Tuesday, 10 January 2023

The AJS 33/7 Trophy Model Restoration: The Crankshaft and Piston/Cylinder Assembly

Up to this point – one year into the restoration – I had not removed the cylinder barrel/piston or split the crankcases. Why has it taken so long? Firstly, there was no necessity to check these before completing the dry build – the bike could be built around these and, secondly, there is always “downtime” in a project, ie a period waiting for other things to happen that could be used to check these components. In the second week of January 2023, I was waiting for the powder coating and chroming to come back from their respective suppliers, I had zinc chromated or chemically blacked all of the components that I finish myself and so it was time to get back to the engine.

Before removing the cylinder barrel I measured the stroke with a vernier; it came out at 101.7mm. This was not what I was expecting! I had already measured the bore and found that to be 77mm (76mm + 0.040" or 3 inches?) and it just so happens that 101.7mm = 4 inches. This gives a swept volume of 473.6cc – somewhat greater than the original 350cc and close to the 33/10 version of the engine.

For the record, the original AJS 33/7 had a bore and stroke of 70mm x 90mm.

You may recall that I bought this bike from a vendor in Canada and with a bore x stroke of 3" x 4" coupled with adaptation to a flattracker suggests that it had been thoroughly modified for racing in North America.

The cylinder barrel and piston were removed and the engine was now reduced to all of its component parts.

The piston was covered in assembly grease

which was washed off with solvent before measurements could be taken.

The piston was brand new and was fitted with a modern ring-pack – relatively narrow top rings and a 3-piece oil ring arrangement. This ring-pack is pretty well identical to the Total Seal rings that I used in the AJcette (see https://www.vintageajs.uk/2020/08/piston-and-rings.html). The die-cast piston itself had minimal manufacturers markings – the number 399 on the underside of the crown but that was all - I have no idea for which engine this piston was originally intended.

The following measurements were taken:

Piston/Liner clearance at bottom of skirt on thrust/antithrust surface: 0.007”

Ring Gaps:

          Top Ring: 0.015”

          Second Ring: 0.018”

Piston (bare) Weight: 336.5g

Piston Rings Weight: 30.3g

Pin + Circlips Weight: 82.4g

Total Piston Weight: 449.2g

The crankcases were split to examine the crankshaft and bearings. The crank is pictured below.

There has clearly been extensive drilling to balance the crankshaft – this needed to be checked and the connecting rod

turns out to be from a Harley Davidson knucklehead/panhead. It will be in good company in the garage with the V-Twin, which also uses HD rods https://www.vintageajs.uk/2021/04/the-ajs-v-twin-crankshaft-part-1.html.

I don’t know the origins of the flywheels but probably also HD.

It was time for a few more measurements. First, the weight of the connecting rod small end:

and, very importantly, the crankshaft runout

which I measured at 0.001” maximum. Very satisfactory.

With the crankshaft mounted on the jig I also attached weights to the small end to determine the mass to be added for perfect balance; this was 248g.

With all the above information, the balance factor can be calculated, as follows:

Reciprocating mass (Mrecip) = mass of conrod small-end (Mconrod) + mass of piston

                               = 189 + 449.2

                               = 638.2g

Mass added for perfect balance, Mperf:

Mperf  = Mrecip x BF - Mconrod

Which gives the Balance Factor as:

BF = (Mperf + Mconrod) / Mrecip

      = (248 + 188.5) / 638.2

      = 0.684

Or 68.4%.

This is a perfectly acceptable value of the balance factor for a single cylinder engine and, together with the crankshaft runout, suggest that whoever put together this part of the engine did an excellent job.

The last dimension to be checked is the crankshaft end float, but before doing this I decided to replace all the main bearings. The bearings that were in the engine were useable except that I prefer to use a seal on the outer side of the outer drive-side main bearing to reduce oil leakage out of the end of the crankcase and the timing-side main bearing did not run quite as smoothly as I would have liked.

I replaced all main bearings with those shown below.

I have found these Japanese made KSM bearings to be of excellent quality and at a good price and I have used them extensively in recent years.

The following measurements were made to determine the end float:

Drive-side crankcase: mating surface-to-bearing = 1.413”

Timing-side crankcase: mating surface-to-bearing = 1.344”

Which gives an internal  bearing-to-bearing distance = 2.757”

Crankshaft mainshaft boss-to-boss dimension = 2.725”

This gives a perfectly acceptable crankshaft end-float = 0.0325”

Another check (not shown here) is that the connecting rod is centrally located – which it is.

And the final check is the compression ratio.

With the crankcases, piston, rings, cylinder barrel re-assembled

It was time to put on the cylinder head, temporarily, to measure the TDC combustion chamber volume.

Using oil from a burette though the spark plug hole, the volume was measured as 59cc.

This gives a compression ratio of:

CR = (swept volume + TDC volume) / TDC volume

       = (473.6 + 59) / 59

       = 9

So, it turns out I have a fairly long-stroke engine (the 500cc model of the day, the 33/10 was 79mm x 101mm – so very similar) and a reasonably high compression ratio. I would expect performance to be “brisk”.  

Overall, I’m very satisfied with the mechanical integrity of the bottom-end, the piston/rings/barrel and the cylinder head and together with the work that I have previously reported on the cambox and the drive to the overhead cam I believe this will be a powerful and strong engine.

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