KMiata Swap – Engine Prep

There it is, the star of the show! The Honda K24a2 from an 06-08 Acura TSX.

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The first task was to strip everything unnecessary off of it. All new wiring is going in, but I’ll likely need some connectors off of the old harness, so everything gets labeled.

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A few mods need to be made for the K-series to fit the Miata. The biggest of those is the oil pump. The a2 variant has a massive pump assembly with counter-balance shafts sitting right where the steering rack wants to live. That’s replaced with a much slimmer pump from a K20, along with a back-cover, pickup and windage tray to work specifically with the KMiata swap.

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The timing cover, chains, guides and tensioners all need to come out to get at the pump. While it was already apart (and since it was 1 extra bolt to replace it), a 50-degree variable intake cam pulley from the RSX Type-S took the place of the 25-degree sweep unit from the TSX.

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Leak testing the pan before it goes on.

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A new gasket & seals set from Honda went in to replace anything worn. The O-rings around the oil system and for the VTEC solenoids were notable in being seriously perished.

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That done, the front of the engine could go back together. Timing chain, guides, tensioner, timing cover, water pump and housing:

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Because I’m using the ATI damper, which is smaller than the stock harmonic balancer, I ended up needing to go with a shorter belt (a 6PK1035, if anyone reading this decides they need to go in a similar direction):

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I love the simplicity of KMiata’s 4-pulley setup, but I’m not really happy with the tensioning setup. As this post is long enough, I’ll dedicate a separate post to that adventure. I’ve no doubt that it works, but I can’t help but picture myself trying to fix an alternator or water pump in a wet / dirt . gravel / grass paddock space at a track or on the side of a mountain, and losing half of those parts in the dirt.

You’ll notice at some point here the engine was installed. That was a HUGE morale booster. There wasn’t much super interesting with that, it slid right in, however I do want to share a tip that’s been passed down to me from others that I found super useful: Making dowels to line up the transmission and engine.

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That made getting the transmission on SO much easier.

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Another quirk of the KMiata swap is that the starter is on the “wrong” side of the engine relative to the Miata’s, so the transmission bellhousing needs to be clearanced for it to fit:

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Next up was to start making and test-fitting cooling system parts. The lower radiator hose (a stock S2000 lower hose) was fairly straight forward:

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The upper hose seems to need to be 3 pieces. I’m still on the hunt for a single hose solution, but for now, it looks like a 72277 and a 72098 hose with a long coupler need to be used.

A 1 foot length of aluminum tube seemed to be just about perfect. A couple of passes run on the bead-roller made a nice looking piece:

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I’m going to put some more anti-abrasion sleeve and a bit of retention where it wraps around the front of the engine, because while it doesn’t touch while not loaded, I imagine when it’s full of water and moving with the car & engine, it’ll definitely rub there. Worst case it’s easy / cheap insurance that I’m not wetting down the course / track.

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Yes that’s a stock radiator. No, it won’t be in the final setup. I’m using it for mockup now to avoid dinging up the shiny aluminum one while still building this setup.

On to the other side of the motor, I can mock up the exhaust manifold. It is on the ‘wrong’ side of the engine (compared to the stock Miata configuration), so it has a crossover tube that bolts on that runs between the bell housing and oil pan.

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Unfortunately here I run into another V8Roadsters subframe issue. The crossover pipe runs into the hoop that connects both sides of the subframe and runs under the engine.

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So now I have the pleasure of taking a freshly ceramic coated to the grinder to gain a couple millimeters of clearance. Which is nice…

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So now it clears, but…ugh.

At least it won’t affect performance, or rattle every time I come off throttle.

Next up, the fuel and accessory belt systems.

KMiata Swap – Suspension

It’s time. It’s finally here. With the Chasing the Dragon Hill Climb complete (and successful beyond my wildest dreams, making the fastest pass by a Miata ever), it’s time to put the bigger, lighter Honda motor in the race car.

Unfortunately however, things got off to a rocky start. Prior to the Hill Climb I started test fitting things to the V8Roadsters subframe and all was not well.

The first bit of prep was to get the steering rack on the subframe. So as to not lose the NB rack bolts I figured I’d put the bolts into the threaded holes in the subframe for safe keeping.
3 out of 4 had powder coat completely boogering the bolt holes. Adding to the fun, the thread pitch of those holes is M12x1.25mm, which is the one M12 tap I don’t actually have in stock.
A quick Amazon shopping trip and a couple days later, the tap comes in.
After chasing the threads in the good bolt hole and 2 of the 3 bad ones, I realize 1 of the engine mount brackets completely blocks access to the 4th. Improvise and overcome right?

Turns out, a 5/16″ 12-point socket makes a good-enough narrow access tap wrench for an M12 tap.

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That’s not great, but a pretty small issue all things considered.
Then a friend (thanks Nick!) who has experience with V8R subframes suggested I check the rest of the holes, and that’s when the big problems started. I could not get the upper control arm bolts through the bores at all.

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I went as far as creating a custom tool with some rod and sand paper to grind the powdercoat out of the bores, thinking it was a similar issue to the steering rack bolts, but no, the bore was shaped like a banana. I still don’t know if it was overheated when it was welded to the frame or what, but it wasn’t great. And given that it left the shop like that, their QC wasn’t exactly giving me warm-fuzzies either.

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To KMiata’s credit, they handled getting a new one shipped out and return shipping for the bad one to V8Roadsters to ‘investigate’. Once we determined just how bad the problem was, I had a new one in hand a few days later.

And check it out: bolts that go in without any hammering or clearance! What a concept!

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Next it was time to get the steering rack prepared. Because of tight clearance to the oil pan (…a recurring theme…), all the hydraulic fittings on the steering rack housing had to be cut off. I used a 3/16″ rivet and some JBweld to seal the holes.

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Next I discovered that Mazda changed the design of their inner tie rod lock washers. The old style washers sat on the OD of the tie rod’s threads, so it was flat between the rack and the tie rod. The newer style lock washers haveĀ a shoulder that goes on the over OD of the rack. That prevents my steering rack limiters from seating all the way at the ends of the rack, taking another 3/8″ or so of travel out of the rack.

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I corrected that by throwing the limiters in the lathe to put a counter-bore in the ID to clear the washer on 1 side, but still ride nice and snug on the rack:

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Time to start disassembling the car:

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Given that I’m going to have to install and pull the drivetrain several times over the course of the coming build, I also wanted to remove the upper radiator support (which doesn’t actually support the radiator in an NA Miata), and make the front bumper bar into a bolt-on piece. As you can see, the engine and transmission will come out of the car practically straight:

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A little bit of cutting and welding later, and the front bar bolts to the chassis legs:

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It’s so much easier pulling the whole thing as a unit:

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With the rack put back together, I could finally put the subframe in the car. Unfortunately there’s evidence that my car has been in a minor front-ender, which appears to have slightly tweaked the subframe pickup points. Because of that, I had to pat my head and rub my tummy and figure out which order the bolts wanted to go in so that all the holes actually lined up, but once I figured out which hole was the furthest out and started there, it all bolted in.

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I found a couple of bad ball joints during disassembly, so I called Mazda Motorsports and re-loaded the parts cannon to get good parts on board, but once that was done, everything bolted up nice and easy (including the steering rack)

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Next, on to the KMiata specific engine mods.