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Fitting close ratio gears

Three speed boxes are easy in this respect, the different gears are simply fitted as replacements.

 

There’s a bit more involved with four speed syncro boxes. Again I would recommend a good read on the various things that have been written on four speed boxes. Ones I prefer for accuracy and good insight on wear and other problems are the ones in the ‘Green’ Companion, the Design for Competition and the original Austin Service manual. Strangely Bill William’s Austin Seven Specials is not very accurate, I wonder why, he must have done a number of boxes?

 

Dismantling is as per the guidance. The input first motion shaft and gear is a straight replacement of the original. Remember to check out the internal core plug and fit a new one if needed. Another area to check is the small syncro teeth behind the bronze taper hub. They are supposed to be tapered at the start of one side of the teeth to help gear selection but not all the way along. There should be a bright ‘witness’ mark but the teeth should still look parallel for most of their length. The bronze hub and the syncro ring can be loose on the shaft. Support the back of the syncro ring across the open vice jaws and knock the first motion shaft through it with a suitable sized socket to dismantle. The same goes for the third speed and second speed syncros. Good news is that they’re easy to change using good bits from another spare gearbox (who hasn’t got a few stacked under the bench?). Degrease and Loctite them in place on reassembly. Don’t get the syncro ring round the wrong way, there’s a raised lip on the inside diameter, that goes on the opposite side to the bronze hub.

The other part to be worked on is the layshaft cluster. The thrust face on the bronze bush in the ‘big’ end is invariably worn, there’s usually a lip on the outer edge. The two halves of the gear have to be separated to fit the new close ratio gear as per the picture above right. This isn’t easy! There’s a circlip behind the thrust face. I prefer first knocking out the bronze bush carefully trying not to damage the end too badly. Use a large diameter drift that has a sharp edge to get behind the bushes shoulder. Once the bush is out carefully expand and prise off the circlip. Take care not to distort this too badly—new ones aren’t available and it has to be refitted. Unlike normal circlips it isn’t made from spring steel and does tend to distort a bit. Once that is off support the third speed gear between open vice jaws and knock the first and second speed end through. They can be a bit tight and have been together for many years so can take a bit of starting.

 

The new close ratio end fits back onto the original first and second speed end, make sure the oil holes line up on the orientation of the splines. Sometimes these are really tight and a little heat on the larger one helps them back together. Make sure they’re both pressed right together, there should be a small gap between the second and third gears and the circlip groove should be clear. Get the circlip back on. Close it up again using grips, vice jaws or whatever pulls it together for you. They never seem to go back as close a fit as they came off, just make sure it’s secure and won’t come off when running.

 

Dress up the small end of the bronze bush and check it is a good sliding fit on the layshaft. Clean up the thrust face and remove the lip. There can be up to 1/16” wear on the thrust and this has to be readjusted back before reassembling the gearbox. The easiest way of doing this is to fit a washer behind the bush, something like thirty to forty thou. Refit the bush and washer, they usually knock in easily with a hammer and flat face.

 

Now the end clearance has to be adjusted before reassembling the box. With the casing, layshaft and end bush cleaned up, drop the laygear into the casing, the big gear goes to the front. Austin’s assembled the box with paper gaskets behind the layshaft and end bush, the thickness of these determines the gear end float. Trial assemble them with no gaskets at all and knock the layshaft into place. If the thickness of the washer you’ve put in behind the bronze bush was thick enough the gear should lock up and there should be a clearance between the layshaft flange and the outside of the casing. Measure this with feeler gauges. Gaskets to be fitted should take up this clearance plus add another five to ten thou to give a running clearance on the gear. If there’s enough clearance fit a gasket at the front behind the bush to start with and recheck. Different thickness gaskets can be made from differing quality papers, ordinary white printer paper seems to be the thinnest with brown paper envelopes being progressively thicker. Fiddle about with this until you get the five to ten thou clearance, preferably nearer five than ten.

 

If the washer you fitted behind the bush wasn’t thick enough there will still be a running clearance on the gear even with no gaskets fitted. Measure with feeler gauges and take a decision—do you want to knock out the bush again (and damage it some more) or run with a wider clearance. If it’s up to about twenty to twenty five thou clearance I’d be tempted to go with it as it is and not fit gaskets, just reassemble the layshaft and end bush with a sealant.

