GT5 small port cylinder pics

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maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
I know I'm damned confused now...
I have 5 motors sitting here, and they ALL seem the same EXCEPT that one weird jug.
All the parts interchange fine except that one weird jug.
It is simply too short for ANY of my motors without excessive base gasket spacing
 

mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
I know I'm damned confused now...
I have 5 motors sitting here, and they ALL seem the same EXCEPT that one weird jug.
All the parts interchange fine except that one weird jug.
It is simply too short for ANY of my motors without excessive base gasket spacing
tomorrow Im gonna do some measurements from base to deck on a couple of the cylinders I have and post those measurements here and if you can measure that odd ball jug you have post that measurements here and lets compare numbers and try to figure out what you have.
 

cannonball2

Well-Known Member
Oct 28, 2010
3,682
221
63
Colonial Coast USA.
Sounds like a plan.

Glad to hear the GT5 is running well. That's what I found on my neighbors build, it was a very smooth runner. I later thought maybe it was the frame combo since it was an aluminum cruiser. I rode it several times thinking this is a damn smooth pleasant ride.

The 38mm stroke should be more willing to rev. Im assuming you have the same engine I have with the FM80 rod/crank(I think that's the one, so many #s). I liked the looks of its internals. Hopefully the Dax engines are similar. Its nice to be able to buy from some one that knows what they are selling, even if its a bit more.
 

mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
Sounds like a plan.

Glad to hear the GT5 is running well. That's what I found on my neighbors build, it was a very smooth runner. I later thought maybe it was the frame combo since it was an aluminum cruiser. I rode it several times thinking this is a damn smooth pleasant ride.

The 38mm stroke should be more willing to rev. Im assuming you have the same engine I have with the FM80 rod/crank(I think that's the one, so many #s). I liked the looks of its internals. Hopefully the Dax engines are similar. Its nice to be able to buy from some one that knows what they are selling, even if its a bit more.
yes the one I have running has the FM80 rod, I bought the lower used from a forum member that estimated it may have about 3000 miles on it, the bearings are tight but very smoothe, the crank has zero end play and no signs of seal leakage, so I flushed it out, cleaned it up and ported a GT5 jug I got from an ebay vendor, shaved it about .030 if I remember correctly and bolted her up with a new NT arb that has the jet soldered and rexrilled with a #71 wire gaugebit, stock type muffler with no internals removed, just port matched the header piep flange best I could to the exhaust port and then polished it glassy smooth.

none of my other engines will rev up with a stock pipe like this GT5 does for some reason, the pipe hurts the torque a little but once it gets to revving it will really wind up, next couple of days I plan to gps the top speed on the flat, it site feels like mid to upper 30's and possibly close to 40mph with the 41T on the rear, but at least mid-upper 30's im sure.

I have run it enough yet to know 100% overall how I feel about it, but so far Im pleasantly surprise and impressed with the balance and the rpm potential with even the junky stock pipe.
 

allen standley

Well-Known Member
Oct 22, 2011
1,126
238
63
Bangor, Maine
All great to know! I got a couple bottoms and a box of pistons. Right now doing bicycle prep I'll be referencing all the above info when the time comes to do the engine. I have put a flying horse piston in a Grubee engine before with great success. The engine and bike were completed 3/4 years ago. It's my kid sisters bike which still runs and goes like crazy. I can't recite the science but I had to plane down the cylinder Head and trim the piston skirt.I got pics of this in my album titled flying horse in a grubee. Be awesome if one of you guys would check it out and see what ta heck I did. Please feel free to leave a comment. Thanks you guys!
http://motorbicycling.com/album.php?albumid=808
 
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cannonball2

Well-Known Member
Oct 28, 2010
3,682
221
63
Colonial Coast USA.
Looks like an excellent job of making something work where its not exactly supposed to!
Im a big fan of that. Don't have a clue how the porting would have been changed, but if it runs good must have worked out pretty well.

My only critique is on the modded pistons skit windows, the cut has square non radiused corners(look at the stock pistons). This is a perfect set up to create stress cracks. Always radius the corners. Must be fine though if its still running after 4 years!
 

mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
All great to know! I got a couple bottoms and a box of pistons. Right now doing bicycle prep I'll be referencing all the above info when the time comes to do the engine. I have put a flying horse piston in a Grubee engine before with great success. The engine and bike were completed 3/4 years ago. It's my kid sisters bike which still runs and goes like crazy. I can't recite the science but I had to plane down the cylinder Head and trim the piston skirt.I got pics of this in my album titled flying horse in a grubee. Be awesome if one of you guys would check it out and see what ta heck I did. Please feel free to leave a comment. Thanks you guys!
http://motorbicycling.com/album.php?albumid=808
After looking at the pix, I noticed something right away, the piston you're labeling as a GT5 isnt a GT5 piston, its a pk80 low pin type piston, one reason I know this is because wrist pin locating on the Flying Horse engine and a GT5 is the same, they both use the high pin piston.

so what you actually built was a super short stroke engine, because the lower you made that engine out of is a pk80 geometry that uses the 110mm short rod and thats why when you put the GT5/Flying Horse TypeB piston in it the skirt hit the cranks counter weights at BDC, what you sorta done back then was make another form of a hybrid/ Half Breed engine we seem to be seeing more of lately, difference is that the Half Breed engine use a 38mm stroke crank that has the 40mm stroke 110mm rod, it uses the low pin piston and this makes the piston .062" shy of coming all the way up to correct deck height when piston is at TDC, so it actually needs the deck lowered right out of the box inorder to get compression up to a good level.

