Friday, 22 February 2019

FH Crankshaft 2 Review

A while back, I dismantled and measured up the small journal crank that came with the 1955 engine I have in my 1958 FH kit, now christened 'Crankshaft 1'. The post is here: https://ariel...small-journal-crankshaft.html.

As we discussed, the 1958 cases should be fitted with BSA's new large journal 'Rocket' crankshaft and in view of the wear on the 1955 crank, considerations of originality and the fact that a well know retailer had a standard large journal crank available at a very reasonable price led me to replace it.

Here it is; no flywheel, somewhat rusty...


Of the three crankshafts I have, this is the only large journal crank. We'll call it Crankshaft 2.

It's got nice looking splines and drive shock absorber thread:


Less nice - looks like the pump drive end has had an argument with Thor, King of Hammers and not come off too well. And why is that bearing stuck half way along the journal?


Other than that, no new horrors, but we need to understand what we are aiming at. This BSA service sheet gives us the dimensions we are aiming at; small journal cranks on the left, large journal on the right:
We need to get it up in the vice and clean it up.


First job is to get some oil on that bearing and pull it off. We need to decide if this crank is a pile of scrap and the ashes of burned currency or whether it will become the foundation of my new bike's engine.

Cleaning the surface rust of the big end journals reveals some smooth surfaces, with no score marks, but with small rust pits:



Both of these journals are 1.655" - 1.656" diameter, measures in several places - which equates to an unworn -0.030" grind; not the standard cranks I was hoping for but usable nonetheless. We'll need to polish them but for now we can tick that box.

Next is the dubious looking oil pump drive thread. It's 9/16" - 20 CEI, left hand - no chance of finding a die for that, unless I buy one and that in itself might be a problem. I could clean it up in the lathe, but that will be another challenge as the end has felt the hammer and the centre drill hole is deformed and probably wont fit the dead centre in the tailstock - so we'll need to buy a fixed steady if we are going to try that approach. Breaking out the thread files cleans the end up nicely enough that the old oil pump drive fits beautifully and reveals their is still enough thread for the lock nut.

Next stop, the timing side main bearing journal. Unfortunately, this measures 1.242" - which is about 120 thou undersize. I can make a bush to fit it, since it's in good shape but is it safe to use? If I use it I will have to recut the oil way, since that has virtually been ground away.


The thrust face is a bit untidy - there is a small ridge which will clean up.

Thinking about this, I've realised that the drive side journal is 1.18", with all the bending load that is subject to from the primary along with the cyclic loads from the power stroke. I guess a 1.242" timing side journal will always be OK.

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