 

OK, once you’ve got to this point it’s a straight forward reassembly. The final part is to adjust the selector fork positions ensuring full syncro engagement. Do reverse gear first and always adjust with the horseshoe gate in place—it’s a good guide to the right positions and you also have to make sure the adjustment doesn’t put the forks hard up against it. Put the reverse fork and rod in on its own and don’t bother with the detent ball and spring yet. There’s little clearance between the gear rubbing on the rear of the casing or conversely snicking up against the first gear, you can just get your little finger in through the filler hole to check it rotates freely. I’ve found there’s little scope for getting this right in the confines of the horseshoe gate and getting the detent in the right place. Once you’re happy with that assemble all the forks and then the rods with balls and springs. Reset reverse back to where you had it and similarly adjust third and top checking by engaging the gears. Check that the syncro hub engages fully with the syncro ring for both gears. A little dentists mirror on a stick helps to see top gear engagement. Set up first and second so that the gearlever slot in the top of the forks lines up with third and top, check the engagement of the second speed syncro but take care it can be over engaged and the fork rubs up against the side of the third speed gear. If it’s OK the sliding horseshoe gate will move freely and stop more than one fork being engaged at the same time and the bottom of the gear lever will move easily from side to side in the gate. If it won’t you’ve got something wrong—have another go at adjusting! Refit the lid and check all the gears engage with the lever.

There’s quite a lot of information in all the usual places about rebuilding axles and trying different ratios to fit. Make sure the gears are meshed properly. Not only is it depressing when all your work results in a big bang and no motion but good crown wheels and pinions are getting hard, and expensive, to get.

 

If you have a standard Austin Seven and want to retain the standard wheel size, 19” for the early ones or 17” for the Rubies try a 5.625 axle and a close ratio gearbox set (3 or 4 speed depending on what you’ve got). The low ratio axle offsets the slightly higher first and second gearbox ratios so getting away on a hill is manageable. The ratios come into their own once you’re into the two top gears.

 

Trials cars need as low gearing as possible on first so go for the 5.625 axle with either a standard three speed or ‘crash’ four speed gearbox. 4.00x18” Dunlop K70’s seem to be the VSCC ‘legal’ standard for trialling A7’s.

 

For a fast road special you want a gearing of about 13 to 14 mph per 1000 revs in top so match your wheel sizes and axle ratios from the charts. Close ratio gearboxes are a good idea if you’ve got small wheels or a low axle ratio to keep first usable and not just a ‘crawler’ gear. This is more so if you’ve got a hot camshaft that only pulls well in the higher revs.

 

The list below are only some typical, commonly used sizes. There’s more information about tyre diameters on the Tyre Company websites. Alternatively it’s quite easy to infer the sizes in between aiming for the combination you’re after.

 

If you want to work it out for other tyre sizes use the sum below to work out the revolutions per mile (WRPM) for your tyre size.

 

Wheel revs per mile WRPM = 534585 / Tyre diameter in mm

 

From that use the one below to get the speed for every 1000rpm in top gear.

 

Mph per 1000rpm = 60000 / axle ratio / WRPM

 

If you’re not sure use some of the numbers in the table and work them out, you should get the same answer as me if you’re doing it right!

A further consideration here is the gearing for the speedometer. This is fixed directly to the number of turns the prop shaft makes, the more it turns to travel a mile as with a lower geared axle ratio the further out the speedo will be. There’s another set of sums you can do for this. Speedo’s often have the turns per mile on the front face, a number around 1000 for later four speed A7’s. There’s a good write up on this on the Dorset A7 site but for completeness here’s another set of sums.

 

Prop shaft turns per mile = Tyre revs per mile x axle crown wheel teeth / pinion teeth

 

From this multiply by the speedo worm teeth and divide by the number of teeth on the speedo pinion to give the number of times the cable will turn for every mile driven. Four speed boxes usually have a two start speedo worm with an eight tooth speedo pinion, three speeds a four tooth worm and seven tooth pinion. Check these gears though as there are other combinations around, the Nippy box has a 9 tooth pinion and two start worm.

 

Here’s one worked out for a Ruby with a 42/8 axle, 4.50x17 tyres and with the 2/8 four speed speedo gears.

 

Speedo cable turns per mile   = 681 x 42 x 2 / 8 / 8

                                                       = 894

 

Typically speedo’s that are marked have 1040 turns per mile on the face which in this case would give an error of 9%, ie reading about 27 mph when you’re really doing 30. The 4.50x17’s are slightly bigger section than the 4.00x17’s originally fitted that some way explains the difference.

 

Speedo’s can be recalibrated but I confess I’m not clever enough to do that. Another way is to make a new face with a more accurate calibration. There’s template faces on the Motoring Pages, download and shift the graduations slightly in the right direction. Before you go to all that trouble though it’s best to check the speedo accuracy against a modern car, their speedo’s tend to be quite accurate. Get somebody to follow you and flash their lights or some other illegal signal when you achieve a steady 30 mph, 40, 50 etc. Compare that with what the speedo was actually reading and that’s what you have to compensate by. The odometer is harder to fix as it’s on set gears within the speedo. If its not far out you could do this mentally as you drive along. If you are making a new face try and get a proper screen printed one rather than a paper one printed direct from the computer. The black from a paper print fades very quickly.

 

 

 

Gearbox ratios are covered in an number of places in the standard A7 references. The tables below are some of the more useful and obvious ones.

