Author Topic: Crankshaft assenbly balance factor C11G/C12  (Read 380 times)

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Offline Rewind

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Crankshaft assenbly balance factor C11G/C12
« on: September 17, 2019, 03:12:33 pm »
Does anyone know what the % balance factor is for the crankshaft assembly (crankshaft, conrod, gudgeon pin, circlips, piston and rings)?

Offline timsdad

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #1 on: September 17, 2019, 03:36:41 pm »
I saw this query on another Old Clunker site and it puzzles me why you'd need it, Mr R.

I get my cranks within about .002" run-out after I've messed around with them and get no vibration, even on a DBD34. My Cs, Bs and Matchlesses I treat the same and they're all as smooth as a really smooth thing. Folk on here have nailed Tiger Cub pistons and other such contrivances into C11s and I don't think they've ever messed around with re-balancing. Billy F may have a few years back, I can't remember, but he went a bit over the top and trawling back may turn up something.


Ray
Just a motorcyclist.

Online Ginge

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #2 on: September 17, 2019, 09:39:33 pm »
I’ve never seen it referenced to a C bike.  Big twins, yes. Short stroke high revving engines, yes.

Long stroke single plodders like ours, no.

How does it get measured? 

Ray, Tongue in cheek I require you to please post a video with pint of beer sitting on the headlight speedo, bike idling.  I believe that to be the traditional test!

Where do you measure run out?  Between the flywheel halves (laterally) or up and down on the flywheel circumference ( longitudinally)?

Ginge.

Online MilitaryRon

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #3 on: September 18, 2019, 05:31:50 am »
Well funnily enough I'm going through this myself and started this thread on the WD bike forum.
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1419649&cmd=show

Ron

Offline timsdad

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #4 on: September 18, 2019, 07:14:35 am »
The flywheels have to be aligned both laterally and longitudinally, Ginge. After assembly with a straight edge across the flywheel sides, and before final tightening of the big nuts, I just sit them in a pair of bearings in a wooden vee block frame in my vice with dial gauges on the sides. It only takes a few taps to get them in line sideways and a bit of tippy tappy to the shafts, with the gauges moved round, to get the shafts as straight as a really straight thing. I then tighten it all up and measure everything again, shafts too for my own satisfaction. These old engines were built to good tolerances and it's no big deal to get it all as good as new.

The only engine I've had to remove the shafts from the flywheels is my Goldie back in the '70s and I had new shafts riveted in by a specialist. Nowadays I'd just do it myself if needed. I had a G80 Matchless forty years ago that I fitted a new big end assembly to that was a problem. It came from Happy Hamrax, if I remember rightly, and was an Alpha jobby that was not fit for purpose. The pin was made in one piece, not sleeved like the originals, so I couldn't get it tight without everything going several thou' out. I then realised there was a slight radius on the inside of the stepped section so I ground a small countersink into the inside of the flywheel holes so it all nipped up snugly. It lasted several years in my ownership and may well still be fine. They don't do miles like they used to!

'No vibration' is a sort of relative term and some of us know what a smooth Brit big single should feel like when it's ridden with great enthusiasm. I'd happily do your test but I realised many years ago that beer and motorcycles don't mix too well together.


Ray
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Offline timsdad

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #5 on: September 18, 2019, 07:32:52 am »
I know it's got little relevance but I built my own engines for racing in earlier days. Jap two stroke twins, and a 750 Laverda twin, didn't seem to need balancing but the 600 Honda and Yamaha Fours did. We tried a practice session on the Yam without handlebar weights once, as a weight saving exercise, but the thing was almost unrideable because of the buzzing through the bars.

I may be a Philistine but I've never felt the need for further balancing on road bikes. My long apprenticeship on old Brit iron may have made me immune because I rode a Laverda Triple for a couple of years before someone informed me that they vibrated and had heavy clutches and clunky gearboxes.


Ray
Just a motorcyclist.

