Grubbe GT5 from the case up ....

EDIT !!!!

i have decided to build this as a super stock motor !!! useing only stock parts that have been moded to get the maximum performance possable !

i am and have been posting videos of the build since the beginning so all can see what i have done starting with the case halves ! from stuffing the bare case halves and porting the transfers all the way up to the bikes first ride i will continue to post videos of my work with explanations of what i have done and what i am doing the whole way !!!

ok guys i am looking for your help here, i am building a china girl from the case up and i am looking for any and all advise on bottom end mods and case stuffing.

so far i have tore the motor down to the bare case and punched out the bearings.
i then ported the transfer case ports to flow smooth out of the case and will port match them to the jug when it comes in
i also stuffed te case around the bearing boses with jb weld and ground it down for crank clearance and better flow in the case.

any and all advice is welcome here, i am looking for the perfect case stuff ratio what should it bee ? how much material do you add to get the perfect stuff ?

also wat would the best crank, con rod and piston combo be on a gt5 motor ?

any way here is a video of the work so far and i am in the process of up loading pics so you can see whats goining on as i build it and offfer advice or scrutany as i go ...


https://www.youtube.com/watch?v=y_4_YBKBqWw

pics of the stuffing and transfer port work to come soon

what i want to know is.... are there self aligning bearings available for this motor ? and if not what are the BEST bearing to use ?
also how much material should i leave around the bosses for a good stuff ?
and can i use the grubee gt5 crank with a dax (longer con rod) con rod to bring the cc's up and give it a longer stroke ? this would mean useing a dax top end also right ?
or should i use the gt5 crank con rod and a gt jug for the build ?

this is going to be the LAST china girl i ever build as i already have a honda gx160 and a honda atc 70 motor ready to be rebuilt and looking at the cost of tottaly rebuilding a china girl for max performance i figure for that cost i could just mod a preddy 212 and use it lol so i am no longer going to be dumping money money into china girls when it could be better spent on a real motor that will be more reliable and have way more power for about the same cost !

i just want this last build to be a ripper and the best one yet !

also what do you guys think about chucking the crank up in my lathe and facing off the sides and outer edge so its perfectly round and true to the shaft ? would this get rid of a lot of the hgh rpm vibes and lighten it up a bit ?

well it was a good runing GT5 but it was my experiment motor... i would experiment with porting and port maps with this motor before transfering the port onto the build motor to get a good idea of how the port would run...

the crank is pretty darn balanced for a grubee and there is not to mutch vibration untill around 9,000 rpms and then you begin to feel it so i might just go with this crank and get a new con rod piston and jug to match...

what i want to do iss smoooth out the crank case interior and port the case transfers to flow up into the jug better and port match the case to the new jug, also i plan on making this a relitivly high rpm motor with a midhigh to high power band so i stuffed the case around the bearing bosses

now that i have a cleaned up port flowed and stufffed case and a good crank i am looking for a set of verry good bearings and a con rod, piston and jug combo i can use to get the most out of this engine... i plan on using stihl chainsaw or moped bearings with less endplay on the piston pin and running a lighter piston pin made of titainium, also wil be lightening the piston and ramping it.

if you know of a set of good bearings for
the case
the clutch shaft
the big end of the rod
the wrist pin

let me know what they are

and if you have any ideas for some case mods or crank, con rod, piston, and jug combos that will work the best also let me know

the bearings that came from the case are loos and the seals are horrible also the big end needle bearing is crunchy and has a ton of side to side and free play !

my idea is to use the crank but run a dax con rod and new upgraded rod bearing and a piston and jug from a dax top end to make the grubee gt5 a true 70 cc 47mm x 40mm bare and stroke if i cant do this i will just use the grubee gt5 crank and rod but get a new grubee piston and rings and a grubee gt5 jug

any ideas on building this thing up to its max would be helpfull even port map deg and numbers or sugested comp ratios and head design would help me out

ohh and i usually runn a strait pipe with a 3/4 in ID about 20 in long for max top end power but this motor is going to be a super high midrange top end monster so i think i might have to go with that wobbly shapped fatty pipe the arrow bikes use or mabey a shortend sbp pipe sugestions would be great

my riding style is pedal up to speed and dump the clutch then bring her up to riding rpm as fast as i can and ride it at a mid to high rpm the whole time untill i have to stop then repeat the proccess there are long winding roads here and hardly any traffic so long wide open fast runs is what i am building it for...

Davezilla said:

The best bearings you can get are the SKF brand part number 6202-2RS-C3, Even if you go with another good brand, the C3 tolerance will be needed to compensate for the thermal expansion as these bearings need to run in a hot environment so they will expand to the right size tolerance when the engine warms up. The tighter tolerance bearings will become tighter as the engine heats up. The case and clutch shaft both use the same bearing, but you need to remove the seals on the case bearings and leave the seals in place on the clutch shaft bearings.
A self aligning bearing would be nice but as much as these cranks can shift and move on the crank pin (they're usually not pressed in as tight as one might think) it could cause the pin to wear prematurely unless you weld the pin into the crank. Even with a welded crank pin, I'd use a self aligning bearing only on one side, if you can find one with a 35mm OD and 15mm ID.

