Help piece together a stroker kit for my build

[kawahonda]

Hi all,

I figured I'd create a separate thread for this topic, because I wasn't getting clarity in the build thread.

My father is doing a total rebuild on my CT70K0. I ordered the DRATV 88cc big bore kit for him to use, as well as the mini-monster cam. It will be using a stock head with new valves. Then I got to thinking about adding a stroker crank. There's a couple things I've learned about stroker cranks:

1) Must have the TB HV Oil Pump, which we already have.

2) Must use the "12v" piston. I "think" the piston that's included in DRATV's kit is the "old" type, which I assume is the "6v" type? I think this is what's included in the 88cc kit
http://dratv.com/repikitfor88.html


Questions:

1) Do I need to use a different piston? If so, which one do I order?

2) Which stroker crank is recommended? Link? Is this all I need? http://www.ebay.com/itm/51MM-Stroke...9-79-CT70-amp-68-81-Z50-TBW0622-/291506360654


Goal of build:

To create a sleeper stock bike using the stock air canister and exhaust. Obviously "stock" can only go so far (I will likely use a 19mm Mukiki carb, and the big bore cylinder does say "49cc", but that's very minor stuff that you'd have to peer into to see). That's acceptable to me for this build. But you guys get the point...I want it to look as stock as possible. I'd want to be able to enter this bike into a show and for it to do good in one.

I'm planning to use the eBay CDI 6V kit. I'd like to keep this bike at 6V because I've already invested in the 6v system.

Thanks!

 

[kirrbby]

I don't know why trailbikes makes it so difficult to find parts on their website. Probably easier to find them by going thru one of their vendor's.

You need the crank made for a 6v 3speed z or CT or atc70 etc. http://dratv.com/trbicrkit.html
Your stator conversion will work with this crank.

You will need to use a oil pump drive shaft like the xr/CRF uses...Skinny shaft, or turn yours down...But I'd recommend the skinny shaft and HV pump. It's on the same link above.

Many say you need a oil cooler if you go up to a stroker. Some say it might not be absolutely necessary.

 

[kirrbby]

The crank above is a replacement for your K0 CT70's crank, and as long as you bought the right bbkit for your K0, the piston that came with the kit will work with that stroker crank. The piston you got should also be correct for your K0 (big dome) cylinder head...Or the TB race head. Both are considered big dome heads. Big dome refers to the size and shape of the combustion chamber.

 

[kirrbby]

Your stock clutch should be fine for the way you're likely to ride this bike...But you'll want to add the HD springs. And they're cheap. If your clutch ain't working, you'll know it, and you'll replace it down the road. A credit/CRF 70 clutch is a nice little upgrade, and they're cheap. HAS to be from a 70 tho, not a 50.

 

[kawahonda]

Hmm. Here's the exact BB kit I bought:

http://www.ebay.com/itm/52MM-88CC-B...ash=item51b812ca5b:g:EO8AAOSwWxNYuXxr&vxp=mtr

If this is the correct piston, then all I need to do is contact DRATV and see if I can just get that crank, and I'd be set?

Any reliability concerns with doing a stroker kit? Sounds like cooling may be a concern, but wouldn't a HV oil pump squash that concern?

 

[kirrbby]

The crank I posted WILL work with that kit. I think dratv has a stroker crank too...52mm
Nope, don't see one for the 3 speed. Go with the one I posted.
You will need the skinny oil pump drive shaft. And you'll have to have the correct pump to match it. The pump you bought might be the wrong one...Old style drive shaft.

 

[kirrbby]

A stroker makes a TON more power. So it's sure to be harder on everything related. Shorter lifespan I'm sure...But worth it.

 

[kirrbby]

Be sure to check to make sure the spigot of that 52mm cylinder will fit into the cases with a little clearance all around...Before, you start assembling the lower end. Min hit, and the cases will need to be opened up a hair.

Also, I would be sure to put the oil pump drive shaft and gear in the cases BEFORE the crank is installed. I've had problems there too.

 

[69ST]

AFAIK, no one ever offered a 52mm 6v/3-speed flavor crank. Closest was the DRP 52mm...that fit the 4-speed flywheel. The rest have either been 12v/CDI, or a different stroke length. The 51mm htb crank is the only one that is available with the 3-speed/6v flywheel taper.

You match the crank to your flywheel/igntion, to get the right taper, then select the piston based on the compression height (to match the connecting rod length) and head type (6v, or 12v). When in doubt, consult the vendor.

A reasonably-well setup & tuned 108 stroker motor will be capable of at least double the peak hp of a stocker. The will generate at double the BTUs, that's basic physics. How effectively increased oil flow volume will keep oil temps under control has generated more than a little discussion of late. The only object answer, guaranteed to match your, specific, situation, is to be found by monitoring oil temps.

