Category: 1978 Isuzu 117 Coupe

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!

This story requires a lot of backstory to fully appreciate it. So, we’re going to go way back to when I had first moved to Japan. I was living in Kitami, Hokkaido (which is nearly the northern tip of the island) and driving an AE86. I, like a lot of people, hated my job and was quickly on the prowl or something better. I found a good little gig down in Nagoya, packed all my stuff into the old 86 and drove all the way down. I completely avoided the highways and spent five days bombing through mountain roads and running along coastal roads in probably the most epic road trip of my life. The scenery was beautiful, the roads were like race tracks and it was one of the most memorable experiences of my life. Just amazing.

Things happen though and after a few years of living and driving the 86 in Nagoya, I spun it in the rain and had a bit of accident involving a pole. The hit was dead square on the A-pillar, cracking the window and even bending the dash support, which in turn skewed pedal placement.

Fast forward a few years and I had bounced through a few daily drivers (’97 Mitsubishi Minica, ’00 Toyota Chaser, ’91 Nissan March Super Turbo, ’73 Honda Life 360) as well as a few project cars. Most notably, I had gotten my Isuzu Bellett running (read about it here), but it needed a full diff rebuild before it could be driven.

Now that you have a bit of background, here’s where the story begins! I was working with a good friend and co-worker of mine on an upcoming overseas project in Myanmar. He was based in Tokyo and I in Nagoya, so there was a lot of phone conferences and trips to Tokyo (keep in mind I could get to Tokyo in an hour and a half on the bullet train). On one faithful trip to Tokyo in early January, we decided to go out for a bite to eat and a few beers. I mentioned to my buddy, Mr. B, that my wife and I were planning a move back to the US, but before we went, I wanted to take a car on a road trip through the south of Japan like the one I did in my AE86 years ago. We went back and forth on good potential road trip cars and I mentioned that I would really like a 117 Coupe. Well, Mr. B happened to have his work computer and USB access key to the USS Auctions in Tokyo, and so a few beers in and we were browsing through potential auctions.

The night wears on and I have to go and catch a bullet train back to Nagoya, thinking nothing of the night. The next day at work however, I get a ring from Mr. B and he’s at the auction looking at one of the 117 Coupes we looked at before. I can’t really remember which one he’s referring to (I blame the beers), but he says it looks really clean and solid and is starting cheap. Unfortunately, being at work, there’s not much I can do. However, my wife is at home. I tell him to send her the pictures. I call her and she talks me through it, giving me an ultimate maximum bid price on the car. I tell Mr. B, he says he’ll bid and then I go about my day. At the end of the work day, after my subway ride home, I give him a ring and ask if we won the auction. Turns out, we did! And for half the maximum bid price to boot!

The car was in Tokyo and had to be processed and then shipped to me in Nagoya. As fate would have it, and I swear I’m not making this up, my newly purchased 117 Coupe arrived on January, 17th (1/17)! It shows up and is surprisingly clean and solid! The major issue I could tell right away (and knew about from Mr. B) was the front suspension made a terrible creaking noise if you leaned on it, but other than that, it seemed like the perfect car for our road trip.

The suspension noise ended up being ball joints, so new upper and lower ball joints were ordered from Isuzu and installed. However, it had a tendency to overheat. The water pump looked like it had recently been changed, so I put a new radiator cap and thermostat in. The car would drive fine for an hour or two and then start to overheat. It was losing coolant and relatively quickly. Finally, I decided to perform the poor man’s head gasket check. I took a piece of paper and held it over each spark plug hole (with the plugs out) while the engine was cranked over. Coolant came spraying out of the number one cylinder, so I knew the headgasket had had it.

I was way too slow on diagnosing this though, as we were now just three days before our planned departure on our road trip. So, I frantically got to work changing the head gasket out. With a new gasket in and just one day left before our planned departure, I took the car on a 100 km test drive and everything seemed to be fine. It wasn’t consuming coolant and she wasn’t overheating! So, we packed our bags and went on a road trip of epic proportions!

The road trip will get an entire post to itself, but the car performed admirably. It consumed about 200 ml of coolant across nearly 2,400km, which I just attributed to it burping up a bit of air (remember this though, it becomes important later). On day three of our 10 day trip though, the poor 117 Coupe suffered a bit of damage from a pole that fell on it at a gas station. The damage was minor, but the gas stations insurance paid to have it repaired. So, the roof, driver door and driver rear fender got repainted. The color is spot on perfect, but the metallic flake is a little heavy in the newly painted panels. It’s one of those things you don’t really notice unless you’re looking for it though.

