Weighing my options....

Now that I am firmly determined to complete the Matchless twin by spring, the time has come to put together the engine.

While I did a dry fitting last year,  I didn't thoroughly measure the crankshaft journals, camshaft lobes or check for 'Ovaling' in the connecting rods.  The motor seemed to be in decent shape when I tore it down; I naively hoped that after a good cleaning and check of the oil passageways it would go right back together and run!

I decided to forgo this flight of fantasy and do the job properly.

I started by looking at the connecting rods I had and weighed them in order to find a balanced pair.

The pair that came with the engine weighed in at exactly 12.84 oz each.  Numbers 2 and 3 weighed in at 14.53 and 14.29 oz respectively.  Lonely number 5 on the end came in at a whopping 15.03 oz.

Interestingly, all Matchless/AJS twin con rods from 1949 to 1964 have the same part number, 014134. In the course of my research, I found a valuable snippet from the Jampot forum which explains the differences.

Early Rods for '49-'51 had a thin 'H' stem section and less material around the small end

Series 2 from '51- Had a thicker 'H' stem section however material around the small end is unchanged.

Series 3 featured more metal around the small end.

Series 4 featured even more metal around the small end.

Series 5 featured the addition of the chamfered shoulders. 

These changes would reflect the heavier load on the rods as capacity increased from 500cc to 550cc to 600cc and finally 650cc. 

Looking at the lightest pair, I would assume they are Series 2 as the 'H' section seems the same as the heavier units.

The stock G9 Crankshaft Journals are 1.625" stock, mine come in at 1.615 according to my micrometer.   This means .010 oversize bearings, fortunately I have a NOS set.

As I went to fit the series 2 connecting rods, I found that one side would move freely and the other would bind.  I attempted to flip the orientation of the rods to see if that made a difference, however there was no joy.  I then fitted the heavier rods and all moved well without issue, however the bearings didn't sit very tightly in the conrods.  The orientation of the later rods is easy to determine due to a chamfer on the shoulder of the rod which needs to face away from the center of the crank.

I did look on the shelf and pulled out a later crank which fit nicely into a nice set of  '54 AJS cases
I had bought a couple of years ago.  These journals also measure at 1.615" however one side has suffered from exposure and is now rusty.

The earlier crankshaft

Later Crankshaft.  Note the corrosion on right journal side. It looked much worse before a bit of WD40 and scotchbrite.

 All hope is not lost though as I found a great YouTube video demonstrating cheap, easy and effective crank polishing.  My next job is to measure the connecting rods properly, save the crankshaft and see what can be put together.

Will it work?  We shall see.

Until next time......