Short Stroke and RX7 Brake Conversion Day 1

With the T3 parts delivered and RX7 calipers in my hands, i got around to start the short stroke and RX7 brake conversion. This covers Day 1 of the conversion. I am still in the middle of the process and reckon it will take 2-3 days to finish.

Short stroke conversion is up first. I will not cover why you need to do short stroke as club4ag has an extensive writeup on that. To give you an idea of how to convert to short stroke, watch this video.

I will be doing it in a slightly different way for safety reason. More on this below. Let's start.

First, cut off the old spring perch as this will get in the way of Ground Control coilover sleeve.

Then smooth out what's left of the old spring perch on a lathe machine.

The next step starts to show where i differ from the video. You'll notice from the video that the strut housing is shortened by taking out the tube near the old spring perch position and weld the two remaining parts together (the bottom strut housing and the top housing where the thread for strut insert lock nut is located). While this way is the simplest, there's slight risk of breaking the weld joint later on. The strut tube is not really that thick to start with and welding just makes it more prone to breaking. I am sure not many people experience the break but why risk it? Anyway, i cut 40mm from the top tube, throwing away the threaded part for the lock nut.

Say good bye to threaded top tube.

Here's the key step and the reason why many chose the easier way. As the threaded part is taken out, you need to make the new threads for the lock nut on the shorter strut housing. Worth mentioning also that the lock nut from the original strut housing has finer threads than the lock nut supplied by my KYB Excel-G strut insert. The new threads will follow the KYB Excel-G lock nut as it warrants perfect fit to the strut inserts.

Look, new lock nut threads!

With 40mm cut on the top strut housing, this is how the KYB Excel-G sits inside. Notice the space. A spacer is needed to push the strut insert up so that it levels with the housing, allowing proper fit to the lock nut and eliminate rattling.

Most just use stacked washers for the spacer but not me. Here's the spacer in the making. Ignore the threads seen. The machinist made the spacer out of some truck wheel shaft, hence the threads. I forgot to measure the thickness but you should be able to figure it out yourself. Always start with slightly thicker than you calculated and reduce thickness as necessary until the strut insert sits perfectly. Reducing height on metal is always less headache-inducing than adding.

Spacer done.

... and here's the final product.

Shorter strut housing that is as strong as the stock one! No worries on breaking any welds. More people should do this! Forgot to take pictures of the coilover sleeve perch ring in the making but you can see it on the pic above. It allows the sleeve to rest firmly on the strut housing. T3 supplied the rings with the purchase but i didn't know what they were earlier and made new ones. I am thinking of putting some holder tabs on the perch ring so that the sleeve won't turn when i adjust the coilover spring perch. This would involve some slight cutting into the coilover sleeve. Maybe on Day 2.

On to the RX7 brake conversion: Day 1 sees the fitting of the Civic rotor to the AE86/Charmant hub. It's a very simple process and should take two hours or less. Civic rotors hub mounting diameter is a bit too small to sit on the Charmant hub. There's two ways to solve this: enlarge the Civic rotor diameter or reduce the Charmant hub's. I chose the later as it allows me to replace the rotor easily later if i need to without modifying again. To my surprise, you need to only take out very small amount from the hub to fit the rotor, maybe less than 0.5mm. If i am not mistaken, i read somewhere that it should involve more work to fit the rotor such as inverting the chamfer on the rotor but this was not needed apparently. Maybe it's not for Civic rotor.

Anyway, here's the hub on the lathe machine. Outer diameter where the rotor sits is being reduced a little.

After the rotor can sit perfectly on the hub mounting face, you need to redrill the rotors so that the hub mounting bolts can be installed. First, using a smaller drill bit, drill through the rotor using the hub bolt holes as reference. Careful not to damage the threads.

Next, enlarge the new holes on the rotor to match the bolts used.

Here you can see how the new holes on the rotor matches to that of the hub's.

Finally, here's the end result..

Note that some people choose to upgrade the mounting bolts to bigger ones. This is a safety measure as the original bolts were used to hold smaller rotor diameter. With upgraded rotors and calipers, bigger force will be exerted to the bolts and they may break. Doing this would involve tapping new threads on the hub to fit the bigger bolts. I chose not to do this for now. If the bolts do break, i will surely do this upgrade.

With those work done, Day 2 will mostly see the fabrication of the caliper bracket adapters and, if i choose to do it, modify the sleeve ring perch with some holder tabs to fix the sleeve's position.