 

[fatcaaat]

I don't know why trailbikes makes it so difficult to find parts on their website. Probably easier to find them by going thru one of their vendor's.

You need the crank made for a 6v 3speed z or CT or atc70 etc. http://dratv.com/trbicrkit.html
Your stator conversion will work with this crank.

You will need to use a oil pump drive shaft like the xr/CRF uses...Skinny shaft, or turn yours down...But I'd recommend the skinny shaft and HV pump. It's on the same link above.

Many say you need a oil cooler if you go up to a stroker. Some say it might not be absolutely necessary.

Kirby, I don't think you are right. The Trailbikes stroker kits are designed to be used in conjunction with Long Rod piston setups. So if you match the piston set that he purchased which matches the KO short rod setup with the long rod TB crank, the piston is going to stick out the cylinder by about 2.5mm.

The crank that works with that particular piston set is this one: http://dratv.com/racr52stforc.html. You can also see that this particular crank requires that you convert over to CDI as well as the stock flywheel setup is not compatible here.

The options are:
1. Replace the piston that you purchased with the correct matching piston for the long rod crank and then buy the TB crank. This piston will work http://dratv.com/52lbio1.html with super high compression. This one will work with lower compression http://dratv.com/pikitg5152tt.html
2. Use the Dratv crank instead at 52mm.

 

[kirrbby]

Fatcaaat, take one more look at the crank that I posted. The picture is of a 12v taper, but I think that pic is generic. The description states that it is for 6v points type engines. The link that Kawahonda posted of his kit, also says pre 81 CT70's.

 

[kawahonda]

Awesome responses guys, I really appreciate it. This is a big help.

What I would think to do if I went with a stroker, is to buy the lower compression piston. Do you know what my expected compression would be with the TB crank and that lower compression piston with the stock head? My goal is under 10.1:1 because I want to keep this thing really street-able.

So with the TB crank that Kirby listed (i'd look for the crank only), and the DRATV low compression piston, I'd be able to get the eBay 6v CDI and be on my merry way (and confirming that the TB HV pump has the correct skinny rod). Is this correct?

Correct me if I'm wrong, but it sounds like the stroker kit, while being a fantastic bang-for-the-buck, there's no guarantee on how well the motor is going to stay cooled, and there's no guaranteed on reliability. So if I choose to ride this thing let's so, at a sustained 45 MPH for 10 minutes or so, I probably should expect to sweat a little bit more (and monitor my oil temp).

Maybe it just goes back to my performance goal. My goal is simply to make this faster, or at parity to a CT90. A healthy CT90 will hold a cruise speed of 45 in most situations on pavement.

Currently, I have the DRATV BB Kit, mini-monster cam, and a refreshed stock head, and the Ebay CDI kit. Tires are going to be unknown at this point, may go with the wings, or may go with a street tire. I'm willing to do anything I can with the stock air canister, as long as I keep the stock canister installed. Obviously will plan to optimize the gearing. I have no problem using a Mukini 19mm carb if necessary, either, if a 88cc requires a carb-up.

It could be that maybe I don't need a stroker to achieve what I'm looking for. And if 45MPH cruise is all I need, it may be better to save $200 and to enjoy more peace of mind from a reliability standpoint?

At any rate, if you guys feel that a stroker crank is a must in order to meet my goals, and if reliability is not really a concern, then based on what I have already, feel free to keep sending me links of what I need to successfully pull this off.

 

[kirrbby]

Kirby, I don't think you are right. The Trailbikes stroker kits are designed to be used in conjunction with Long Rod piston setups. So if you match the piston set that he purchased which matches the KO short rod setup with the long rod TB crank, the piston is going to stick out the cylinder by about 2.5mm.

The crank that works with that particular piston set is this one: http://dratv.com/racr52stforc.html. You can also see that this particular crank requires that you convert over to CDI as well as the stock flywheel setup is not compatible here.

The options are:
1. Replace the piston that you purchased with the correct matching piston for the long rod crank and then buy the TB crank. This piston will work http://dratv.com/52lbio1.html with super high compression. This one will work with lower compression http://dratv.com/pikitg5152tt.html
2. Use the Dratv crank instead at 52mm.

I'm looking at the BB kit that he linked in post #5.
The piston that he linked in post #1... I'm not sure about.

Kawahonda, the crank that you linked in post #1 is the same one I linked, but without the extras. TB part number TBW0622

 

[fatcaaat]

Points or no points...don't let that confuse the matter. The connecting rod on the TB crank is a long connecting rod...that means that you need to match a long connecting rod piston so the deck is correct. I'm not even talking about the flywheel taper...that's a different issue.

Dratv no longer carries the points taper 52mm crankshaft. If you want to run the trailbikes crank with the points taper, you need the piston to match, and the one that Kawahonda links to is a short crank piston.

