Z50R head vs XR70R head questions

916mestizo

916mestizo

New Member
Forgive me if there is a previous thread about this, I did some searching but on here but couldn't find what I was looking for.

I picked up a '91 Z50R and plan to do a TBparts 88CC piston/cylinder... I have a spare '97 XR70 head and wanted to know if that head would be considered an upgrade or in any way better than the 50cc head.

I read that the 70cc heads have bigger valves and flow more, but I have not seen any info as to whether or not it would be a direct swap or if I need a specific piston for that head/cylinder combo.
 
kirrbby

kirrbby

Well-Known Member
I believe the 70 head will have bigger valves and bigger ports...intake and exhaust. And bigger IS better here. I would go with the xr70 head, then buy a GOOD quality BB kit to match that head. Basically buy the kit that's made for a 97 xr70. Maybe add a cam and a carb...exhaust..?
:)
 
motodevo

motodevo

Active Member
The xr70 head has a larger dome than the late z50 head. I know BBR recommends not using a 70 head for their 88 kit as it reduces the CR. You might want to send TB an email.
The XR70 head has the same size dome and valves as an early model (pre 82) Z50 head. Late model Z50R head has the smaller valves and dome like a CRF50 head.
 
916mestizo

916mestizo

New Member
Ok thanks for info, i think I'll send tbparts an email and see what they.

I read the same thing both of you said and never could find a definite answer as to which piston/cylinder/head to go with.

Lol so much terminology to figure out, long rod, short rod, old dome, small dome, etc.
 
6

69ST

Well-Known Member
49cc and 72cc flavors of theses motors only differ in top end assemblies. Short rod = 6v = pre-1982. Later motors were long rod.

Can't run a 70 head with a 50 cylinder, the valves won't clear. If you want to increase displacement...72-88cc...just source the head, cylinder, piston for the desired displacement, as am assembly. Choose piston by compression height (short or long rod) & head...EZPZ. Then match the intake & carb. The 50 carb is way too small for anything over 49cc.
 
kirrbby

kirrbby

Well-Known Member
motodevo said:
The xr70 head has a larger dome than the late z50 head. I know BBR recommends not using a 70 head for their 88 kit as it reduces the CR. You might want to send TB an email.
The XR70 head has the same size dome and valves as an early model (pre 82) Z50 head. Late model Z50R head has the smaller valves and dome like a CRF50 head.
Click to expand...
Recommending a 50 head over a 70 head on a 88cc kit makes no sense to me. I'm thinkin Tbolt must only have a one size fits all type piston for their 88cc kits. I'd look for a kit that has a high compression piston for a xr70 head, if high compression matters to you.
 
kirrbby

kirrbby

Well-Known Member
hmm...Tbolt shows a high compression 88cc kit for a xr70, so I don't know why they'd recommend using the Z50 head. Hopefully they'll explain.
 
916mestizo

916mestizo

New Member
racerx said:
49cc and 72cc flavors of theses motors only differ in top end assemblies. Short rod = 6v = pre-1982. Later motors were long rod.

Can't run a 70 head with a 50 cylinder, the valves won't clear. If you want to increase displacement...72-88cc...just source the head, cylinder, piston for the desired displacement, as am assembly. Choose piston by compression height (short or long rod) & head...EZPZ. Then match the intake & carb. The 50 carb is way too small for anything over 49cc.
Click to expand...

Ok got it!

Some of the stuff I read was saying that all 50cc versions were a different rod length. Most of what I have dealt with up to this point has been ATC/TRX 70 motors, which are their own lil world as far 6v/12v/short rod goes. o_O

So if I am not mistaken at this point, I can just order an 88cc kit for the XR70 and I'm good to go?

As for carb, I plan to run the Mikuni 19mm, ordered a few of these for my ATC 70 drift trikes with 88cc kits and work great so far.
 
916mestizo

916mestizo

New Member
kirrbby said:
Are you the 816 guy over on planetminis? Over there, they are talkin bout 80 z50. and 85 atc 70 heads
Click to expand...

Nope, I go by this username on everything, the only other name I use is just plain "mestizo".

Let me know if you want any pics or specific questions about the TRX 70 motors, I'm no expert but I have one of both an 86 and 87. So far after doing the 88cc kits on both the only major differences between the two is the foot brake pivot stud and extra electrical plugs for the key switch on the 87. The both have a plastic "spacer" between the head and cylinder on the cam chain tube and also a metal sleeve in the oil supply hole between the head and cylinder, that I have not seen on any of the other 70cc motors I have worked on.
 
motodevo

motodevo

Active Member
Just for comparison, here is a 12V stock 50 head 88 piston next to a minimoto 12V high compression piston. I bought the minimoto piston for my z50r i am slapping an 88 kit with an XR70 head. Only cost $30 Australian (considerably less in US $) from Webike.

I have no personal evidence , just what i read here about the 70 head
https://bbrmotorsports.com/Products/Products.aspx?Prod=411-HXR-5500
 

Attachments

  • IMG_20180613_11353.jpg
    IMG_20180613_11353.jpg
    125.8 KB · Views: 127
Last edited:
kirrbby

kirrbby

Well-Known Member
916mestizo said:
Nope, I go by this username on everything, the only other name I use is just plain "mestizo".

