Dennis Neville writes:
For those of you who may feel a bit embarrassed asking for tools using jargon you know nothing about, here is a decode:
Whit stands for British Standard Whitworth or BSW, a coarser thread form than British Standard Fine or BSF. Each Whit spanner will fit and probably be marked with an equivalent BSF size that will be 1/16in greater and may only be marked BS!
A/F stands for Across Flats and is used on American (Unified) threaded nuts and bolts, ie. UNC and UNF.
BA stands for British Association and is for smaller sizes and was used widely in electrical components. It is based on a metric system but Anglicised to Imperial units. Numbering starts at 0 which is the largest.
I am sometimes asked exactly what size is a spark plug spanner? A good question easiest answered by taking a sample of each type of plug and seeing which spanners fit! I searched through my tool kit and found the following:
- 14mm British unscreened or screened (7/16 Whit or 13/16 AF or 21mm.)
- 14mm Champion REL 37B (13/16 A/F.)
- 12mm British unscreened or screened (7/16 Whit.)
- 12mm NGK D6HA (18mm or 3/8 Whit.)
NB. 18mm is 0.001in smaller than 7/16 BSF or 3/8 Whit.
As British sizes are hard to find, a 13/16 A/F is a snug fit alternative to a 7/16 Whit and 18mm, an equally snug fit alternative to a 3/8 Whit.
However, it is not as simple as that: all plugs require a deep socket but, not all deep sockets or sockets marked ‘12mm or 14mm Spark Plug’ are deep enough to fit a screened plug. Different manufacturers machine-out their plugs and deep sockets to different bores and depth. It follows then, that to save a wasted journey, take a sample plug with you when you go to buy a plug spanner and try it for size.
To get at the front port plug you cannot afford too bulky a spanner as the air scoop gets in the way. Consider a socket with a hexagon machined on the outside so you can get an open-ended spanner on it if necessary. To save on bulk and weight, buy 3/8in drive sockets rather than a 1/2in drive.
Before you can take a screened plug out you have to disconnect the HT lead. This requires a 3/8 Whit open ended for British plugs or a 3/4 A/F for the American Champion.
Whilst on the subject of alternative spanner sizes, I will run through the usual ones:
Magneto adjustable points are listed as 4BA but 6mm is often a better fit. If you have a set of Terry magneto spanners the No 5 fits the points, but the rest do not fit anything else except the No 7 which will substitute for a 3BA.
The circuit-breaker assembly is held in place by a 3BA domed bolt which will take a 9/32 A/F or, maybe, a 7mm spanner.
If the safety cut out is fitted, a 1/8 Allen Key is required. Note: this is an Imperial size, not metric.
Remember, spanner sizes and nut sizes can vary depending on the manufacturer. This is not that noticeable on the larger sizes but on the smaller ones a few thou can make the difference between the spanner not fitting and rounding off the corners of the nut. You may find you need three different spanners to adjust the points on one engine.
To fit a magneto you will require a 3/8 Whit open ended, ring or socket.
Distributors should require a 2BA or, maybe, a 9mm or even a 1/8 Whit socket or 11/32 A/F to remove, depending on whether it is screened and what nuts are used. For unscreened magnetos the half-knurled, half-hexagon nut requires a 3/16 Whit. The thread stays the same at 2BA.
The slow running mixture strength and throttle stop are adjusted with a 3BA or 9/32 open ended; you will need two, one for the lock nut.
The cylinder heads are check tightened with a 3/8 Whit open ended or slim combination ring. An 11/16 A/F or 18mm may do. You can get at some nuts with a slim jawed crow’s foot. The later service tool for torque loading is a much ground down ring. Due to differences in manufacture, both of the nuts and spanners, it is worth while shopping around trying AF and metric sizes to get a good fit.
The pressure oil filter is released by a 1 13/16 A/F single hexagon socket deep enough to clear the filter turning handle. The filter hexagon is somewhat tapered so depending on the quality of fit, several different spanner sizes will do, eg. 46mm is 0.002in smaller than a 1 13/16in A/F. Originally, it was a Whitworth size, probably a 1 1/8 Whit. For big spanner sizes like this, use a 1/2in drive socket and drive. Caution: Do Not Overtighten.
The suction filter is primed by removing the brass nut with a 5/8 Whit open ended or ring. The tacho drive cable requires the same size, shortened for ease of access, and a 28mm should be an acceptable substitute. A multi-hex open ring or’ flare nut’ spanner would be ideal, if you can find one.
The exhaust and inlet manifold nuts are tightened with a 3/16 Whit thin wall socket or open ended. A 7/16 A/F will be a good substitute.
Rocker boxes are removed using a 3/16 Whit open ended.
The tappets are adjusted with the same 3/16 Whit open ended or combination.
The propeller is removed with a 3/8 Whit ring or socket if you wish to refit and torque load.
The hub is removed with a 1 3/4 Whit ring and "one yard" extension for a Gipsy Major 10 Mk 2! In theory the nearest alternative is 2 3/4in A/F or 70mm. For a GM 1 a 1 1/2 A/F or 38mm may be better for a closer fit.
If you are removing the oil hose connections from a Chipmunk oil tank you will require both 1/2in and 9/16in Whit open ended, shortened to get at the tank connections if the engine is still in place.
The manifold drain will require a 7/16 Whit to undo the drain pipe and a 3/16 Whit to undo the brass elbow. On the other hand both can be done with, dare I say it, an adjustable.
Fitting the bottom bolt on a starter motor on a Gipsy Major 10 Series is probably the most challenging job. If you can find or make up a 1/4 Whit crow’s foot it will eliminate some of the frustration.
Whatever you do, do not think you can fit Whit or BSF nuts on a Gipsy engine. As explained in Technical News Sheet G.E.6, de Havilland used a modified SI metric thread on their piston engines but modified the hexagonal head forms of the nuts and bolts to suit British spanner sizes.
Do not forget that the bolts holding the propeller hub together and the slotted nut holding the spinner in place has a metric thread form like the rest of the engine.
In their original engine tool kit, de Havilland made great use of box spanners and these are still made by Melco. According to their catalogue, King Dick still make Whitworth and BA spanners, or find a retired British Rail engineer and ask him if he still has his toolkit. For those who have difficulty finding anything to fit, try converting an 11mm spanner to a 3/16 Whit by spending some time with a small, coarse stone, and gently open it out. This is easy with an open ended; the same can be done with a single hexagon socket and, if ground down on its outside diameter as required, will be useful on the inlet and exhaust manifolds.
Finally, if you have to use alternative sizes, use single hex (6 point) sockets as there is less chance of them slipping if oversize and damaging the nut/bolt.
©Dennis Neville 2020