CT70H stator flywheel mystery

I’ve been a member for a little bit, but this is my first time posting. I’ll try to make this brief:

I have been doing a ground up CT70 build, and was looking for an engine or engine donor bike. I found a running 1997 XR70 on CG, and picked it up for $500. Once I got it home I realized the bike had a CT70H engine. I felt like I had won the lottery.

So the stator does not have a lighting coil, so I’ve been looking for stator rebuild kits. I’m not positive that this is the original stator, and when I ran the part number on the flywheel it came back as a ‘79-85 Z50. Can someone speciate these parts and give me direction on this? I’d prefer to use what I have since it works but I need the 4 wire stator setup.

The motor is a CT70H but I think the flywheel is not original to the motor. Lots of experts here to help you past this roadblock. I’m just not one. It is interesting and makes me wonder if the crankshaft has been changed. Or, if the case was replaced. Do you know if it has 4 gears?

The outside of this engine has been wet-blasted...assembled (look at the screw heads) and, based on what you've posted so far, it's likely a mishmash of parts...not all of which are compatible on the same engine..."fluffed for a quick sale".

First off, there are three alternator types: 3sp/6v, 4sp/6v & 12v CDI. The parts that can only be used as an assembly are crank + stator + flywheel and this applies to each of the three alternator types. Flywheel taper is matched to crankshaft taper...and length; stator is matched t the flywheel. There is one more fly in the ointment...4-speed alternators were made by Mitsubishi & Hitachi and there are no parts that interchange between them. Looks like you have the Hitachi flywheel, which is a lucky break...if it's a CT70H, SL70, or XL70 - and - you also have the matching stator assembly. There are plenty of Hitachi parts available, including coils. Mitsubishi owners aren't so lucky.

Post a photo showing the inside of the flywheel. It should have a centrifugal advance mechanism. Next up, figure out if it fits your stator & crankshaft. A lot of these engines were cannibalized for their flywheels, over the years.

You have a number of options, anything from sourcing a lighting coil to going to CDI, in 6v or 12v flavor. There are enough possible combos to be confusing so, best to begin by determining if you can use this alternator before moving onto ordering parts. Once you've identified the flywheel/stator/crank, it'll be time to split the cases...to see what you have to work with.

I probably should repeat, the engine runs. It is definitely a 4-speed. I rode it all over the place before I removed the engine, so there's no doubt the flywheel at least matches the stator. Not much went into this bike to resell, I mean, the coil was attached to the frame with a zip tie.

The flywheel, I don't believe to be Hitachi, or at least it looks nothing like the Hitachi on my '72 3-speed. Fortunately the crank threads are in great shape, as well.

What you have is an Hitachi Flywheel from a 1979-1985 Z50R, part number 31121-165-005. They fit & work fine on an H style crank (no timing advance). The stater is also from the same period and is probably part number 31141-181-671 or 31141-181-672. Both work fine on a H crank but as others have pointed out, no timing advance. You can add a lighting coil to stator plate with ease, or install a Malaysian CDI kit, or source a correct Hitachi flywheel.

I can help you out with a lighting coil (cheapest solutions to get lights) and maybe a correct flywheel (will know in a few days).

Here's a pic of one that I have.

Thank you for your response. That was my opinion as well.

I'm an old hot-rodder, so I understand the value of timing advance. That said, I do like the idea of having a 12v system.

The funny thing of this, is that my son and I been building this 1974 CT70, and we were just looking for a motor. I already have the suspension installed, but no wiring. I've been trying to figure out which direction to go... Now I'm probably going to drag out my 6/70 H frame and start on it and use this motor, then maybe do a China motor on the other project.

allenp42 said:

What you have is an Hitachi Flywheel from a 1979-1985 Z50R, part number 31121-165-005. They fit & work fine on an H style crank (no timing advance). The stater is also from the same period and is probably part number 31141-181-671 or 31141-181-672. Both work fine on a H crank but as others have pointed out, no timing advance. You can add a lighting coil to stator plate with ease, or install a Malaysian CDI kit, or source a correct Hitachi flywheel.

I can help you out with a lighting coil (cheapest solutions to get lights) and maybe a correct flywheel (will know in a few days).

Here's a pic of one that I have.

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If I went with the Malaysian CDI kit, could I use the flywheel I have? Also, could I use a stock wiring harness with a 12v rectifier?

Also, what lighting coil would I get to go onto this stator if I went that route?

racerx said:

Post a photo showing the inside of the flywheel. It should have a centrifugal advance mechanism. Next up, figure out if it fits your stator & crankshaft. A lot of these engines were cannibalized for their flywheels, over the years.

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Thanks Racerx for the info on the advance mechanism, that was something I didn't know.

Brofessor said:

If I went with the Malaysian CDI kit, could I use the flywheel I have?

