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HGT sequential gearboxes, performance upgrades

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HGT Precision Sequential Gearbox | well built, ready to go

Maintaining and Rebuilding the HGT Sequential transmission | what to look out for

The HGT sequential transmission is a fine piece of machining and ingenuity.

Every year, it is necessary for us to take a look inside and check wear components to see how the box is holding up to road racing conditions. This year is no exception.

HGT Precision Sequential Gearbox Mazda RX7

HGT Precision Sequential Gearbox Mazda RX7

Now, for the street guys, this isn't absolutely necessary unless there is noticeable issues. I would still change out the fluid regularly, remember if you are putting this in a car, it isn't one that is going to be driven cautiously.

What to look out for:

Worn dog rings, these are supposed to wear out before the dogs on the gears themselves, so that is the first thing I look for mechanically.

during the year, every two events, I change out the fluid, it is cheap maintenance for a gearbox, plus it lets me clue into wear patterns of the fluid, does it smell burnt, does it have a large amount of shavings in it?

  • Check the grooves and pins on the tumbler, are they in good shape, does the box move up and down through the gears smoothly?

  • Check the o-rings, this is an easy fix if they are ripped, or damaged, I would hate to destroy a box because of a $10 o-ring failure (all the fluid leaks out and the box self destructs).

  • Check the operation of the lever to the shift drum, very critical as this moves very fast, a worn one will fail (it is not if but when).

Road Racing is a sport of the extraction of power at the racing surface, that means stressing out everything in the car, including the transmission. Most people will wait until it is broken, well that gets seriously expensive.

Our gearbox has some minor wear, but nothing that was critical, the other thing on this set up is the electronic tuning via the ECU for full throttle upshifts and rev matching on downshifts. This gearbox shifts in the 40-45 millisecond range on upshifts, still the wear is within limits, I did have to smooth out a couple of dog rings with some emery cloth sandpaper, but nothing I didn't expect.

Now, comes the hard part, getting all the parts back in the box correctly.

This box is a seriously well built sequential gearbox, with that comes tight tolerances and the expectation of a proper maintenance schedule.

After the first go around putting it back together I had to pull it apart because there was something not correct.

Mazda RX7 Sequential

Mazda RX7 Sequential

It turned out to be one of the collars on the main shaft wasn't pushed on all the way (we are talking barely noticeable, but when I pulled it out, I could move the gear ever so slightly (fore and aft on the main shaft) and that meant it wasn't going to line up once the bolts were all tightened on the main case. The first symptom was that when I got closer to the 40 nm torque spec, the shaft didn't rotate smoothly, it did at 25 nm, so I took it apart and spent the time to find the issue. My understanding was that it moved when taking out the other components.

I visually noticed and measured (of course) the lower bearing for fifth gear didn't line up directly with its counterpart (very hard to see, but when I put the dial caliper on it, it was off and that meant that the collar had pulled itself away from the intermediate case.

  • A typical shop would just pound through it and expect it to tighten up on use, why damage the box because one is lazy, do it right, or don't do it.

Inspection of every gear, spacer, needle bearing, spline etc. is relevant for any gearbox to operate properly, yes it took a good 2 more hours to pull it down, check everything and use the bearing press to put collars and splines back on, when I put the 8 bolts in for the main case, snugged them up and started the torque sequence it all worked properly as it should. That alone tells me this is a very fine piece of hardware.

#HGTPrecisiongearboxes | Sequential, quiet, fast shifts

#HGTPrecisiongearboxes | Sequential, quiet, fast shifts

The reality is that details matter, the more precise it is, the better the mechanic has to be to be as good as the part being worked on. Have the right knowledge on what to look for, the tools to disassemble and reassemble and above all, know when something isn't correct, take your time and understand the situation before moving forward with a plan of action. If it doesn't feel right, it isn't.

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Topics:   HGT sequential gearboxes, performance upgrades

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