'72 CT70 winter restore project

Discussion in 'Projects/Builds' started by Kennysyeti, Dec 28, 2019.

  1. racerx

    racerx Administrator
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    Just pay attention to the type: 3-speed/6v, 4'speed, 12v. Each requires its own alternator type. 12v is considered "long rod".
     
  2. kirrbby

    kirrbby Well-Known Member

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    I've seen the listings on the dratv site for "short rod" 52mm cranks but they say they're discontinued. Being a "short rod" type, stroker crank, they must act a little differently than every other, "long rod" stroker crank.
     
  3. OLD CT

    OLD CT Well-Known Member

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    The funny thing is Webikes does not mention that the 52mm crank is 6v or 12v. I am going by the snout taper. It ''looks'' like a 12v to me. Not sure if it is something faatcaat is interested in, but worth mentioning.

    Webikes is not very informative to say the least.
     
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  4. racerx

    racerx Administrator
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    The trailbikes 51mm crank is the only one of which I am aware, that fits the 6v/3speed alternator. Here's the brain-twister, the same 52mm piston that fits the stock crank, also fits the stroker cranks...should be the same "short rod" compression height, right? Yet, as I recall, a 47mm piston with "long rod"/12v compression height is needed. That's never made sense to me. However...there is a means of figuring this out for oneself. Start out by dry-fitting the original piston (rings removed for ease of installation); if you end up with negative deck height (piston standing proud of the cylinder at TDC, a 12v/long rod piston is needed.

    Few websites are very specific when it comes to crank/piston/alternator fitment. Dratv is an exceptional resource. Some of the EU vendors go by key dimension(!). Part of the problem is that Japanese doesn't translate directly into western languages, including English. Running it through a program like Google translate, Babelfish, etc, transrates Japanese into Engrish. And, I agree, Web!ke is in a category of their own when it comes to lack of details.
     
  5. Kennysyeti

    Kennysyeti Member

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    Well I tore apart the 2nd motor and it is surprisingly a lot better condition than the first. Ir still had oil in it after 30 years. So now my focus is getting it up and running. This one will get the 52mm cylinder and hot cam. I'm a little worried about the stator as there was no cover on either motor and plenty of rust has formed. Can the flywheel be polished up where the points run to work? I still plan on building the first motor with the stroker crank and stock cylinder but after the 2nd one is complete.
     
  6. racerx

    racerx Administrator
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    As a mental backstop... as long as the transmission isn't rusty most of the lower end should be usable. Worn shift forks are common, with everything. Worn gears, otoh, are rare. If the primary drive gears & shift linkage are rust-free, you've got a viable lower end that only needs a few parts to bring back to as-new mechanical condition. This is my basic approach, beginning from the worst-case scenario. It can be a bit optimistic but usually isn't. These engines are remarkably rebuildable, no matter what the issue(s). The ones that turn out to be true paperweights are the rare exceptions.

    Breaker point cams can be polished, successfully. The removable 4-speeds are far easier to work with. But, 3-speeds can be done...slowly, by hand. I'm reluctant to media-blast a flywheel and wouldn't recommend this to anyone who is inexperienced. Chemical immersion isn't a great idea, either; there's no way to be sure that no residue is left between the hub & the aluminum body. For the novice DIYer, Scotchbrite pads + mineral spirits are much safer. You can use coarse to clean the magnets...and to begin cleaning the point cam, if it's significantly rusted. If you just have a heavy "haze" of surface rust, better to start with a fine Scotchbrite and penetrating oil/PB Blaster, or similar. That'll help remove rust and leave a finer finish. If you get a uniformly clean, shiny, surface, you're home free. If deep pits remain, it might be better to source another flywheel. That's not likely...and...3-speed/6v flywheels are not in high demand, IOW easy to find & cheap, if you need one.

