Small Journal Crankshaft

[Ron B]

I have a A10 engine with severally worn crankshaft drive end splines. It Rattles.  I have a good splined small journal crank.  Since this crank is lighter will my selection of pistons factor into a vibration problem?  I would assume this crank was mated up with 6.5 - 7.0 compression ratio pistons.  Any thoughts would be appreciated. Thanks Ron B.

[baz]

If your large journal crank is good apart from the splines why not get them re built?......baz

[KiwiGF]

Hi Ron,

I've got a SJ 650 engine in my '56, well I will have, when I put the bike back together. Here's some comments, but not necessarily answers....I think there are differing views regarding the desirability of LJ versus SJ cranks, some think the LJ is stronger and most agree it is less prone to whipping, but the big end bearings might actually be stronger on the SJ (they are smaller diameter but wider). LJ cranks are far more expensive to get hold of.

1. I suggest you check/measure the engines balance factor (to ensure the vibes are not excessive) whether you decide to use a new LJ or your SJ crank. I know many people do not bother, probably as it's not so easy to do and the calculations are a "challenge", but I personally could not put an engine together without having a pretty good idea of what the balance factor is. The other option is to give the job of balancing the crank to an engine builder (but again I personally would still check their work/calculations!). There's a couple of recent threads regarding balancing.

2. You will need SJ con rods to match the SJ crank (I'm using SJ billet rods, which are said to be significantly heavier than OEM rods).

3. I'm not sure what the total weight diffences of the SJ versus LJ crank is, but I would expect the weight you are really interested in (for balancing purposes) will be the counterbalance weight difference.

4. The counter balance weight might very well be the same for the 2 cranks? If the weight of the rods/pistons/pins/rings is the same for the SJ and LJ engines?

5. I'm not sure if BSA altered the balance of the cranks to suit the various pistons the bikes were sold with? Probably not? There is a service sheet on crank balance which from memory states the counter balance weight at 32oz, no mention of differences between the models or SJ or LJ.

6. I suggest you weigh your pistons/pins/rings, you might be surprised at the results, for example, I've found Hepolites to be similar in weight to JP's, and both being heavier than BSA OEM pistons. Generally lighter parts are better for keeping vibes down, assuming the crank has the standard counterweight.

Just my 2 cents worth!

Edit:

Here's a link to the service sheet 712x

http://www.petercomley.com/712X%20Flywheel%20balancing%20static.pdf

[Sparky]

I think BSA replaced the small journal crank with the LJ for good reason.  I rebuilt my A10 the first time in the early 70's.  It had a SJ crank.  I had it balanced and it ran smoothly and reliably, until about 1000 miles post rebuild. I was on an evening ride, about 5 miles from home, and noticed a lower end knock.  It progressively and rapidly worsened to the point I shut the engine down and pushed the bike the last 1/4 mile home.  On teardown, i discovered one of the big end journals was cracked all but about 45 degrees of its circumference.  A disaster was narrowly averted!

Obviously, not all SJ cranks broke, but it is worth noting that BSA only advised one regrind on them, so there wasn't much extra "meat".  I feel much more confident with an LJ crank knowing I probably won't be having to push my bike home again some night... or at least not for that reason.

[chaterlea25]

Hi Sparky,
Small or large journal cranks will break if the regrind is not done correctly
The important part is that the radaii where the journal and crank meet are done correctly
I have seen an A10 crank break after only a few miles after a regrind ???? the corner between journal and crank had not been radiused at all 

Regards
John

[KiwiGF]

H Sparky I had a 59 super rocket LJ crank break back in the seventies, on flat top pistons. Probably due to the radius not being done right I found out later. It's certainly not an issue limited to the SJ crank.

There is SJ v LJ "controversy" though....

The BSA service sheet list the SJ big end sizes to -030?

[muskrat]

 G'day Ron,
               I've had 2 SJ's go (they were in the weapon [racer]) both due to poor radaii. A good one lasted years. The one in my '51 is SJ and done right 30 years ago is still fine, I run 8.5:1 in her.
The radaii is important but so too is the surface finish of it.
Cheers
 Is that how radius x 2 is spelt?

[KiwiGF]

And the other thing to note is that if the radius is made too small by mistake that's the crank ruined, you can't put the metal back!

[Rocket Racer]

Back in the early fifties, a kiwi (the late Erle Culver) built a racing golden flash outfit, yes thin flange/small journal with 8/8.5:1 and a 358 cam -he used to rev it to 8000 rpm and had two full seasons out of it before selling it (it did subsequently blow up but not surprising)

The early road rockets (40hp) with 8:1 used small journal in 1954/55 and I have one on the shelf still standard.

For a golden flash with 7.25 or 8:1  ridden normally small journal should be fine, the lighter reciprocating weights should make a sweeter motor, but yes more fragile.

BSA apparently rolled the radii with ball bearings on the SJ cranks, the radii being the weak point. For a road motor I would have no issue with SJ, but I would be carefully checking my rods, crack testing and shot peening and replacing the rod bolts.

personally I wouldnt run a hotter cam than a 356 though.

[Sparky]

Obviously, not all SJ cranks broke, but it is worth noting that BSA only advised one regrind on them...

I know I had read something about this somewhere, so I went back to my Service Sheets and found that this applied only to some early A7 cranks and NOT to the A10 or later A7 cranks which as KiwiGF noted allow for 0.30 under.