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I have been building a new four synchro remote gearbox for my old Group N race car and I decided to try something that I had been curious about for a long time – a sprung gate conversion to help improve the shift quality. I thought this was the ideal time to bench test the sprung gate conversion and see if it feels worthwhile.
I had read about similar conversions years ago in an old book or article (maybe Clive Trickey?), and more recently, Phil from Steveston Motor Co demonstrated his interpretation of the concept in a Youtube video several years ago. However, I couldn’t find any technical information about how to go about this modification. Now that I have done it, I thought I would share my approach for anyone else who is interested in giving it a try. So, here is a description of how I tackled a sprung gate conversion on a 3 or 4 synchro remote change gearbox. Hopefully it might be interesting and useful for anyone thinking about trying a similar mod.
This modification is simple on the surface - it simply involves adding a spring inside the gearbox on the selector shaft that runs diagonally down into the box from the external shifter knuckle linkage. The toggle lever at the bottom of this selector shaft engages with the three selector forks that respectively engage either Reverse, 1-2 gears, or 3-4 gears.
I found a reference on Instagram to a person (@thatyellowcooper) who did this modification about 5 years ago. After some trial and error with springs, he reported that the mod has been working well.
All the (limited) reports I have found about this mod indicate that, despite being very simple on the surface, the installation is very sensitive to the selected spring tension. Based on my preliminary experience, too much pressure is bad. I would say definitely pick the lowest spring pressure that seems to work for your application… The first spring that I trialled was 45mm free length, 2mm wire diameter stainless. I used a stainless spring simply because that was what I could find at the time on eBay.
Spoiler – that spring didn’t work. It was too stiff. When I bench tested the shift with the remote gear lever assembly, the shift was inconsistent and troublesome.
Next, I tried a longer 50mm spring, and a 5mm shorter tube spacer to match. For my application, this spring combination seems to work a treat. I now have a nice, positive and very slick gearchange – probably the best I have ever had in 40 years of playing around with Minis. I am using a standard ratio shift lever and the shift throw is so short and precise that I can’t imagine why anyone would want a quick shift lever on one of these gearboxes (other than to mask excessive wear in the OEM gear shift linkages and selector forks etc…).
I haven’t got an accurate means to measure the spring rate. However, in any case, the required spring rate may vary depending on how new/worn your gearbox detents are. So, what works for my new gearbox, (with all new detents and springs) might be too much for a worn box.
The spring I used has an internal diameter of 16mm which just moves freely over the thicker upper section of the gear selector shaft.
To support the spring base, I used a flat machine washer taken from an A series rocker cover shaft spacer. It was 1.3 mm thick.
You will also need a short section of tubing to complete the assembly in the box. Mine measured 56 mm with the 50mm spring but your mileage may vary. This tube is required to support the spring and keep everything from touching the differential pinion retaining nut and locktab on the end of the mainshaft.
I measured the maximum assembly length with some inside callipers and came up with a distance of 106.5 mm.
This was the dimension when reverse is selected. (ie this is when the selector shaft is pushed down into the box).
In neutral (ie the 3-4 gate), this dimension reduced to 97.4 mm.
When the 1-2 gate is selected, this dimension reduces to 89 mm. (This is when the selector shaft is pulled out fully). These measurements give the total spring travel (compression, or max lift in valve spring terminology) of 17.5 mm.
• Spring free length = 50mm
• Spring fitted length = 50-1.3mm (thickness of washer determines preload). = 48.7mm
• Spring max compressed length = 32.5 mm.
When you go to assemble the conversion, there are a couple of things to consider.
There is a 1/8” wide woodruff key in the lower portion of the selector shaft to index the selector toggle.
It is not feasible to remove this woodruff key and then replace it inside the gearbox case when assembling the sprung gate.
Therefore, it is necessary to size the support tubing with an ID of around 16mm so that it can pass over the lower section of the selector rod with the key in place. I used 1mm wall thickness tubing (OD 18mm).
I measured my shaft with the key in place and a 16mm ID tube was a neat fit. You may have a problem if your woodruff key sits higher than mine.
The other fitting tip, assuming the selected washer ID is a close fit over the lower shaft diameter (which I recommend) is that you will need to cut (file) a small keyway into the washer so that it can slip over the woodruff key.
How to assemble the parts:
You should have a spring, a support tube, and a washer to fit between the spring and tube.
With the selector shaft removed from gearbox, partially reinsert the shaft into the gearbox case.
Thread the spring onto the upper shaft, ensuring that the spring ID does not bind on the shaft.
Next, fit the support washer, then the spacer tube below spring.
Slide the selector toggle onto shaft over the woodruff key and then fully insert shaft into gearbox. You will have to overcome the tension of the new spring assembly to do this, but it should not be excessive.
Secure the fork to the shaft with its locking set screw as normal.
Fit the remote gear selector housing and complete gear linkage.
Bench test the operation of your new sprung gate.
Spring tension could be increased if needed by varying length of support tube and/or packing with plain washers at base.
If necessary, a stiffer spring can be substituted. This will require trial and error. I would personally use the weakest spring possible.
There should be just sufficient preload on the installed assembly so that the spring is still under very light compression when reverse is selected. Otherwise, it is possible that the assembly will rattle and vibrate. This may not cause any operating problems but could be disconcerting and could conceal other unwanted mechanical problems.
I used the thickness of the selected washer to achieve some light preload. Adding another washer, or using a thicker washer, would easily adjust the preload incrementally
Rotate the mainshaft and check for clearance to the final drive pinion nut and locking washer with the selector shaft in all three gate positions (ie R, 1-2, and 3-4).
Risks:
This modification adds non-standard parts into your gearbox. I’m not a mechanic or an engineer. I am just sharing how I approached this modification. If you want to embark on something similar, you do so at your own risk.
It is up to you to ensure that the added parts are fitted and are secure so that they clear the differential pinion nut and locking tab, and your crankshaft rotating assembly.
If the selected spring pressure is too high, I imagine there is a risk that this modification could backfire and lead to problems selecting gears, or even aggravate problems with a worn gearbox jumping out of gear. This risk is probably small, but don’t blame me if it gives you grief…
This modification is completely reversible. It should be possible to do so by just removing the front casing from the box (with the speedo drive housing and engine mount), but if that doesn’t work you will have to open up the box. Removing this modification may require separating the engine and gearbox.
That is why I recommend bench testing the gear selection before assembling with your engine and installing into a car. If it doesn’t feel good on the workbench, it certainly won’t feel any better in the car…
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