pinion opinion

[RupertW]

Can anyone explain in simpleish terms why it's so bad to mismatch CWPs? I can fully understand why a matching pair is preferable, but I would have thought an unmatched pair would soon enough wear in to match each other without terminal damage?

Given the price of new CWPs and the relatively plentiful supply of unmatched pairs I would have thought it's a viable option even if not the ideal?

[Stuart Giles]

Can anyone explain in simpleish terms why it's so bad to mismatch CWPs?

I'll try, but the geometry of gears is not an especially simple subject. The actual shape of the teeth in two CW & P sets may not be the same. Gear teeth have something called a pressure angle as part of their inherent design; this angle needs to match across any pair (or train) of gears. Even if Austin didn't change the geometry of their gears during production, some Austin factory supplied gears were reputedly made by ENV; so unlikely that 'one of each' would make a good pair.  pretty well all spiral bevel gears made at the time (and quite a lot since) are matched after manufacture, in simple terms, oversize pinions are matched to undersize  crownwheels and vice versa. after the pairs are matched up, they were lapped together to ensure a good mesh and silent running.  If you had a load of unpaired CW & Ps in your garage, selecting the ones that give a reasonable 'print'  when assembled and then lapping them together using some fine grinding paste should work. The ideal print (one I did for a mate's Westfield) looks something like this. This print comes from painting a thin coat of  chrome yellow on the pinion. I find it's much more visible than using Engineers Blue.



If you find you're starting off with toe contact on the drive side and heel on the overun side, or the other way round, the gearset is likely to be really noisy and due to the high 'point loading' is unlikely to live for any  length of time.

[Parazine]

In the post vintage years, new CWP's were manufactured by a variety of firms and some in-house. Pinions and crown wheels were inspected and those with matching tooth forms and characteristics were "matched" as pairs and then run together in a polishing/lapping operation, using a fine abrasive to finalise the tooth form and contact area. They were then assessed and if found to be quiet in operation, marked with the matching numbers and their setting distance, for use with the assembly gauges.

Apparently, vintage CWP sets were all made by ENV and are more reliable at random matching successfully.

So you should be able to see that selecting a random post-vintage crown wheel and a random pinion, possibly from different years and potentially different manufacturers is a lottery. I won't deny that sometimes, they can be carefully assembled, occasionally quietly, sometimes reliably but they should at least be run in a bath of mild abrasive to emulate the original lapping procedure before being carefully assembled with engineers blue to assess the mesh pattern, which is unlikely to be correct.

Of course, you can just "slam 'em together" and run them but we wouldn't do that these days, would we? 

I personally, value my time higher than the several tens of hours required to do this properly, with a only small chance of success. If your time is free and you don't have any other call on it then go ahead but I'll be fitting one of the readily available, excellent quality replacements which can be installed and set up in a small fraction of the time. It's a small investment if you put a monetary value on your time......

[JonE]

That last paragraph is so true. I was actually thinking that (after rebuilding an engine replacing just the broken piston rings to see what would result) the pinion that started the thread would be the sort of thing that something like "Henry's Ruby" might take on, with a suitable clean crownwheel, should it ever need such. i.e. where there is time and interest just to play and learn and make mistakes along the away, but where final success is hard won. I'm enjoying the playing and learning but am getting too encumbered by sheer practicality as months and years pass.

[RupertW]

Thanks all, that's been interesting to read, and certainly appreciate the point on the investment of time. I've got to a point in my life where I work out how many hours a job will take me, give myself an hourly rate, and then work out if it's actually 'cheaper' to either use new parts of pay someone else to do it properly!
Main reason I asked is that I have what I think is an unmatched axle and torque tube and have often wondered if it might do as an emergency backup if I put it together.

[Bob Olive]

Didn’t Colin Chapman (back in the good old days of well-stocked scrapyards) assemble what ever CWP bits he had, and fill the axle with Bluebell metal polish and then run it until it all ‘matched’ (more or less!)?

[Charles P]

Didn’t Colin Chapman (back in the good old days of well-stocked scrapyards) assemble what ever CWP bits he had, and fill the axle with Bluebell metal polish and then run it until it all ‘matched’ (more or less!)?

Yes, but he never worried too much about how long his cars lasted.

[Bob Culver]

I recall reading that Chapman mixed ratios!!
From experience with two Regent sets the cws seemed soft. The pinion wore hollows in the cw making reassembly tedious.
Normally any wear is very slight.
I have seen lapping demonstarted on a Gleason machine. A firm used it to try and cure gaurantee comebacks for noise. The pinion osciallted about over a small range so the final mesh was not hopelessly critical

[Ruairidh Dunford]

Dave Williams mixed ratios with some success and also some, not so…