Let me know if you want any pics or specific questions about the TRX 70 motors, I'm no expert but I have one of both an 86 and 87. So far after doing the 88cc kits on both the only major differences between the two is the foot brake pivot stud and extra electrical plugs for the key switch on the 87. The both have a plastic "spacer" between the head and cylinder on the cam chain tube and also a metal sleeve in the oil supply hole between the head and cylinder, that I have not seen on any of the other 70cc motors I have worked on.
Click to expand...

The head gasket bits sound just like what the trx90's have. And I'll bet they would use the same gaskets.
I'm mainly interested in the stator. It has coils like the older 6v points bikes. But it's a 12v CDI system. It seems to be a odd duck "tweener".

Tweener being...somewhere between the 6v points type, and the common 12v CDI that has the 5 small coils, there is this trx70 stator that I've only seen on trx70's.
Trx70's didn't come with lights, but there is a lighting coil that is unused.

They also have a 12v type crankshaft, with a old...big dome, 6v type head....odd duck.

"Extra electrical plugs on the key switch" I didn't know about. That's a good bit of info.
 
6

69ST

Well-Known Member
With cranks, 3 basic flavors... 3-speed/breaker point a.k.a. "6V"; 4-speed w/spark advance: "12v". Each of these has its own flywheel taper. The "12v" type also comes fitted with a longer rod, thus requiring a shorter piston (reduced compression height) to keep the squish band at "zero deck height" i.e. flush with the top of the cylinder, at TDC.

Pistons are a little more involved. However, most of the crank assemblies out there either match "6v"/short rod, or "12v" long rod, compression height pistons.

Where things get complicated, in a hurry, is when stroke length is altered, or head type is changed. Increasing stroke, all else the same, dramatically increases static CR. That's where some of these aftermarket pistons can allow one to really dial-in a combo...if one knows what he's doing. Sounds a little complicated...and it is...to a point. There really aren't all that many different pistons out there, off-the-shelf. Ultimately, that saves a lot of bacon...sanity, too.
 
916mestizo

916mestizo

New Member
kirrbby said:
I'm mainly interested in the stator. It has coils like the older 6v points bikes. But it's a 12v CDI system.
Click to expand...

Yes this is correct, it looks like the older 6v points setup without the condenser but with an added trigger on the outside of the stator plate. Also it does have the extra coil for lighting as you said.

So the email I received back today confirmed what was said earlier... XR70 head, 88cc cylinder and he recommended the high comp piston.
 
kirrbby

kirrbby

Well-Known Member
racerx said:
With cranks, 3 basic flavors... 3-speed/breaker point a.k.a. "6V"; 4-speed w/spark advance: "12v". Each of these has its own flywheel taper. The "12v" type also comes fitted with a longer rod, thus requiring a shorter piston (reduced compression height) to keep the squish band at "zero deck height" i.e. flush with the top of the cylinder, at TDC.

Pistons are a little more involved. However, most of the crank assemblies out there either match "6v"/short rod, or "12v" long rod, compression height pistons.

Where things get complicated, in a hurry, is when stroke length is altered, or head type is changed. Increasing stroke, all else the same, dramatically increases static CR. That's where some of these aftermarket pistons can allow one to really dial-in a combo...if one knows what he's doing. Sounds a little complicated...and it is...to a point. There really aren't all that many different pistons out there, off-the-shelf. Ultimately, that saves a lot of bacon...sanity, too.
Click to expand...
racerx, 916mestizo was asking if he should use the stock 91 z50 head on his future 88cc big bore kit...or, use a 97 xr70 head with his BB kit.

It sounds like Tbolt is recommending using the xr70 head, with the matching piston. That makes the most sense to me too.

BBR is I think telling us (in the link above) NOT to try running their kit with a 70 head because of the bigger dome of the 70 head. That is because their kit is made to be used with a 50cc head only. They must not make a piston for the 70cc head.

916mestizo, when you step up to that 70 head, you're really going to need to go with a bigger carb and intake too. It would also be the best/easiest time to add a performance cam...if you're interested.
The carb/intake from the xr70 would probably be a bit undersized, but work, with just a larger main jet...if you have that carb.
Otherwise, member dirtbikr188 really likes tbolts carb with the 88cc kit. I think it's their 18mm shengway carb.
 
6

69ST

Well-Known Member
kirrbby said:
racerx, 916mestizo was asking if he should use the stock 91 z50 head on his future 88cc big bore kit...or, use a 97 xr70 head with his BB kit.

It sounds like Tbolt is recommending using the xr70 head, with the matching piston. That makes the most sense to me too.

BBR is I think telling us (in the link above) NOT to try running their kit with a 70 head because of the bigger dome of the 70 head. That is because their kit is made to be used with a 50cc head only. They must not make a piston for the 70cc head.
Click to expand...

Agreed, sounds to me like BBR is approaching this from a marketing standpoint, i.e. can't sell what they don't have. I think I'd just go with the trailbikes 88cc kit, in 12v flavor, with the 70 head. That'd deliver the proper compression height & piston dome to fit the 12v era CT70/CRF70/XR70 head. IMHO, 18mm is a good size metered air & fuel leak (a.k.a. carburetor:sneaky:) to feed 88cc of rip-snortin' horsepressure.
 
916mestizo

916mestizo

New Member
Top Bottom