    I'd recommend splitting the cases. There's a lot of scrubbing to be done, inside. I can almost guarantee that the shift forks have the old, dreaded, "smiley faces" and need to be replaced. You also want to be sure that the crankshaft assembly is within wear limits, i.e. no more than 0.018" side clearance, no detectable radial play...and the wrist pin fits snugly in the small end. New tranny bearings are cheap insurace, too.

    As for the stator, the business ends of the coil armatures can be sanded clean. There are also stator rebuild kits available...new coils/points/condenser.
     
  7. fatcaaat

    fatcaaat Well-Known Member

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    Back to the cranks, I was really interested in putting a 51mm tb stroker in my recent 95cc engine to bring it to 117cc but it wouldn't allow me to use the piston I have for it. The piston I have is an old school 12.5:1 high domed racing piston that is designed for the short crank, and thus, not a match. I decided I wanted the high compression mated with the big cam, over the extra stroke, and smaller domed piston...at least for this particular build.

    The trailbikes team did something interesting on their designs...they made their stroker cranks use the same piston deck height as the xr50's or the "long rod" cranks that started in the 80's. This, in essence, removes all the confusion about what works with what...instead all you need to consider is matching the head to the piston dome profile...Old head, new head, and z50 head...three different pistons, all the same deck heights matched to the stroker crank. And three cranks...12v, 6v 3-speed, and 6v 4-speed.
     
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  8. racerx

    racerx Administrator
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    To paraphrase fatcaaat, TB stroker cranks all use 12v/"long rod" compression height pistons. Go with that, then match the piston dome to your head..."big/6v" "small/12v".

    I'll add one side note. On a stock-bore stroker motor, static compression ratio tends to be high, since the combustion chamber volume remains the same, but displacement increases. All else the same, adding a 51mm crank to a stock 6v motor will increase static CR from 8.8 to 10.75:1. That's a healthy increase and pairs well with a "hotter" cam that has more overlap (which bleeds-off more cylinder pressure). Each 1-point increase in CR can be expected to increase efficiency by 5%. It's all about balance and approaching an engine as a balanced system makes things easier to understand.

    Pairing compression ratio and cam profile is a longer discussion than we need to have, at least for now and in practical terms. The point I was going for is that a small dome piston, would drop CR back toward the stock 8.8 and be perfectly livable. Stock combustion chamber volume, at TDC is 8.18cc (nominal). With 88cc displacement and the "6v/big" dome piston, you'd have 10.75:1. If one is "stuck" for a piston choice, it very well may be possible simply use a "12V/small dome" piston with the 51mm stroker crank and 6v head. The small dome piston has a nice "squish band" that the big dome piston lacks so, while CR would be lower, that squish band makes the combustion chamber more efficient...a nice tradeoff, if there must be a compromise.
     
  9. Kennysyeti

    Kennysyeti Member

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    So the TB stroker crank is bringing the piston down the cylinder farther but not any higher. I will be running the stock head so do you think I can use the stock piston and can it take that much compression?
     
  10. racerx

    racerx Administrator
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    Octane requirement is a tricky thing. I'll spare everyone yet another longwinded dissertation, with this. First off, 10.8:1 static CR should be quite compatible with 92 octane pump premium. Due to the tiny bore diameter, there's some question as to whether premium is even needed. Cutting to the chase...at 100mpg, running premium is cheap insurance.

    Let's be clear about the term "compression height". This refers to the location of the wrist pin, relative to the top of the compression ringland, i.e. the area above the top piston ring. Dome is another element. You're correct, longer stroke does pull the piston lower inside the bore. But, it's possible to have the same stroke...with different connecting rod lengths. And that's where compression height matters...a longer rod needs a "shorter" piston, to keep it from standing proud of the cylinder at TDC (a.k.a. "negative deck height") and hitting the head. Likewise, that shorter piston, combined with a shorter rod would leave too much deck height at TDC, leaving the piston way down in the bore...killing compression.

    Honda went to a longer connecting rod, with the 12v era motors. The wrist pin had to be move up, closer to the piston crown, to match.