Offline Rewind

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #6 on: September 22, 2019, 07:02:43 am »
Well funnily enough I'm going through this myself and started this thread on the WD bike forum.
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1419649&cmd=show

Ron
Thanks Ron, that's a really useful link. I have built and balanced a couple of Triumph 750 twin (T140) engines in the past and had great results with very smooth engines.  The balance factor I used was 85% and I found significant differences in the piston assemblies (piston, rings and gudgeon pin) weights.  Also in the conrod weights, I trimmed metal off the inside of the piston skirts and as I had a selection of gudgeon pins to choose from I could select ones which were near enough identical.  The conrods were not identical in weight either, I removed small amounts of metal from the end of the big end cap bolts and from non load bearing parts of the caps and ensured that the complete rod assemblies were balanced weight for weight and end for end.  The balance weights I used (clamped round the big end journals on the crankshaft) were the sum of the conrod big end weight plus 85% of the small end weight plus the piston assembly weight. I also lightened the valve train (pushrods, rockers, tappet adjusters and valve retaining caps) and the engines were reasonably smooth up to over 7000rpm (not that I kept them at those revs for any length of time.....!). From your link it seems that 55% to 60% would be the norm for a road going single.   I just ordered a new barrel, the old one was very badly worn and the crank big end feels very dodgy, it was noisy from the bottom end and vibrated quite a bit although it was still starting easily and running quite well with ring gaps around 0.055".   Thanks again, I'll report back with results once I've finished the rebuild.

Offline chaterlea25

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #7 on: September 22, 2019, 01:43:31 pm »
Hi All,
From memory there is a service sheet on balancing the bigger B series engine cranks?
If I was going to the trouble with a C crank
I would aim for a mid 60's factor
I would try and find an original BSA piston
to use If possible, or just to weigh and compare with what ever replacement that's going to be used
.003in per inch of bore size is correct for ring gap, so 63mm = .0075 near enough
Err on the larger side for safety

John

Online MilitaryRon

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #8 on: September 25, 2019, 07:29:56 pm »
I collected my crankshaft from Ainsley today. The outcome is that when he first checked it, he calculated the balance factor at 43%. After drilling the flywheels at the big end area, he's achieved a balance factor of 58%.

My 'RAF' crankcases arrive from the Netherlands yesterday and I've fitted the new main bearings and ordered new gaskets and small parts from Hitchcock's, so I can proceed with the engine build tomorrow. Ron

Offline Rewind

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #9 on: October 11, 2019, 11:25:41 pm »
Thanks Ron, 58% sounds about right, see my latest post about using an earlier C11G crank assembly in a C12 engine.  Hope your rebuild is going well?
Best regards,
Les

Offline Rewind

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #10 on: October 12, 2019, 09:00:27 am »
I’ve never seen it referenced to a C bike.  Big twins, yes. Short stroke high revving engines, yes.

Long stroke single plodders like ours, no.

How does it get measured? 

Ray, Tongue in cheek I require you to please post a video with pint of beer sitting on the headlight speedo, bike idling.  I believe that to be the traditional test!

Where do you measure run out?  Between the flywheel halves (laterally) or up and down on the flywheel circumference ( longitudinally)?

Hi Ginge, reference where to measure run out I have done it two ways.  I rebuilt many (more than 20) two stroke race kart cranks and always put the assembled crankshaft between centres and measured run out at the bearing surfaces.  When I rebuilt my BSA B31 engine 20 years ago I aligned it on the flywheel outer diameters with the main bearing journals supported on rollers.  Both methods worked fine.

Offline camman3

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Re: Crankshaft assenbly balance factor C11G/C12
« Reply #11 on: October 12, 2019, 12:36:25 pm »
His profile if you want to pm him.
https://bsac10c11c12.co.uk/smf/profile/?u=1140
Best to do this as he will receive email notification while not on site.
Graham
1957 C12
In sunny (sometimes) Christchurch, Dorset, UK