Z50k2 with a Trx top end ??

[Cleato904]

I want a little more than stock from my z50k2. If I'm reading (comprehending) correctly by the measurements and the data provided by Beatrice cycle. Looks like a Trx70 cylinder and head will bolt right up, Since the piston to be used is from the Trx70, the cylinder is the same height as the Z50k2 and both motors have short con. rod crankshafts and also the head/cylinder studs are the same length. Just wanted to know if anyone has went this route before instead of the 88cc kit. Since I've got a couple of spares in the floor. I figured a budget build??

 

[kirrbby]

It does look like the TRX 70 top end will work. I thought the crank would be different than your K2, but the doc says it's the same. It should bolt up and work well. You might have to find a little bigger carb that will fit under the tank of your Z. Or the stocker might be ok for 72cc's with just a bigger jet.

 

[kirrbby]

I only worry about your rear fender. With all that new power, it'll be in the danger zone :--)

 

[Cleato904]

Thanks kirby for the confirmation. I thought I was reading correctly on the specs for the combination. Rear fender needs replacing any way. If it gets to Harry I'll put a wheelie bar on I don't want a repeat of my past when I was racing limited modified gocarts and put one of my alcohol 5 hp briggs Limited modified motors pushing 12 Hp in a thunder bird mini bike. I looped out ending up tearing my ACL in My left knee. But that death trap was faster than a kx65. But wouldn't stop for crap with scrub brakes.

 

[69ST]

The 72cc engines are based on the 49cc, Z50, lower end. Anything up to & including the 54 x 51 TB117 is possible without losing the outwardly-stock appearance of the motor...aside from the alloy cylinder jug & external plumbing, of course. A lot of owners have swapped-in a complete 72cc top end (cylinder, piston & head)...it's the original bore-up kit, using all OEM parts. The Z50 carb & exhaust will definitely limit breathing and power. However, it should be possible to rejet the carb, retain both and gain a nice bit of torque.

One caveat...test-fit the new cylinder in the block. Some Z50 cases have just enough casting flash to prevent a CT/TRX 70 cylinder from fitting. It's easy enough to correct but, the cases have to be split.

 

[Cleato904]

I test fit the cylinder and it slid right in like the 50 one that came out, that's what really go my gears turning. Between my research and the confirmation from you guys, that'll be the route I'm going to go with since I've got enough parts to build it laying under the bench, and gaskets on the shelf. Thanks

 

[69ST]

Cool biz! Last thing to verify is compression height of the piston. Once you get it, do a test fit...clean & oil the cylinder, fit the piston to the rod, but don't install the rings. That'll allow you to verify deck height. From there you're home free.

The ball bearing TRX head is 12v flavor, a nice upgrade to a 6V motor.

 

[Cleato904]

I'll get some molding clay and put a small ball on top of the piston and see how much room between the piston and head dome there is. Plus I think I'll get my dragon race cam out my daughter's Trx70 and put her stock cam back in and use it I'm my 50/70 build since her motor is all stock.

 

[69ST]

You'll be checking valve clearance with the cam & chain installed, correct? Not being insulting here, just wanna be sure that we're both on the same page. I've no idea how much engine building experience you have. IOW...mo`better ask what might be a stupid question than take a chance of setting you up for a disappointment, just when you're expecting to enjoy your newfound power.

 

[Cleato904]

That's correct I'll be making Shure there's no contact with valves and piston which there should not be by other people's homework. Just making Shure. Kicking over your first build and it locking up and bending a valve is a sound/feel you don't forget. Ive been working at my current Honda dealership for 14 years and 3.5 a the old dealership. I won 1st place in the GA. V.I.C.A SKILLS in air cooled gasoline engines in high school in the 11 the grade and placed 13th in the nation. I'm not bragging it's just what I've accomplished so far.

 

[Cleato904]

The stock TRX70 head isn't a ball bearing head it's a 6V style.

 

[69ST]

Well, in that case, it's virtually the same as a CT70. For some reason, I thought you had a 12v style TRX head.

 

[Cleato904]

The TRX90 has the bearing cam. It's been 31 years since Honda intraduced the the Trx70 for 86-87 and I swear 90%of them were 86 models and 80% are still running. I see 10 trx70s to 1 trx90 (20+year production)come into the shop