Author Topic: C11G-C12 Crankshaft swap?  (Read 184 times)

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Offline Rewind

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C11G-C12 Crankshaft swap?
« on: October 11, 2019, 11:20:36 pm »
I recently bought a 1956 C12 which was starting and running reasonably well but was noisy, vibrating and burning oil and with a badly worn +0.060" barrel. The piston ring gaps were all around 0.050".  At the same time I also got most of a 1954 C11G engine including crankcase halves, crankshaft assembly, head etc.  In fact, most of an engine except the cylinder barrel.  The crankshaft assembly from the earlier C11G seems to be in good condition with very little side to side play and no detectable vertical play whereas the C12 crank assembly has significant side to side play and some vertical play.  I am hoping to use the C11G crank in the C12 cases but having carried out some dimensional and weight checks I have found the following:-

1 The C12 crank assembly (crankshaft, flywheels and conrod) weighs approx. 7.5kg, the C11G assembly weighs 6.9kg.

2. The C12 flywheel diameter is 6.75"whereas the C11G flywheels are 6.625". Most other dimensions are similar, bearing journal diameters etc.

I would like to use the C11G crankshaft in the C12 crankcase pair so that I can keep the original cases with the engine number which  is the engine that came with the bike from new.  Has anyone else done this swap?  Any problems experienced? 

I asked earlier on this forum what crankshft balance assembly should be used with these engines, one of the most credible answers was 60%,can anyone confirm what the original factory balance factor was?

I have already got a good cylinder barrel (currently on +0.040") which I will bore out to + 0.060 and fit a new piston and rings.

All advice gratefully accepted.

Regards,

Les

Online Ginge

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Re: C11G-C12 Crankshaft swap?
« Reply #1 on: October 11, 2019, 11:50:29 pm »
They’re the same part number in my book Les.

Both sides, drive and timing side halves.

Don’t know why the weights and diameters are different. Any casting numbers on them?

Ginge.

Offline Rewind

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Re: C11G-C12 Crankshaft swap?
« Reply #2 on: October 12, 2019, 08:52:14 am »
Thanks Ginge.  Interestingly the timing side casting is the same number for both cranks DA9093.  Similarly the drive sides have identical casting numbers: DA14351.  The only thing I have noticed is that the later crank flywheels don't have the holes through the crank webs which can be used to help lock the crank whilst undoing the crank pin nuts.  I'm wondering if the earlier flywheels have been machined down to lighten them because the inner and outer corners have been radiused as well as the outer diameter being slightly reduced.  I just inspected them both with a watchmakers glass and the machining does not look quite as "factory finish" as the newer ones.  I might finish up swapping the good earlier crank pin, bearing rollers and rod assembly to the later crank halves.  I'm a bit reluctant to shell out for a complete new rod/pin/bearing assembly at over £400!

Offline ianinglis

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Re: C11G-C12 Crankshaft swap?
« Reply #3 on: October 12, 2019, 04:53:18 pm »
both engines are the same just the frame is different (swinging arm C12 or plunger / rigid C11G)



ian
BSA C10L 1956 - JFO 588
BSA C10L 1954 - MVE 107
RM6 RALEIGH RUNABOUT
ENFIELD DIESEL KUBOTA OC 95
BSA M21 RIGID RKN 816
CBF 1000
BSA ARIEL 3

Online Ginge

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Re: C11G-C12 Crankshaft swap?
« Reply #4 on: October 13, 2019, 07:13:00 am »
Les,

I am about to build a side valve engine up from parts, so thought I would explore what I have. I have two complete cranks and one in bits. One has been fitted to a '51 C11 engine and failed in the crankpin.  The others are unknown.

All of my flywheels are marked DA-9093 on both primary and timing side flywheels with one exception.  The oddball is marked DA-14351.  It is different in that it has a raised boss in the centre. The boss extends beyond the outer edge of the flywheel towards the primary case by about 1/4".  I believe this to be a C10L flywheel.

I have no idea why it is different.  Someone will know the answer.

My flywheel shaft diameters on the primary side measure 0.9825", 0.983", and 0.9825".  The last measurement is the oddball one with the raised boss.

My flywheel shaft diameters on the timing side measure 0.978", 0.9775" and 0.978". Again the last one is the oddball crank, but it is very similar to the others.

It would be fair to say all of these shafts have been to the moon and back.  I checked my books and can't see what the nominal diameter should be.

As regards conrod and pin, you can exchange like for like through Draganfly if they have the stock.  It is not cheap.  I had a clever chap machine the rod ring internally to clean up the burrs and fitted oversize rollers.  I think he turned the crankpin down a shade as well.  Nothing was re-hardened. We didn't have to remove much material and the bike is not worked hard.  It was a patch up job.

Still going after about 500 miles. Fingers crossed it hangs together.   It's not ideal but it's an option.

Keep at it!


Ginge.

Online Owen

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Re: C11G-C12 Crankshaft swap?
« Reply #5 on: October 13, 2019, 08:01:06 am »
The G & L models engine casings and flywheel were modified to include the engine breather.  This change did away with the oil flinger spacer that was on the earlier engines. They added the raised boss on the lhs flywheel to move the oil flinger back in position.
1940 C12 (350cc)
1945 C10 & C11
1953 C10 & C11
Anyone want a B20 in need of a lot of tlc

Online Ginge

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Re: C11G-C12 Crankshaft swap?
« Reply #6 on: October 13, 2019, 09:27:17 am »
So the G, L and the 12 need a flywheel half on the primary side with the extended flywheel boss.

DA-14351 on that side for those bikes.

Timing side should mix and match between C10/C11 and the later 10L/11G and 12.

Good info, thanks Owen.
Ginge.

Offline Rewind

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Re: C11G-C12 Crankshaft swap?
« Reply #7 on: October 15, 2019, 06:28:37 am »
Thanks for some really useful information Owen and Ginge, back in the garage this morning with the mic and vernier, will keep you posted on progress.  I'm quite tempted to try using the lighter crank assembly which all appears to be in good condition.