1978 Isuzu 117 Coupe: The Swap Begins

When we last left off, I had just gotten the new Mazda Miata 1.8 twin-cam engine – read up here for more info on how we got to that point.

First things first, I laid the Miata transmission side by side with the 117 Coupe transmission and the Miata transmission was a bit longer, meaning the engine would need to sit a little further forward than the stock 117 Coupe engine. This actually tuned out to be a very good thing for reasons that will become apparent later.

I measured up the engines next, or rather the lengths of the oil pans, since they are a pretty good indication of overall engine length. Here are the measurements:

117 Coupe pan: 545 mm

117 Coupe transmission: 755 mm

Miata Pan: 460 mm

Miata transmission: 820 mm

117 Coupe total length: 1,300 mm

Miata total length: 1,280 mm

As you can see there’s just 20 mm of overall length difference between the two!  Awesome, this plays well into my overall ethos, which is to get this swap done with zero modifications to the 117 Coupe. So, the next step was to just lift the transmission into place for an initial test fit. For now, all I did was line the shifter up, jack the transmission roughly into place and pop a jack stand under it to see how it sat.

So far so good! The first major problem in this mad experiment soon showed itself. The 117 Coupe uses a ball-nut type steering setup, which necessitates the use of a center sump oil pan. The sump rests in between the crossmember and the steering linkage. The Miata engine uses a rear sump oil pan, which normally wouldn’t be too much of a problem to modify, except the Miata oil pan is aluminum and structural, meaning it also bolts to the transmission.

You can see here that the Miata oil pan is resting on top of the steering linkage, so the entire pan needed to be modified extensively. It was time to call upon my favorite fabricator, Dashbuilt Performance.

All I did was give him a printout of those dimensions with a general idea of what I wanted to achieve and he got work!

The results are stunning!  I actually had to send it back for a revision and the pictures above show the revised pan, but that was because my measurements were off.  That adage of measure twice cut once should be modified to measure 50 times and still have to cut twice. However, the new pan now lined up perfectly, so I slapped it on the motor and lowered the engine into place. After some fiddling and fighting with it, I got the transmission bolted up so I could start locating things properly.

Of course, it doesn’t always go super smoothly. The Cam Angle Sensor located on the back of the cylinder head is just too large and was butting up against the firewall.

In order to keep the transmission optimally located as well as keeping the pan from running into the steering linkage, that sensor will have to be removed. This means I can no longer run the factory ECU, which isn’t necessarily a bad thing. I was planning on running a set of ITBs in the future, which would necessitate the use of a standalone ECU, so this is just moving that idea forward a bit.

Once I was happy with the engine position, the first step was building a transmission mount. Using the factory body mount locations, I built the above piece to run from left to right and hold a factory Miata engine rubber mount. The bottom half of the Miata engine mount bolts onto the piece I made above and the top half bolts onto a piece that connects to the transmission using the factory bolt holes for the piece that connects the Miata trans to the diff in the Miata.

With the transmission located right where I wanted it, it was time to work on engine mounts. I reused the factory 117 Coupe engine mounts to assure that the entire drivetrain is rubber isolated. Then I built adapters that go from the factory engine mounts to the block.

Now, all the mounts are going to come back out and get reinforcing braces welded to them where it counts to make sure everything is plenty strong. However, for the time being, the engine and transmission are in!

With the engine in its final location, I could start work on the next major hurdle I had to overcome. The 117 Coupe uses a cable operated pull clutch while the Miata uses a hydraulic operated push clutch. I tossed around a lot of different ideas of how to overcome this issue, which was compounded by the fact that I refuse to cut the firewall to mount a clutch master cylinder. Ultimately, I decided to build this contraption.

This piece mounts in the front driver side area of the engine bay and uses a lever to operate a BMW 325is clutch master cylinder with the stock cable. The reason it’s shaped so… elaborately, is because it mounts through bolts and holes that were already on the body (for the crossmember, condenser and headlight). I haven’t tested it fully yet because there’s no clutch or flywheel on the engine right now, but I’m quite happy with it!

The Miata slave cylinder uses a 3/4 in. bore size and the BMW master cylinder also uses a 3/4 in. bore size, so we get a 1:1 movement ratio out of that. Through various reading, it seems that the Miata uses anywhere between 15 and 20mm of travel at the slave cylinder, which means we want to aim for 17mm of travel at the master cylinder. The 117 Coupe factory clutch cable has about 30mm of travel. So, the cable is placed 90 mm from the pivot and the master cylinder is placed 50mm from the pivot. Since these draw two different arcs, it means that for about 30mm of travel at the cable, the master cylinder travels 17mm. Of course, I made this whole thing with a sawzall, drill press and an angle grinder (save for the two round pivots I machined on the lathe), so there’s going to be a little inaccuracy, but it should be close enough.