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i just might go with a single self aligning bearing on the crank to acount for any misalignment in the case casting and machine work i mean damn man those things are nice as heck when building a preddy or a honda gx !!!!

i understand the thermal expantion and the heat rating and i guess i will go with the sks bearings you mentioned thats a good co. but i will see if i can run one self aligning on the other side and see how that goes with this motor, if i get a wobble or it seems not right i will just go with all sks.

any ideas on a crank conrod and piston and jug combo ? or should i just go grubee gt5 all the way and play it safe with a strait up 66cc rebuild and not opt for the 70cc top end and con rod ?

i took a bunch of pics and they are too big lol so i am going to have to take some more tomorow at the right size to make it easy for me to load them and not have to fool around resizing them and crap, any way its all stuffed and ground down for clearance and port flowed and ready for a jug ...

i am ordering a jug conrod and piston along with the bearingss tomorrow so all advice is welcome

To put in perspective what i want from this build .... I have a dax gen iv i left the case alone and did a lot of top end work and some heavy top end porting cause like i said i get up to high rpms and leave her there for quite a wile so a top end bike with a,lil bit of a meaty mid is what i am after ... Anyway the current dax build i am running is linked in my sig and that is an aluminum frame cruiser with mtb suspention and mtb 7sped shifter on a lever runnong a puch high comp head and dexked jug with an ultra thin head gasket and a thicker base gasket with a large bore intake and a 16mm carb....

The bike will do 35mph all day on a 44t rear and burst up to 40 for a bit but i wont hold her up that high for to long... She has no low end but once up high in the rpm range there is no hill that can stop her as long as i hit the hil in the upper rpm range... Srs she wont even bat an eyelash as she flys up the steepest hill as long as i am up in rpms ....

Now i want another bike like this but with more midrange and about the same top end , i do not care about down low as i never ever drive that slow lol she will need a bit more mid cause i plan on running a 38 or 40 t rear instead of a 44 this time i want to make 45mph on this new build so i am going from the case up everything new .

I made it to 40 on the dax build woth no case mods and the first port map i came up with for high end power high rpms and used a large 44 t rear to gain back power and to have more power when up in the band ... I want the same setup only better and with a smaller rear

One of the reasons that dax builds hits 40 so easy and has so mutch power when its up there is i ran a strait pipe on it and tunned the length for top end but in doing so robbed the low end to add to the top ... On this build i have to come up with a pipe that scavenges and adds to the midrange but will not afect the top end speed i dont care if it robs low end

The port map will be like so

Verry large intake port and durations something like 110 deg ... My dax is like this and low end suffers and idle suffers but good god once ur up in rpms it has unholly power for a small motor !!! I plan to raise the jug with a .5mm thicker gasket and lower the intake port as much as possable and widen it as much as possable then cut the skirt and ramp the piston

The exhaust will be a lil differant in prder to maintain useable cilynder compression space so i will only round out the port roof a lil and round out the floor a lil but i will widen it as far as it wil go and hope that the roof of the port is not raised verry much so i can maintain more dynamic compression and midrange power

The transfers will be raised up .5mm by the jug lift so all they will need is a good clean ip and to be aimed at the center of the jug .

A high cpmp head will be used prob a puch 70cc hi hi or that diamond head from jnm motors i am looking for around 12:1 static comp and a squish band of course i will deck the jug for proper deck hight and squish clearance too

The carb will be a 18 mm mikuni or phgb delloto with a power plenum intake or hog snout

And the exhaust will be that squiggly one that is on all the arrow racers ....

Wow jnm motors selles a top end kit just like what i am after but for 400$ lol i might as well do it all myself ,!!! Now they sell a third transfer port jug with this kit and,its a nom reed kit ? So how well would a third transfer work out on a stuffed case piston port motor ? Would the tird port lower,velosity to mutch ? Wouldnt 2 large ports and a stuffed case have highr transfer velosity than a stuffed case with 3 transfer ports ?

I wonder if it would be worth chucking a new jug up in the bridgeport and using a round nose carbide miller milling out a tird transfer port in my new jug .....

here are some pics i could get loaded let me know what you think how does the case stuff look ? is it too much ? do you think the case preasure will be too high ? i only used half a tube of jb weld for bolth sides and ground it down to clear the crank and matches up with thw gausets around the bearing race

Davezilla said:

$400 for a top end kit on a China Girl? Ouch!! For that kind of money ya might as well spend just a little more and get a KTM clone and have about double the power that top end kit will make... Even a Puch Athena kit or the Hebo Manston replica kit on a China Girl bottom end will get you some serious power for about half that price. If ya got the skill to assemble and install that $400 kit, set the right squish & port timing to make it work, it's not That hard to make one of these other jugs work for a lot less $$...

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No no no lol lol they sell a kit all pre prepped and ported and all that for 400$ roflao....
For that price i would just dup the money i.to my gx160 and smoke everything with pedals !

I would rather just buy a stock piston jug and puch head and dech and grind and trim it all down myself !!!! For 1/10th the cost !!!

I definatly have the skill lol engine building has been my hoby for 20 years ! I am,32 lol

And holly crap that plate idea is great for extra stuffing but i thought of filling the crank pin with jb weld and also filling the drilled crank holes with jb weld i mean that is in the case too and would displace even more volume right ? And i dont think it would add that mutch lb that it would through off the crank balance noticablt