The Kit you are linking to, Kirby, will work...but not the crank independently with Kawahonda's piston...

 

[kirrbby]

I got ya now. The TB crank is a "long rod" replacement...Even tho it's made to replace a stock CT70 K0 "short rod" crank...6v 3speed taper.

So, Kawahonda, you would need the crank that you linked in post #1. But you will need a different piston. You need a 52mm piston that is made to use with a long rod crank, and a big dome head. The options posted by Fatcaaat above. Maybe if you email dratv they will let you return the piston you have, and send you the one you need...Maybe they have the, crank only, too.

In your dad's thread, the oil pump looks like it came with the skinny drive shaft, so you should be good there.

This is why it's so confusing trying to piece together a "kit". And why it's so nice to have the help of lh to sort it out. Sometimes the best way to get the right information, is to post the wrong information

 

[69ST]

Fatcaaat was a lot clearer on the piston & crank issue than I was. And that's the operative word here "was"...the DRP 52mm/4-speed type crank joined the NLA parts list about 2 years ago. That is part of what makes the trailbikes 51mm arm the go-to stroker crank. The 3-speed flavor is the biggie, for most.

FWIW, I'd convert to 12v/CDI while doing this build. You're married to the long-rod/12v compression height piston, regardless of which trailbikes crank you use. So, with the piston selection sorted out, the last choice becomes ignition type. It's 2017, 12v/CDI is not only a huge improvement over the old 6v/breaker point setup, it's also the norm. For a road bike, that you're going to ride, do you really want to be tied to 6v bulbs and ignition maintenance?

As for reliability, imho, 45mph sustained should be no problem. That's close to the level which Honda took the venerable 49cc-based motor, with the C90...which never made it to North America. The biggest differences, with the 108 tune, are the torque output (big increase) and top speed potential which should be close to 60mph. Going by the convenient 80/20 rule, that should give you solid, usable, road power into the low 50s. It's that 50+ range where the stresses reach the steep portion of the curve. Going from 50 to 60mph requires ~60% more hp. IOW, while the lower end will definitely be worked harder, you're not really talking about riding with the throttle held wide-open all the time. 40-50mph really isn't asking the motor to work a lot harder than the factory engineers felt was viable.

In terms of estimating service life between rebuilds, I'm not an engineer. Best I can offer is an educated guess, based indirect experience & observation. The common denominator is odometer mileage. While it's relatively unusual to see a CT70 with 5-digit mileage showing, over the years I've seen a surprising number of SL & CL70 models with ~15K on the odometers. Likewise with S90s (different engine but close to the same era, displacement and hp). I've seen CT70 engines that were all done by the 2K mile mark, others that were still healthy in the 6-9K range and a few (less than a handful) well into 5-digit mileage. How the motor is treated makes all the difference. Get the setup, break-in & tuning optimized and keep clean oil in it and you're likely to get a lot of happy miles. And be certain you're running the proper oil type...wet-clutch compatible and plenty of zinc, to protect the transmission.

 

[kawahonda]

Thanks everyone. For now, I think I'm going to pass on the stroker build. This thread should be very valuable for someone who is looking to piece one together. It was valuable for me, too.

Racerx, sounds like you think 45 MPH sustained is not going to be a problem with an 88cc kit, 18-19mm carb, CDI kit (with an advance curve), a modded air box, a mini-monster cam, and good all-around gearing.

 

[69ST]

With that setup, you might see low 50s, under ideal conditions; those last 3-4mph will be fragile. 45mph sustained, from 88cc + stock crank, is going to be edge city. Expect the horsepressure to start getting thin around 40mph, with about the same seat-of-the-pants dyno reading (acceleration) as a really healthy stocker...only the speedo will indicate a few more mph before leveling-off.

 

[fatcaaat]

My trailbikes 108 will top out at 67mph tucked. Granted, this engine is sporting some top notch portwork, matched intake, and a perfectly tuned vm20 carb with a stepped exhaust to provide just a tad back pressure running on nice Molas street tires and geared to absolute perfection with that top speed hitting at 10krpm. Cruising, I like to run it at 50mph but it requires downshifts on small hills.

My experience with 88 kits are wanting...I think I got 55mph out of one once...40 is a great cruising speed with it...it may do 45...I don't remember...

 

[kirrbby]

The 88 kit gives a noticable boost. Makes the bike much more interesting to ride...But that's about it IMO. I know some of the guys over on planetminis tune them to rip, but I think it takes some money, and some talent that I don't have. The 88 is super easy and certainly worthwhile...But it don't really compare to a big stroker engine. I'm not sure what a 88 will cruise because I've never cruised on one for very long. It's not really a street machine. I think you'll feel small on the road with it.