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Discussion Starter #21
Keep us posted! Also what carb did you get?
I got the 20mm carb that comes with the kit - I believe its the one listed above ^

Anybody in this thread done the CRF70 Head & Piston, and do you think it's a good in between the stocker and race head?

I'm sure I'll ride this new setup around a while, and then just like all my other projects I'll of course need more power and do one of the stroker setups.. :alien:
 

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The 70 cylinder and piston is 47mm, the TB Parts 88cc kit uses a 52mm cylinder and piston. I'm not sure if you can use the 70 head on the 52mm cylinder, it is an improvement over the 14/17mm valves of the stock 50 head, as they are 23/20. The TB Parts race head has 25/22 valves.
 

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The kits we both, both 88cc are meant to work with the stock 50cc head. I’d have to think it would work with the 70 head, unless I’m missing something.

Really good to have those valve sizes though. Helpful to see them side by side.


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Also, the 52mm piston for the stock head may hit the valves on the 70 head, the flycuts are not as deep as on the race head piston, as you can see below.


273234
 

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Oh!!! I bought mine as a kit. Didn’t realize the pistons were different. I mean, obviously they would have to me. Dumb.

So, if you want to put a race head on an existing TB 88 bore kit, you need a new piston as well? Good info to squirrel away


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From what I've read, and seen...the tb race head is a kinda cross between a 6v CT70 "big dome" head, and the 12v xr/crf70 head. The combustion chamber is similar to the 6v 70cc head, but it uses the newer style bearing cam, which gives you more choices in cams. It has big cooling fins similar to the trx90.
I've read where someone said it was patterned after the trx90 head...which I think DOES look to be the case.
The valve and port sizes are pretty close between ALL 4 heads that I've mentioned. The tb race head may have somewhat improved porting...shape of the ports, for better flow..? (I'm not too sure about that one)

A 52mm big bore piston, made for a 50 or 70 head, might run with the tb race head, but the compression would probably be low since the piston dome wouldn't fill the big combustion chamber as well as a correct piston... with the "big dome".

No matter what configuration you're building, you ALWAYS want to match the piston to the cylinder AND head AND crankshaft that you're using.

Some smart guys will run special pistons, for special reasons...modify pistons for, performance changes and such. I don't know much about that smart guy stuff tho :)
 

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the kirbbster said:
Some smart guys will run special pistons, for special reasons...modify pistons for, performance changes and such ...
Like the time I chucked up a 6V short-rod CT70 piston in a lathe and turned it down to fit an XR/CRF 70 head just to see if it would work?

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I recently went dow this path...and still kind of am. Rebuilding a salt waterlogged 6v ct70 motor for a friend. He provided me with a Chinese cylinder and head. I put it together and realized it’s a 12v, flat piston. Sat WAY down in the bore. I went to on dratv and bought the “correct” piston. Slapped that in and it hit the combustion chamber. New to this, I didn’t realize all of the above. I needed a “special” piston that allows you to run a 6v motor and a 12v head. A pain and some money lost but could be worse.

Oh wait!! It is. Put the whole thing together for the zillionth time and there is none, or little compression. Hooked up a leak down and possible both but for sure the intake valve is leaking. Wonderful


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Like the time I chucked up a 6V short-rod CT70 piston in a lathe and turned it down to fit an XR/CRF 70 head just to see if it would work?

View attachment 273237
I recently went dow this path...and still kind of am. Rebuilding a salt waterlogged 6v ct70 motor for a friend. He provided me with a Chinese cylinder and head. I put it together and realized it’s a 12v, flat piston. Sat WAY down in the bore. I went to on dratv and bought the “correct” piston. Slapped that in and it hit the combustion chamber. New to this, I didn’t realize all of the above. I needed a “special” piston that allows you to run a 6v motor and a 12v head. A pain and some money lost but could be worse.

Oh wait!! It is. Put the whole thing together for the zillionth time and there is none, or little compression. Hooked up a leak down and possible both but for sure the intake valve is leaking. Wonderful


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A while back, a buddy told me the 52mm pistons Dr.ATV sells produce a really high compression ratio with a 6 Volt big dome head, like 18:1. I have not verified this, but can attest to how hard they are to kickstart, having broken two kickstart gears trying to get a bike going. My solution was to work the combustion chamber a little bit, de-shrouding the valves, and take a little off the piston top (I just held the piston top against a stationary disc sander, and took about a millimeter off the peaked top portion of the piston.) It was easier to start and actually seemed to accelerate better afterwards. Not exactly scientific, but it worked.
 

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I wish everything could just work together, simplify things for everyone. Hell, I'd pay someone to write a guide on what works with what with PN's and pictures. That would be dope.
 

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Discussion Starter #36
Just checking in after getting everything installed and taking it for the first spin yesterday.

Very happy with the power over stock provided by the 88 kit, stock head, stock pipe, 20mm carb. Started 2nd kick and just took some minor carb tweaks to have it ripping around

I often take my 6 year old on rides with him sitting in front of me, and it definitely has more power to get through some sandy spots and I can leave it in 3rd instead of constantly downshifting.

Thanks for all the advice everyone. Going to enjoy it like this for a few months until I for sure get the itch for more (and more, and more) power like I do everything else.
 

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Discussion Starter #37
Let me add the only gripe I have is that the throttle cable with the TB kit seems a bit long, and the screw that goes into the twist grip, and the middle adjuster are both almost all the way out to get into adjustment. Runs fine but just awkward.
 
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