Grubee GT5B - A Pursuit of Reliablility

weefek

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(Warning probably will be kind of a long post. Rebuilding a GT5B engine for reliability).

My new bearings have finally arrived so it's time to get some work done. Still waiting for the main seals which will be here next week.

I originally bought a GT5B kit back at the beginning of May and threw it together because I needed a way to get into town. Didn't do anything primarily aside from taking off the head and jug to make sure nothing looked amiss. It didn't, and the original engine now has 278km (173 miles) on it. It's getting progressively louder (clanky) but I have a feeling it's the clutch causing these noises (clutch is addressed at the end of the post).

My goal here is reliability. I bought another GT5B kit so I could pull it entirely apart and rebuild it properly from the ground up, swap it onto my bike, and then do the same to the original motor. Not looking for performance, just reliability.

I used Al Fisherman's guide for tearing the motor down (https://motoredbikes.com/media/54-remove-clutch-drive-gear-nut.39510/) with a few minor differences, so I won't be going over that. The engine is now completely disassembled, minus the rod from the crank because there isn't a need to do so.

Finally got my replacement bearings (SKF 6202 2RSH C3) thanks to @ImpulseRocket89 for the recommendations on which exact bearings to buy.

1.jpg


That being said, time for some work to get done. This thread will be a journal of sorts for this job and hopefully will help somebody in the future.

First, the case halves. Completely taken apart. Took a dremel to any excess flashing left behind. These engines are decent build quality but definitely not perfect. Original bearings and engine had random metal shavings in random places, the bearings felt gritty, and generally bothered my OCD.

Took a small wire wheel to internal surfaces, and mating surfaces. Used 600 grit wet paper on the bearing mounting surfaces and the seal mounting surfaces just to clean them up.

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5.jpg


Brake cleaned and wiped out. I'm going to wait to install the bearings until the main seals arrive.

Next, head work. I already cleaned up the new head, but needed to install it on my existing bike engine because the head gasket wasn't sealing very well. So I took the original head and did the same thing.

Wire wheeled the combustion chamber, good enough for me. I have no reason or need to spend time polishing the crap out of it.

To me the main thing is making sure the mating surfaces are flat. Used the same 600 wet with a pane of glass that I used to use for CPU heatsinks back in the day.

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About 20% of the way there. You can see the pitting from casting.

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The key to this is to rotate 90 degrees whenever the 'lines' from the previous sanding are gone. Around 20 seconds or so for each 90 degree turn.

Good enough:
8.jpg



Then, I did the same to the jug. Head gasket mating surface, and exhaust and intake mating surfaces.

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I highly doubt I took enough material off to make any difference with regards to timing / compression ratio. I literally just made sure the surfaces were flat, and then stopped.
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Still need to take a bit more off the intake, you can see bottom right there's still a low spot.

I also lapped the "intake manifold" which was a lot flatter than the port on the head was. I also took a dremel to the inside of the "intake manifold" to clear out some flashing left over from casting. I fear some of the small pieces of casting flash can and will flake off eventually , leading to small metal pieces being introduced to the engine eventually / over time.
10a.jpg
10b.jpg



With regards to the piston, I removed the rings. Checked the retainer pins to make sure they're tight. There was a lot of manufacturing garbage in the ring lands so I'm glad I removed them and took the time to clean the piston up. I deburred / sanded the wrist pin holes and transfer port slots with the 600 wet sand paper. There was a pretty good edge on them. Now they're nice and smooth. Still have to do a final cleaning on the piston and rings before install.
10c.jpg



Next, the jug. The port openings weren't terrible, but there was extra flashing. I removed the excess material. I DID NOT open the ports up horizontally or vertically, just removed the excess material. First with a dremel bit, and second with the 600 grit wet paper. They're nice a smooth now. I still need to polish them up a little bit with the scotbrite stuffed in the drill chuck trick, but that will probably happen tomorrow.

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The transfer ports don't look the best, but the chamfers from the factory are all in and not out, so I just lightly sanded the edges w/ the 600 wet, removed any excess flashing, and called it a day.

I'm also rebuilding the clutch. The stock clutches on these are sloppy as hell. At least 2mm of play in the stock bearings. I took the clutch apart, and am waiting on the 1/8" G25 stainless bearings I ordered from ebay to arrive before I put it back together using bearing grease. I don't know if it's apparent in the pictures but there was a decent amount of surface rust on the parts. I have a feeling these are leftover parts from the original GT5A runs back in 2010ish. They look like they've been sitting on a shelf for awhile. The gear teeth themselves also had some surface rust, so I took a small dremel wire wheel to all the surfaces, then lightly sanded w/600 wet the surfaces that the bearing balls sit on.

I bought these red pads from amazon but I don't think I'm going to use them. The width / height is so inconsistent and they don't really fit the openings properly so I think I will clean up the original clutch pads and throw them back in.

12.jpg


The next part of this story, once the main seals arrive, will be reassembly of the case halves. I will update when that process occurs.
 

ImpulseRocket89

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Nice start to the thread and good approach. This is how I approach my new motors because I have learned not to trust the quality control lol.

Pertaining to the chamber polishing. The reason I do this, and others do this is 2 fold. The more polished the surface the less inclined carbon will stick to it which reduces buildup, but a highly polished surface also has a higher thermal emissivity (Reflects and radiates heat much better) which helps improve thermal efficiency and reduce heat transfer from combustion to the head. If it makes a huge difference in the latter is likely arguable as not being worth the effort, but it's still a thing. That doesn't mean carbon can't build up, because that is also very dependent on how rich of a tune is being run, but long term on a well tuned motor you can see the difference.

