

That's the set I used. With a set of Hastings rings (also sold by MAP). Stock compression, basically stock pin height, basically stock skirt length.

For me I've definitely actually had luck with Emgo pistons in plenty of british stuff before, really anything that has decently designed expansion grooves, and has a reasonable thermal expansion coefficient that doesn't need 0.005"+ of clearance works fine for me. As mentioned by X-File, these are the same pistons as JCC, and modern hepolites, and comparing the two pistons, the machining is definitely top notch compared to a NOS set of AE's I did accquire (which were also 5 grams off, but sold in factory box with rings on the side as a "pair"). These run happily down to 0.004" PTW clearance, and Ring sealing is also top notch, take a nice plateau honed bore and have little to no issues getting up to full compression and bed in quick.

That was my confusion WZ507, as for the mine, I used a standard rod weighing jig, checked the that the rod was level, from P-P and double checked the weight. My castings are the R9R and R10R. Weighed before and after I polished the shanks. I'm a little neurotic about this stuff, but I've always found it doesn't take more than an extra day to spend some time polishing up any stressed alloy parts and weigh/balance whatever you can before to minimize any unnecessary vibes. As mentioned, with everything from the crankpin up being weight matched, the only rebalancing done is on the two outer counterweights to make it even left/right.

The +5grams from +0.02 makes sense.