GT5 small port cylinder pics

Seems that these are coming over on multiple engines now, I don't know the reason for making the ports so small in these other than intensionally trying to lower performance and rpm capability for longevity boost possibly or for trying to get better compliant to EPA standards or something...Shrug...who knows for sure.


I have two of these now and kinda unsure what I should do with them.

Transfers tiny

Exhaust port very small and then opens up like a funnel as portflows out to flange area.

Tiny intake port, narrow and short.

Casting on jug is fairly clean and straight looking but the ports are just horribly small in my opinion.

seems these are the jugs coming with the dax GT5 and the half Breed engine I got out of Hong Kong a while back.

Not hating on the jug and I appreciate very much that Duane sent it to me, just curious about whay all the sudden some of the other small port jugs are looking like large port jugs now compared to these.

cannonball2 said:

Thanks for taking the time to do the show and tell on the jug and the Dax lower in the other thread. Its nice to see whats going on with the CGs as of late. If Im not mistaken the rod in the Dax lower is the missing rod in Freds vid, right. If so then it should be a 38mm stroke long rod bottom end and not a half breed?

One way to look at the current engines showing up is they are not made for tuners. Im theorizing that these are perhaps the latest generation and maybe one of the best yet to be used as a bicycle engine. The porting and sizing of the ports should yield a nice running engine with good bottom to mid range and an acceptable top for general bicycle use. The cranks are certainly improving in engineering and balance, and there are some better looking magnetos showing up with some of these engines. From the manufacturers point of view, a smooth reliable torquey engine to power bicycles for transportation is probably the goal. There are probably many other markets in which making it from point A to b safely and reliably trumps our hobby tuning. Im finally liking what Im seeing when I open one of the newer engines.

Didn't mean to editorialize, Im just thinking these could be pretty good engines if they run like my neighbors did right out of the box. I got one coming and I will find out soon. Short of checking the head bolt/nut/torque scenario Im going to run it as is to see if its the out of the box runner I hope it is.

Now it time for you experienced tuners to get the most out of em!

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Yes these newer dax engines are true GT5 high piston pin engines, Duane told me he's running one of them right now on a bike with a 36T rear sprocket and a Puch head (no head gasket) a d he's getting 40mph top speed.

not about the rezt of his setup

yeah lots of work will make them better, personally as long as i can get jugs with close to ideal size ports from people who are selling them on ebay and else where I'll buy good lowers from dax and others and just build the engines up that way for myself.

builds for sale dont concern me much as long as the engines are of reasonable quality and balance.

im gonna try one of these jugs, just to see how the engine will perform with minimal port work, just cleaned up but no making them any bigger,im thinking it will be a dog and be very rpm limited but i want to know how much and I will shave the deck a little to get piston where it needs to be at TDC.

maniac57 said:

The single biggest difference I have seen in bone stock untouched chinadolls, is the ones coming across with marginal timing. I ALWAYS look at the magnet keyway when inspecting a new doll!
There have been several motors come across my bench that were simply dogs from day one. A simple magnet change and they woke right up.

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yes that is a consideration, I have heard Fred speak of that on one of his videos also.

which type is best maniac?

look closely and you'll see the intake port is small and oval shaped, that what the third pic in my post was included for, I havent removed the intake spout yet.

The exhaust port is also a small oval shaped port inside at cylinders edge, it gets larger toward the outside edge of it, but its a small outlet.

frank66 said:

what rod does it have?. the gt5 i see lately have a round inlet at the manifold not a rectangle.

pictures do not show the inlet just the intake port. look under the manifold...... how much cash was that?

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rod looks like rod in older 40mm stroke GenIV engine nice bronze color and olier holes drilled.

I took some pix of intake port, Ill post here when I an load off my phone and onto my PC, the exterior portion of the port is semi round/oval, its not rectangular like some of them are, Im not sure it the ideal height jug for the GT5 either, I used calipers to measure deck to base on it and its only about .010 less in height than my pk80 jugs, it would be fine with the deck height brought down as for as getting compression up is concerned but I still dont are for the extra small ports.