Another little thing I find worth doing is to open up the oil holes for the wrist pin on the underside of the piston, and put a bevel to the edge. This improves oiling to the wrist pin surfaces on the piston. They don't need to be huge, just a touch bigger.

Now is a good time to rough up the intake port and the manifold as well. 60 or 80 grit sandpaper or a light touch with a double cut burr is just about perfect. You don't need to remove much material, just rough it up. The boundry layer effect from doing so really makes a difference in both cold start/run as well as idle quality, power, and responsiveness. This also applies to the surfaces for the transfers.

Setting a good squish gap is also worth taking the time. My favorite range for a typical engine is around .8-1mm on anything that isn't trying to be a race engine.

While I know you said you aren't chasing performance, none of these upgrades are really "high performance" upgrades as they are about improving efficiency. They all make small improvements that collectively can take an "ok" engine and turn it into a great one with minimal effort.

Anyway, that's my few cents. I look forward to following this build process.
 

weefek

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Well, the 1/8" bearing balls are not a direct replacement on these clutches. 3.02mm (original) vs 3.18mm , they BARELY fit, if you force them in. Going to mill out the center by .10mm at my machine shop next week and they should be good.
 

weefek

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Nice start to the thread and good approach. This is how I approach my new motors because I have learned not to trust the quality control lol.

Pertaining to the chamber polishing. The reason I do this, and others do this is 2 fold. The more polished the surface the less inclined carbon will stick to it which reduces buildup, but a highly polished surface also has a higher thermal emissivity (Reflects and radiates heat much better) which helps improve thermal efficiency and reduce heat transfer from combustion to the head. If it makes a huge difference in the latter is likely arguable as not being worth the effort, but it's still a thing. That doesn't mean carbon can't build up, because that is also very dependent on how rich of a tune is being run, but long term on a well tuned motor you can see the difference.

Another little thing I find worth doing is to open up the oil holes for the wrist pin on the underside of the piston, and put a bevel to the edge. This improves oiling to the wrist pin surfaces on the piston. They don't need to be huge, just a touch bigger.

Now is a good time to rough up the intake port and the manifold as well. 60 or 80 grit sandpaper or a light touch with a double cut burr is just about perfect. You don't need to remove much material, just rough it up. The boundry layer effect from doing so really makes a difference in both cold start/run as well as idle quality, power, and responsiveness. This also applies to the surfaces for the transfers.

Setting a good squish gap is also worth taking the time. My favorite range for a typical engine is around .8-1mm on anything that isn't trying to be a race engine.

While I know you said you aren't chasing performance, none of these upgrades are really "high performance" upgrades as they are about improving efficiency. They all make small improvements that collectively can take an "ok" engine and turn it into a great one with minimal effort.

Anyway, that's my few cents. I look forward to following this build process.

Good point on the chamber polishing. I agree with what you said, however I don't have the proper polishing compound and am not going to spend on the money on it. Like I said this is a reliability build, and on my stock untouched engine I haven't had any temperature issues. I did have metal polishing compound at some point in my life but it has removed itself from my life, I have no idea where it went.

As for the oil holes on the piston wrist pin. I will do this. Thank you for the tip.

I was under the impression that roughing up the port was beneficial for intake but not exhaust. I have done some for intake so far but I will dremel / rough it up a bit more. I was never going to polish the intake port, only the exhaust.

Thank you for the recommendations , definitely going to take these into account. With regards to the squish gap I am looking into it. These came stock with 2 jug to case gaskets. Same with my stock motor.

Just trying to make this thing last as long as possible. Efficiency isn't even really that much of a concern, as the MPG is ridiculously high whether or not the engine gets touched or not. For example I do a 12~ish mile run into town and back every other day and I barely have to add any gas to the tank. I haven't done the math but I know it's pretty good.
 

Chainlube

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Any upgrades you do are only going to add to the longevity and efficiency of the engine. The most important thing is a balanced crank, cuts down on vibrations which can lead to all kinds of failures.
 

weefek

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Any upgrades you do are only going to add to the longevity and efficiency of the engine. The most important thing is a balanced crank, cuts down on vibrations which can lead to all kinds of failures.
That's 100% the point. If i could balance my crank I would but I have no idea how to do it properly.
 

DAMIEN1307

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That's 100% the point. If i could balance my crank I would but I have no idea how to do it properly.
Any upgrades you do are only going to add to the longevity and efficiency of the engine. The most important thing is a balanced crank, cuts down on vibrations which can lead to all kinds of failures.
The Zeda 80s cranks on the Zeda motors, like mine, seem to come fairly well balanced right OOTB...Next to zero vibration through the whole throttle range.
 

weefek

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The Zeda 80s cranks on the Zeda motors, like mine, seem to come fairly well balanced right OOTB...Next to zero vibration through the whole throttle range.
My first grubee crank did too which is why I'm not so worried about it. If i had the means to balance it properly I would.
 

ImpulseRocket89

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I was under the impression that roughing up the port was beneficial for intake but not exhaust. I have done some for intake so far but I will dremel / rough it up a bit more. I was never going to polish the intake port, only the exhaust.
I never mentioned roughing up the exhaust port. I said intake and transfers. ;)
 

weefek

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Finally finished rebuilding the clutch with 1/8" bearings. I had to seriously open up the inner diameter on the clutch basket (58.10 to 58.40mm roughly) compared to 58mm stock. From what I've seen most clutches accept these bearings without modifications, but the grubee clutches are not the same. Rigged up a broken clutch shaft to fit in a drill so I could use the drill to sand the surfaces.

There was a lot of left / right slop in the original clutch. After rebuilding there is very minimal play left/right but a little front/back which i guess makes sense as it needs to disengage / engage.
 

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