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Yes, you can use that flywheel. And if you're wanting to go with 12v, that flywheel works in your favor. The magnets are much stronger in the Z50R flywheel you have than a standard Hitachi or Mitsu. Since I have plenty of time on my hands at the moment, I'll do a little testing and tell you how much juice you can expect. Off the cuff, I would guess 50-60 watts. Give me a day or 2 and I'll let you know.

allenp42 said:

Yes, you can use that flywheel. And if you're wanting to go with 12v, that flywheel works in your favor. The magnets are much stronger in the Z50R flywheel you have than a standard Hitachi or Mitsu. Since I have plenty of time on my hands at the moment, I'll do a little testing and tell you how much juice you can expect. Off the cuff, I would guess 50-60 watts. Give me a day or 2 and I'll let you know.

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Allenp42 is the authoritative source for info on this topic. I strongly recommend searching the recent archives where his projects have been detailed, at every stage of development.

You guys are awesome; thank you for the help. I'm glad to finally be getting this far with my build. Having two high school student-athletes in the house, there is always something that seems to be a higher priority.

allenp42 said:

Give me a day or 2 and I'll let you know.

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Thanks again, I'll be standing by.

Here are some numbers from 3 coils I tested with a Z50R Flywheel. 2 of the coils are from Hitachi Kits supplied with Malaysian CDI kits for the CT70 or ATC70; the 3rd one ("my coil") is one that I wound and found to be the best for a standard flywheel. The wire gauge for both coil A & B are the same. "My coil" uses a larger gauge than either of the other 2.

This should generate some healthy debate. Coil A has fewer turns than the other 2, which is why the power is greater at higher RPMs. When limited to a single coil, it's a compromise situation. You can have more power at lower speeds or more power at higher speeds. It all depends on turns on wire size and number of turns assuming everything else is the same.

These numbers are the maximum you can expect to achieve. If you go with LED Lighting, any of the coils will work fine. The benefit here (my opinion), is that you can use the coil supplied with the CDI kit and get reasonable 12v juice when when using a Z50R flywheel or a 80-82 CT70 flywheel (Hitachi).

Holy cow, outstanding information!

I think I like the idea of more power at higher RPMs (Coil A), because my experience has been that I always want to see farther down the road at night at WOT. Since I have a Frankenstein of sorts anyway, this sounds like the best direction.

Do you have a link for the Malasian CDI kit?

I'm assuming once I go down this road I'll be using a 12V conversion wiring harness as I see listed on the CT70 parts sites?

The most complicated wiring job I've done to date is installing an MSD ignition in my 1979 Trans Am. This little bike is going to provide opportunity for wood-shedding my electrical skills. Thankfully I have another bike to use as a reference, but I'm sure I'll have a steady flow of questions for the group.

Food for thought...

Even for the purist, converting to 12v/CDI shouldn't be a deal-breaker. First off, it's reversible. Second, and more importantly, it can be "invisible", i.e. outwardly stock appearing.

These little alternators cannot generate many watts. So, getting 400%+ the stock DC output to the battery is a huge improvement. Going to LEDs can easily triple lighting output, for any given bulb wattage. Huge gains in lighting and maintenance-free ignition, what's not to like?

Do you know if the original XR70 wiring harness is installed on the bike? It may be hacked up a little to allow the engine to just use the kill switch, wire for the coil, etc. I did a quick search of the wiring diagram for a XR70. Not much in the original harness.

I assume that you want to add lights and maybe turn signals? This may turn into a sticky wicket. By that I mean you'll have to purchase a HL, TL, battery box, engine kill switch of some type, horn, HL switch, and a wiring harness for a CT70. The harness you need will depend on your plan, as well as the other stuff. I'm not sure what fits a XR70?

Or maybe the previous owner added some of this before you got it?

I lost focus for a minute that you have a XR70 bike with a CT70H engine...and the all the wiring in a stock XR70 would fit in your pocket.

That poor little XR... yes it still has parts of the harness... It doesn't even have a kill switch lol. This old retired guy bought the XR to resell because he's bored, but he wasn't that much of a mechanic. I'm pretty certain the CT engine was in it when he bought it, which was smoking badly so he installed a TB 88cc kit prior to selling. He barely broke even on it.

Since I have a matching frame, my thoughts are to go with what appears to be a mostly stock HKO setup, just running 12V with a race head/cam and 88cc kit.

Is the 6V harness different than the 12V harness?

Again, if I make comments out of ambiguity or due to lack of clarity please call me out on it and instruct me. I want this to be a well thought-out project.

Brofessor said:

That poor little XR... yes it still has parts of the harness... It doesn't even have a kill switch lol. This old retired guy bought the XR to resell because he's bored, but he wasn't that much of a mechanic. I'm pretty certain the CT engine was in it when he bought it, which was smoking badly so he installed a TB 88cc kit prior to selling. He barely broke even on it.