That brings us up to speed. The garage is quite full at the moment, so it may be a while before I get back into the 117 Coupe. In the meantime, keep an eye out for upgrades to the Bellett!

4 thoughts on “1978 Isuzu 117 Coupe: The Swap Begins”

  1. Wow! You are doing some really neat modifications to the 117. I love it. Let me ask, do you have any tips on what to look for when searching for a 117 in Japan and what it takes to maintain (fluids, ease of service, and parts availability) in the U.S.?

    1. Thank you so much!

      The 117 Coupe is a great car to hunt down, because there’s so many models that you can almost certainly find one that matches your budget perfectly. I’ll break down the models briefly right quick:

      1968 – 1973: Zenki
      These are the early, handmade models. The design was so complex but beautiful, that Isuzu didn’t actually have the manufacturing capabilities to produce the car at the time, so they handmade these! You can pick these out by quickly glancing at the headlights and taillights. They have the round headlights and slim, small taillights. These also demand the highest price, usually over $30,000 USD!
      Here’s an immaculate model for sale for about $45,000: https://www.carsensor.net/usedcar/detail/VU1461192557/index.html?TRCD=200002

      1974 – mid-1977: Chuuki
      In 1974 GM and Isuzu penned an agreement and part of that agreement was GM helping to update Isuzu’s production facilities. From 197 on, the cars were capable of being mass-produced. These still have the round headlights of the earlier models, but they changed the taillights to a long piece that spans the width of the car. Because these were mass production cars, they’re considerably cheaper than the early handmade cars, usually for sale between $10,000 and $20,000 USD.
      Here’s a good example for sale for about $14,000 USD: https://www.carsensor.net/usedcar/detail/VU1971219394/index.html?TRCD=200002

      1977 – 1982: Kouki
      These are the late models like mine. Isuzu updated the design to match the changing times, giving it square headlights and a fairly different interior. They’re also the least popular, but most widely available models in the entire 117 Coupe lineup. But, that means they’re also the cheapest! Some models can be found for as low as $4,000 to $5,000 USD. Although, they have been coming up in price a bit lately, especially the twin-cam models.
      Here’s a very clean twin-cam model for about $13,000 USD: https://www.carsensor.net/usedcar/detail/CU5803648923/index.html?TRCD=200002

      As for parts availability, I would personally shy away from the twin-cam models, even though that Isuzu twin-cam is a peach of an engine. The SOHC models came with either a G180SS, G180Z or G200Z engine. The G180SS is nearly identical to the G180S that was available stateside in the ’72 Chevy Luv, so decent parts availability there. The G180Z and G200Z were used in a massive number of Isuzus that were sold in the US, from the Rodeo to the Impulse, so tons of parts availability there.
      I can’t speak too much for the Chuuki or Zenki models, but the Kouki model like mine also shares bits with other cars. For example, the front brake pads are shared with the SA22C RX-7, although the calipers are a bit different.

      The only thing that I came up against a brick wall for was the A/C compressor. But, that wasn’t something unique to the 117 Coupe, an A/C compressor for anything with a G180Z or 200Z would have worked (so Luv, Trooper, Impulse, etc.), but I just couldn’t find anything new or re-manufactured. Given that I was jonesing to engine swap something, that was kind of the catalyst to start this whole project of mine!

  2. Wow, I completely missed that you were engine-swapping this! Seems like a very good choice, the Miata engine. Almost seems to fit perfectly as well! That elaborate brake mechanism is a work of art, though I imagine you’ll have to keep it well-greased, etc so it doesn’t foul up over time.

    Kudos to you for being unwilling to modify the car that much…that way if you or a next owner ever want to go back to stock everything is always as it was…smart to re-use the old engine mounts with an ‘adapter’ to the new engine as well!

    1. Thanks!
      That was my primary goal when going about this whole thing. The engine bay is fairly large and I could have fit nearly anything in there, and I’m not gonna lie, I thought about a 1UZ-FE V8 swap a bit. But, I wanted to keep it completely reversible, and it’s hard to reverse holes in the firewall and sliced up transmission tunnels. I genuinely wanted it to be a bolt-in affair and the Miata engine was physically very similar in size, which made everything a lot easier to work around. Also, it’s a 1.8 twin-cam, like the original 1.8 twin-cam that came in the high grade model of these cars, but with modern fuel injection and reliability (and parts availability, Isuzu G180W parts are non-existent in the US).
      We’ll see how the clutch mechanism works out. It needs a bit more filing and finishing work to be right, but at the moment it seems like it’ll get the job done. Once I get fluid into the system, it may be a different story though.
      I actually just pulled the engine and transmission back out yesterday. The engine will be going off to the rebuilders in a week or so and I’ll get to work on finishing out the engine and trans mounts so that way when it gets back, it can go in for good!

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