    With aftermarket cranks, you have to pay attention to which configuration is used...6v/short rod or 12v/long rod and match the piston accordingly.

    I suggest ordering your stroker crank, then test-fitting the original (6v) piston, sans rings (to make things easier), to check deck height. The original cylinder could be used for this, even if you replace it, the same, stock, 63mm length is required. If any of the top ringland protrudes above the deck, at TDC, you need a 12v style piston. Then, you order the correct piston without guessing.
     
  11. Kennysyeti

    Kennysyeti Member

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    Thanks for the info. The first engine I tore down was under a shed but with no carb mounted on it and the piston was stuck hard( busted the cylinder pressing it out) and a light layer of rust on everything inside(stroker candidate). The second engine was not under cover and without a sparkplug in the hole but a dirt dobber(wasp) filled the hole up with dirt and everything inside this engine is still has a film of oil on it( 88cc cylinder on this one). The cylinder honed out nicely and even the piston looks good but with stuck rings. So that's the one I will put together first and swap it into the candy yellow frame engine while I am waiting on the ruby red paint job so I can tear into that crappy running engine and try to figure it out.
     
  12. fatcaaat

    fatcaaat Well-Known Member

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    FYI, I was on the Dratv page yesterday and noticed that they now have a 52mm "short rod" crank listed out there for CDI. This is a new item that they were not previously selling. It is not the dragon racing parts porkchopped version, and I have no clue where it comes from or what the details are, but it is out there. Now...if he decides to bring one back for points cranks, we might be in business.
     
  13. racerx

    racerx Administrator
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    Agreed...but...there are positive sides to this, regardless. A short rod/CDI crank has a place in the overall (global) parts catalog, too. That makes the piston selection easier/more straightforward...and allows 12v/CDI conversion without any other internal mods. There's a tradeoff, as with anything else, namely the flywheel cover; that boils down to a choice between a CDI flywheel cover or fabbing a spacer plate. A 51-52mm stroke "sleeper" build with 12v/CDI would be cool setup, in its own way.

    BTW, there's a slight chance that the DRP cranks may come back. I've noticed that some of the DRP stuff has returned over that past year. Not a guarantee but a good sign, imho.
     
  14. Kennysyeti

    Kennysyeti Member

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    I'm almost done with the first engine(running it with stock bore first then adding the 88cc cylinder and monster cam after knowing engine is fine) which I'm thinking of swapping into the candy yellow special frame to test ride and was wondering what is the best after market exhaust with stock look out there. I have a Chinese copy on there now. Trail Buddy has a couple of nice looking pipes(Thumper 2) and CHP also has a few stock copies. Also I'm still struggling with the candy special yellow engine(weak and won't rev out). My 71' HKO will run circles around it. Someone else on here with a SL70 had the exact same symptoms.
     
  15. racerx

    racerx Administrator
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    Best guess is that it's way down on compression. Take a peek inside the intake port; if it's anything but clean, grey, aluminum the intake valve hasn't sealed properly in a long time.

    With an 88cc tune, the stock exhaust is adequate. For that matter, it's good enough to handle 50mph horsepower, regardless of displacement...it's just that bigger displacement tunes make more power than that.

    Regardless of how you end up building these engines, take the time to split the cases for thorough cleaning & inspection. The odds of finding worn shift forks as sky-high. Those are easy to replace, once the engine is open...and cheap insurance against having to go back in before the rebuild has seen less than 5-digit mileage.
     
  16. Kennysyeti

    Kennysyeti Member

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    20200117_100031.jpg 20200117_100152.jpg 20200117_100137.jpg Well, I swapped out piston, cylinder and head on the Yellow 72' CT70 and it seems to run a lot better. I need to run it longer(later today) to make sure but what little I did, it revved out faster. It still doesn't idle right so looks like that may be in the carb(acts like an air leak). Also waiting on some head parts but almost got the other engine finished which will have the 88cc kit with monster cam. That's going into the candy ruby red frame that's at the pain shop.
     

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