Spotting close ratio gears

There seems to be loads of spare gearboxes about perhaps a testimony to their general running reliability. Occasionally one of these has Nippy or other special gears in and it’s useful to know how to easily spot them. Of course the most reliable way is to count the teeth on the constant mesh gears but that isn’t easy or quick especially if the lids on.

 

Nippy sports boxes had a letter S stamped next to the box serial number. These are at the rear of the box just behind the filler hole. While the guts may have been changed it’s probably a good bet as being Nippy ratio—ed.

Standard ratio on the left, Nippy in the centre and the straight cut one on the right is a close ratio. Note the relative position of the outside of the gear to the bearing.

Rawson 4 speed close ratio gears. Note that the new constant mesh gears are straight cut unlike the third gear. This makes the gears noisier in first, second and third. Once the drive is straight through in top it quietens down. Straight cut gears are regarded as stronger. SPEEDEX close ratio gears were straight cut like these whereas the ‘Super’ ones were helically cut (and hence quieter) like the original Austin ones.

Gearbox miscellaneous

· The gearbox input shaft has a core plug seal in the centre. An oily clutch can be from this leaking rather than the rear engine seal. It’s a normal 7/8” dished core plug fitted from the gearbox end. If you do like using a sealant on the plug beware that the inner diameter of the shaft is a bearing surface, smear a slight smidgen on with a cotton wool bud from the front end of the shaft.

· The very last 4 speed boxes had oil seals on the input and output shafts. The input one is Payen part number B122 and the rear is NA320/C529. Any bearing supplier will be able to cross refer these numbers to modern seals and get them for you.

· The 750MC ‘Design for Competition’ has a couple of articles on overhauling the four speed box and another one in the ‘Green’ Companion. A Digest of Technical Articles from the PWA7C Magazine published by the Pre-War Austin Seven Club has similar notes on stripping three speeds.

· Building Austin Seven Specials by Bill Williams also has info on gearboxes and also tells of and has a photo of rotating the speedo cable drive on the later 4 speed boxes to bring the cable out upwards. This is a worthwhile mod for a low special or trials car to avoid trailing the cable along the road, it can come straight up and into the back of the speedo like the early three speed ones. Obviously you’d have to have a shorter cable.

· Syncro and fork detent springs seem quite fragile in the four speed boxes, this is the usual reason for jumping out of gear. They and the associated balls (which always seem to jump across the garage when removed—wrap a rag round the hub before dismantling) are available from the standard A7 spares suppliers, replace the springs even if they look OK. They aren’t the same or interchangeable between the two applications. Fork detent balls are 9/32” diameter and the springs are 0.270” diameter by 0.650” long. Syncro detent balls are 1/4” diameter and the springs are 0.235” diameter by 0.630” long.

· Output flanges on the four speed boxes are interchangeable between the early crash box with the three legged spider and the 6 or 4 bolt Hardy Spicer flange. It’s a common conversion of the crash box to have a Hardy Spicer flange to use the later UJ prop shaft.

· UJ joints are the same as used on A35’s, Morris Minor’s etc, they are still easily obtainable from good local spares outlets.

· The pudding stirrer gear change can be a pain on a low special, it has to be bent to fit under the dashboard. It can end up being not easily to hand and also a bit vague in getting the right gear—especially reverse. After another crunched gear in Chris’s Nippy the cry goes up; ‘they’re all in there somewhere’! Specialists in the sixties made remote change conversions to overcome this. The simple ones had the lever cut off and connected with a parallel linkage to a new stubby lever mounted on the transmission tunnel. These are fine for three speed boxes or the forward speeds of the four speed. But they are an abomination getting reverse on the four speed, you can’t lift the lever to get across the gate. The better linkages like the Speedex and Cambridge had the new parallel link in a tube with the new gear lever spring loaded. By pressing down the lever the parallel link pivots on a button in the tube and lifts the lever at the gearbox end to get over the reverse gate. If you can’t find a good sixties one or don’t want to make one, John Barlow has recast the three and four speed Eltos remote change.

One interesting possibility is the Austin 10 gears, the gearbox internals are nominally the same as A7 although the input shaft will be different. The ratios are very slightly wider than Nippy ones, could the internals be crossed to provide an alternative set of ratios?

Another way to count the teeth without opening up the box is to insert a clutch plate and select first gear. Turn the tail shaft flange slowly and count the number of turns the clutch plate makes. A standard 4 speeder will make 44 turns for every 10 turns of the output flange. A Nippy 39 turns for every 10 and a close ratio 32 turns etc.

If the lid is off the difference in the diameter of the input constant mesh gear can be seen. Standard gears only go about halfway across the bearing width, Nippy ratios reach the inside of the outer bearing race and closer ratios go across the outer race some way. See the photo below.

 

A final thing about choosing a gearbox to use is to check the syncro teeth, general lack of corrosion and dirt inside and no undue movement where it shouldn’t be. Ensure the gears engage and ‘feel’ if the detent springs click into the grooves when the gears are engaged.