I put it up beside the other small port jug I have that ame on the Halfbreed engine I got recently and its identical to the point its obvious it came from the exact mold, the one came from dax and the other from a kit I ordered directly out of hong kong.

If the jug Duane sent me came on a GT5 type engine then Im gonna say these are being used on the GT5 and some of the Half Breed engines also.

Ok Here is a look at the intake port on the jug Dax sent me which is on the right and the jug on the left is one I have had a while that came from an ebay vendor, not 100% for sure who it was from actually, but anyway here is a comparison that shows intake port shape and size.

I have two of the jugs, the one Duane/dax sent me and another which looks Identical that came on the Half Breed engine I ordered out of Hong Kong, casting looks to me like they both came from the same mold, same unique markings on both jugs.

Personally, I dont thing the smaller intake port will have much performance difference than the jug with the larger rectangular port, the port is still much larger than the intakes we typically use to link the carb to the jug, the intake itself will create more restriction than the smaller intake port in jug.

The thing that I am more concerned about with these small port jugs is the transfers and the exhaust port being so much smaller, that is what is gonna limit RPM potential because you cant get as much air/fuel mix into the cylinder and you cant get it out as quickly or as much crammed back in when using a correctly tuned expansion pipe, the intake port isn't gonna create an issue I dont think unless a person is using a large carb with a larger that normal intake.

frank66 said:

thats not bad. im seeing gt5 w small 3/4" round inlets at the manifold. would work great with the round cns manifolds. there are gt5 with rectangle inlets aswell. when dealing w grubees there are many build choices to make and not all of them are clear.

i never tried a pk80. but im looking forward to hear of your maiden voyage results.

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I might get a chance to put one of the dax GT5 lowers together this weekend if the honey do's dont stop me, I've got my Puch 70cc Hi Hi head cleaned up a bit since that is what I plan th put on the engine, I will need to clean the ports up on a jug, I cant decide on which one I want to use on the engine, I have some really nice jugs with the larger rectangular transfers and medium size exhaust and intake ports that are both a nice oval shape, Im seriously thinking of shaving the deck on one of them for it or posssibly using the one jug I have that uses the 40mm spread intake bolt pattern since it has nice size intake and exhaust ports and fairly good transfers also, they're the cat eye shaped ones and position a little lower in the cylinder than the ojugs that have the larger rectangular transfers, honestly, Im not sure which will give best performance, but I think the rectangular port jugs are gonna be best for higher rpm potential.

I'll be reporting what I get done here and which jug I decide to use, main thing is I just hope the new dax lowers have a good balance to them like the original GenIV 40mm stroke lower have.

Im not looking to set speed records, I just want my personal bike to be capable of 40mph and run comfortably smooth doing it, so I can have good comfortable low to mid 30's cruise speeds with the ability to hit low 40's on the flat when I need to.

Duane told me he's hit 40mph on his bike with one of these newer GT5 lowers, he's running the Puch head with no gasket and just one of the small banana expansion pipes and I think the RT carb and a 36T sprocket, Im not sure what jug it has on it, but I assume he at least cleaned the ports up on it, he said the balance of these 38mm stroke engines is as good as the others he sold before so that is very good to hear since the others he sold were the best balance of all the engines I've had.

I'll be posting what I get done.

cannonball2 said:

Heres another GT5 jug. Notice the different casting around the exhaust port. Also the transfer ports. Theres a nice pit below the intake port(holds extra oil-lol).
This if off my $60 engine. I just couldn't leave it alone and run it right out of the box. Its a fine threaded engine(cylinder studs) with the FM80 rod/crank. Has the new style magneto, the wider gear cover and the lower compression slant plug head which is missing a few of the smaller fins thanks to no packing material and Fedex treating it like a football..

Will trim the flashing from all the ports, but wont match the transfers as I don't want all the metal in the crankcase. Basically want to see how it runs stock(more or less).

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Best wishes on the engine, sure hope that nikasil plating doesn't start chipping off where that pitted spot is, if it doesn't then you should be just fine with it.

curious to hear how it turns out, $60 for an engine is a good deal if it turns out to be a good smooth runner.