Crankshaft for 650SS 1968 Mercury

The 650 flywheel is peculiar to that engine size, the Atlas and 750 Commando one is different, heavier, I think, and balanced differently,  it would 'fit' to your 650 cheeks easily enough, but you would only find out what that engine would run like by trying it, very roughly, I wouId imagine. I suppose one could be modified if you had a drawing of the original but it would be an awful lot of trouble to go to to make a standard bike, and would probably cost more than getting one sent from over here.

Good luck.

Hi Dennis.....your Mercury 650 does indeed have an Australian heritage having been imported by Burling & Simmons into  New South Wales. However my records show that this happened in March 1969.

The crankshaft will accept an Atlas (and even a Commando) flywheel but will need to be rebalanced to suit your conrods and pistons. If used, a Commando flywheel will need some serious machining due to its balance factor being considerably different to that of a 650.

The other point to check is that after mounting the pistons on their conrods they clear the shoulder of flywheel at BDC.

Hi Denis,

               I have an Atlas crank. I reside in Melbourne. If you are interested I will check the journal size.

When the Atlas motor went into the isolastic frame to make the Commando, a completely new primary drive with triplex chain was introduced, the drive side shaft being redesigned to suit, hence the change to the part number.

Denis I have a complete Atlas crankshaft assembly. It appears to be 20 thou under.

Hi Dennis......The attached photos should help to ID any crankshaft chunks you are offered.

The Timing Side cheek, as you have discovered is almost the same for all the bigger engines. The 650 & Atlas flywheels were slightly different in terms of mass and machining but with some engineering can be used in either engine.  The photos show the difference.

The Commando Drive-Side cheek has a different shaft to enable the Triplex sprocket to fit. A quick 'spot the difference' check is to look for the alternator key-way. On the Atlas & other smaller Dominator cranks this is placed next to the sprocket taper. On Commando shafts it is further outboard next to the crank nut thread.

Hi Denis, why not install my crank complete in your cases. The crank would need to be re-balanced. If you go to the forum archives the correct balance factor is available. Philip Hannam probably knows the figure without looking it up !

Cheers Andrew

Denis.....get all the crankshaft parts in a bundle and take to a recommended company that knows how to balance a crankshaft. So that is the refurbished crank, the rods, big-end bearings and pistons. Some of the better companies can regrind the crankshaft and then balance it as well.

The balance factor should be around 70% for a 650 engine.  That is for a static balance with a dry crankshaft. Some engineers like to fill the shaft with oil to replicate running conditions. Dynamic balancing, where the crank is spun at speed, is also another option. Everybody has their favourite.

The attachments are words of wisdom  from John Hudson, plus a DIY version offered years ago in Motorcycle mechanics.  Plus more....the Steve Robinson Notes on crankshaft balancing.

I had a crank ground and balanced at SRM recently, £460. Including return postage. 
Regards, Al.

Denis......The Atlas and 650 crankshafts are identical apart from the respective flywheels being machined and balanced a little differently.

SRM are a good company but very pricey.  I think the postage costs from Australia would also be off the planet.  Go local.

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