Fast forward another few months and the 117 Coupe gets shipped home in the same container as my Isuzu Bellett and Mazda Cosmo Sports. Once in Texas I collect the cars up, bring them home, and register the 117 Coupe as soon as possible. It served as my daily driver in Japan and did so again here in Texas. It performed beautifully for three years as my daily, only requiring regular maintenance work.

That is, aside from the fact that it would consume coolant at faster and faster rates. At first it was 200ml in 2,000 km. Then it was 200 ml in 1,500 km, then 1,000 km. I kept it topped up and it was consuming at such a slow rate that I never really worried too much. However, I would always just top it up with whatever water bottle I had laying around. This was a big mistake it turned out. We had a particularly cold winter where it got down to about 5 degrees Fahrenheit. The constant addition of regular water had diluted my antifreeze enough that it actually turned to slush during this freeze! After it thawed out and I got the 117 Coupe running again, it was overheating again, this time even quicker than before.

Thinking it was the head gasket again, I decided to pull the head and inspect. Unfortunately, in the process I miraculously managed to drop an Allen key right down the front of the engine. I heard it bouncing off the timing chain as it fell further and further until it made the unmistakable clink of hitting the oil pan. And in order to get the pan off, the engine had to come out. So, out the whole thing came. While it was out I decided to polish the crank, pop a new set of main and rod bearings in, along with a light hone and new rings. While doing the bottom end, I took the head to get refreshed at the local machine shop. Turns out the cylinder head actually had a crack that started under the exhaust valve spring seat and spread nearly over to the other side. My best guess is this crack was always there, hence why it was consuming coolant. Then during the freeze, it caused the crack to spread violently, bringing us here.

So, I tracked down a used cylinder head off an old Chevy Luv, had it rebuilt and then slapped it on my refreshed bottom end. The whole engine went back in, fired right up and sounded smooth as can be. I started the break-in procedure and put around 300 km on it with no issues. I decided to take it on a pretty long drive of about 100 km round trip, to really get the break-in going in earnest. On the way back I noticed it was starting to idle a little rough, and by the time I got home it was running really poorly. Not good. I checked the compression and sure enough, cylinder one was at 50 psi. The engine came out yet again.

So, you see, when I was refreshing the bottom end, I pulled the rings out of the package, measured the end gap on cylinder four and it was perfect. No need for filing the rings. Being the impatient idiot I am, I figured they were all perfect and just slapped the rest of the rings in. (Cue Morgan Freeman narrator – “They weren’t perfect.”) Cylinders one, two and three all had cracked rings. The end gap was too small and the long drive put enough heat into the rings to expand the gap to the point of touching, causing them to shatter. This threw junk in the oil and destroyed the main and rod bearings and scored the crank.

So, now I’m back to square one and I think to myself, “What’s the one thing I want, but don’t want at the same time? A twin-cam.” I want the old G180W/G200W Isuzu twin-cam engine because that was the penultimate engine for this car. I don’t want it because parts are exceedingly rare, particularly Stateside. So, I say to myself, “What if I could get a twin-cam engine, and still have massive parts availability.” So, I purchased this on eBay.

Yup, that’s a Mazda Miata BP 1.8L twin-cam engine and 5-speed transmission. Now, this was actually a swap I had been thinking about for a while now, and I specifically chose the Miata engine over other options for a few reasons. The main reason being its size. With the transmission attached, the engine/trans combo is 20mm shorter than the 117 Coupe engine trans combo. That’s a pretty negligible difference. I do have some pretty big hurdles to overcome though. The chief among which is the oil pan. The Miata oil pan is an aluminum, rear sump unit that is structural, meaning it bolts to the transmission. The 117 Coupe oil pan is a stamped steel, center sump unit. The reason it is a center sump is the steering link has to fit between the sump and the transmission and the cross member has to fit in front of the sump. So a custom oil pan is currently being made. There’s a litany of other hurdles that must be overcome, but we’re taking them one step at a time! Here’s a quick pic of it before I tore the engine out of it the second time parked next to my Bellett (so we can end on a happy picture, haha).

And that brings it up to date! Thanks for reading and check back for more updates to this ongoing build!