What you did is use a lower assembly designed for being a 40mm stroke and you did all those so that you could make a GT5 geometry upper work on a pk80 lower, you could take that same lower, put low pin piston in it and use a standard pk80 jug/cylinder on it and you'll have a standard 40mm stroke pk80 geometry engine.

Not a thing wrong with what you done here, and what I think you may have had originally if im not mistaken was a GT5A type engine which I think was a 40mm stroke engine, I cant be possitive and if Im wrong someone can correct me here, but either way the fact is that you forced a GT5 38mm stroke setup to work on a pk80 40mm stroke lower engine assembly and if it works as good as you say, I see nothing wrong with that at all, what you may have here is a 34mm engine now....

at any rate the fact that you got it all to work and run good is a cool thing to me, Id call that your "Mad Scientist" Geometry engine.....lol!!!

I remember when you did all of this, but back then I wasn't as familiar with the different engines and there geometries as I am now so I didn't realize what it was you had actually accomplished.

One thing you can remember is that if the upper ever gives up the ghost all you'll have to do is drop a standard pk80 type jug and piston on that lower and she'll be ready to go again without having to do all the mods unless you just reall wanted to do the tinkering.

looks like a fun project, at somepoint I might even make me one of them just for the fun of it.
 
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mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
Well I'll be darned! I'm gonna need a couple days to digest this thread. I appreciate you guys! See that's why I do this. Thanks again!
lol.....!!!

these type discussions is why this forum is so great, we all can share and learn together con erning many things, the forum is just as addictive as the bikes we talk about for me, I really enjoy it and I have learned so much from many of the excellent members we have here.
 

mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
Best I can get my GT5 jug measured with it on the engines it is roughly 68MM in height.

I measured it before I shaved the deck but I didnt write it down and now I cant remember what the original height was, I did measure one of my untouched pk80 jugs and it is a tad over 70MM
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
Well I found a motor that it fits properly.
It's a older 40mm stroke engine with the zl rod. As you can see, fits perfectly.

I was not aware there WAS a 40mm stroke crank with bolt on counterweights! I got this engine from bikeberry a few years ago when I ordered a standard chinadoll kit and it was running good until the owner stripped a head stud. I swapped it out to save time and only discovered the stroke after beginning teardown to swap cases.
The GOOD news is that now I have a bottom to use with the ported jug without buying any parts! Simple case swap and it will be up again.

Thanks for all the help mapbike! I would have never discovered the stroker without your input making me curious.
 

cannonball2

Well-Known Member
Oct 28, 2010
3,682
221
63
Colonial Coast USA.
You still see those bolt on weight/ZL rod cranks for sale. The Flying Horse I have is relatively new. It is still 40mm with the ZL rod but the crank/counterweights look like the GT5 crank and its a pretty smooth engine. There is some mixing of parts for sure with these engines. Never sure what you are getting until you got it.
 

mapbike

Well-Known Member
Mar 14, 2010
5,502
109
63
Central Area of Texas
Well I found a motor that it fits properly.
It's a older 40mm stroke engine with the zl rod. As you can see, fits perfectly.

I was not aware there WAS a 40mm stroke crank with bolt on counterweights! I got this engine from bikeberry a few years ago when I ordered a standard chinadoll kit and it was running good until the owner stripped a head stud. I swapped it out to save time and only discovered the stroke after beginning teardown to swap cases.
The GOOD news is that now I have a bottom to use with the ported jug without buying any parts! Simple case swap and it will be up again.

Thanks for all the help mapbike! I would have never discovered the stroker without your input making me curious.
good deal, thats great....

I have two 40mm stroke engine with the ZL rod and both have the crank with bolt on weights, mine came from BGF back in 2010, one of them is in need of repair and the other one is running good on my second build, only thing about the one thats still going is that it vibrates like a mad man above 32mphwith a 36T one the wheel, its always been a very torques engine, pulls hard even with a 36T but kinda peters out at some point due to the really bad balanced crank.

well, maniac Im just happy to hear you found a lower for that cylinder, hope it makes a great running engine for you sir.
 

exokinetic

New Member
Mar 18, 2016
108
4
0
Lake Forest, CA
I wanted to try to clear up some engine geometry confusion.

Connecting rod length has no effect on stroke length.

It does however effect where in the cylinder that stroke happens. A longer rod moving the whole 38 or 40mm's of stroke up in the cylinder. This means the piston still moves the same distance (38mm, or 40mm) with a longer rod, but at TDC the piston is higher (relatively) in the cylinder (witch effects compression). It also will not go as low (relatively) in the cylinder at BDC (witch effects port-time-area).

Same deal for the piston wrist pin location, high or low.

The ONLY part of the geometry that effects stroke length is the connecting rod pin location in the crank.

As far as I know there are two possible pin locations in all of the cranks out there. One results in 38mm stroke, the other results in 40mm stroke.