Since I have a matching frame, my thoughts are to go with what appears to be a mostly stock HKO setup, just running 12V with a race head/cam and 88cc kit.

Is the 6V harness different than the 12V harness?

Again, if I make comments out of ambiguity or due to lack of clarity please call me out on it and instruct me. I want this to be a well thought-out project.

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My general approach to engine rebuilding is: if it's running well, has decent compression, maybe stab-in a hotter cam but leave well enough alone. At the other end of the spectrum, if the piston needs to come out of the bore the cases should be split...to allow complete inspection of the lower end. Intact, healthy, shift forks in used engines are the exception, not the norm. If the rod is worn, the cases must be split. At that point, shift forks are dead-easy to replace...and crank replacement becomes an easy option, as well. H-model cranks can be had from dratv and on the cheap. If you want a lot more power, a tbparts stroker crank costs less than a replacement stock 3-speed crank, or rebuilding one.

I like the 12v head, because of the ball bearing cams used and the bigger selection of cam profiles. That said, the old 6V CT70head is a pretty decent casting, with large valves. A little port cleanup on the exhaust side and one of the few hotter cams available will make good power, comparable to what can be done with a 12v head.

With a 52mm bore, stock stroke and a more aggressive bumpstick, you should be able to get 50-55mph out of an SL, or XL, 70. Those taller tires roll with less resistance.

It boils down to your goals and, of course, budget. The more miles you want from the machine, the more it pays to be thorough from the get-go.

The wiring isn't as complicated as you may think. You could adapt a CT70H wire harness. Not sure how it would fit. I expect that some leads would be too long, others not long enough. It's possible to fab a custom wire harness, from scratch, if you know how to read a schematic. It's not the complexity of the project, it's the sheer awkwardness...sourcing spools of wire with the desired color-coding is a logistical pain in arse, for a single project. The actual work...soldering, installing the proper connectors, wrapping, routing, is fairly easy...but painstaking, more of a test of one's patience than electrical expertise. I'm not suggesting any particular course of action here...only outlining some options. The point being that both the engine and wiring can be sorted, with no overwhelming difficulties...even under a worst-case scenario.

Agree 100%, and that's pretty much where I am. I live in the suburbs of north Atlanta where my county's population is 250,000+, so I need to have all of my CTs to be able to get 50 mph minimum. As an example, I'm currently doing a little brush up on my 2200 mile K1 with a port job, cam upgrade, 88cc jug, 20mm carb, exhaust upgrade and 16t/35t sprockets. The HK0 will be more of a hot rod (at least conceptually at this time).

To clarify, the XR70 will be rolled into the barn for either parting out or maybe take a chinese motor later as a beater. It is but a footnote as an engine donor, lol. I honestly just bought that thing to put in my K3, but then things took a turn when I saw the serial #.

I also restore and repair guitars, and some of the wiring in those can be pretty complicated with onboard preamps and the like. I got into the CTs because building cars has become exorbitant, and then being OCD, my tendency is to build something too nice to drive. Also it has given my son and I something fresh to connect with.

I appreciate forums such as these where folks already know the nuances so I don't have to relearn a lesson. I built the car pictured below from scratch which was a 100% restoration, and I made some great friends on the TA forums while building it.

Also, a CT70 was the first bike I ever rode when I was a kid, as I'm sure many of the folks on this forum.

Everyone loves the Trans Am! That looks nicer than I remember them new,nice work

Brofessor said:

Agree 100%, and that's pretty much where I am. I live in the suburbs of north Atlanta where my county's population is 250,000+, so I need to have all of my CTs to be able to get 50 mph minimum. As an example, I'm currently doing a little brush up on my 2200 mile K1 with a port job, cam upgrade, 88cc jug, 20mm carb, exhaust upgrade and 16t/35t sprockets. The HK0 will be more of a hot rod (at least conceptually at this time).

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That's pretty typical suburban road conditions and very familiar to most of us. And, FWIW, I agree with your mph requirements; they are reasonably...and achievable. Cutting to the chase, at a minimum you need a stroker crank and 110cc displacement. If you want to limit the bucks, invest in a tbparts 51mm stroker crank, HV oil pump, Takegawa HD manual clutch, and a hotter cam for the existing head. Feed it with a real Mikuni VM20 carb. That should be enough to reach 60mph. More accurately it's 60mph potential. It's enough to deliver a sustained 50mph, probably a little more, plus adequate pulling power to keep pace with traffic, deal with modest grades and headwinds.

If your budget allows, go for a 54mm crank. Nothing adds roadworthy power like a longer stroke. That said, beyond a 52mm stroke, the build gets more involved and costs more.