Grubee GT5B - A Pursuit of Reliablility

Put the bottom end together with the new SKS bearings and seals.

The seals I ordered from Amazon for like 10$ a set were not very good quality at all. I ended up ordering some OEM seals from motorizedbycicle.ca (where I got my engine kits) and they are the same ones that came on the engine, but they look a lot better than the originals, and we're like $1 or $2 a piece. I will take pictures of both and post tomorrow. Regardless I used the "OEM" replacement ones instead of the random ones I ordered off amazon (they looked terrible).

Still looking for a better replacement for stock seals, I've got a few leads to a few sites and I think I may try them in the future, although they aren't the 100% oem sizes (if they can be trusted).

Lubed EVERYTHING up with slurry mix of 2 stroke and vaseline because I don't know how long this engine is going to sit before I end up installing it.

Ended up having issues with the clutch bearings / clutch shaft seating properly so I removed it from one side for the time being. Sanding down surfaces (15.02 to 14.98) (haven't measured outer bearing surfaces yet) After mating both case surfaces one of the clutch bearings got pushed slightly out on one side which was causing binding. Not playing around with that. Once I get the tolerances a little better it'll go back together (probabably this weekend).

The paper towel is just to stop the rod from clanking around.
 

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I'll upload pictures tomorrow (nothing special or revealing) but the engine is 100% put together at this point. All the bearings and actions feel great. I have the 2nd rebuilt clutch ready to put on the new engine but haven't installed it yet. Everything else is put together.

One thing I want to do before I use the engine is fix the squish gap. I haven't seen very many comments, if any, regarding the Grubee GT5B squish gap on a stock motor, but both of mine are around 2.6~mm which is a bit much for me. I'll be re-decking the jug at some point before I install it. Low and mid end torque aren't the greatest from my experience, and this may be partially why.

I ended up using the replacement seals I ordered from motorizedbicycle.ca which are basically the same as stock but they looked a lot better than the crappy ones I found on amazon. There are still some alternatives mentioned previously in this thread that I have not tried (It was something like $10 a seal vs $0.99). The "stock" seals have worked fine for me on the 'throw it on the bike and go' motor, so hopefully these hold up the same way. Regardless I bought 5 each of the thick vs thin seals vs 1 aftermarket/other brand seal so I'm ok with it.

I used the 2-stroke and vaseline mix on all the parts of the engine liberally because I'm not entirely sure when I'm going to install it. The engine I have currently is around 600+ miles and is making strange noises (like piston slap) above 7000rpm so I have no idea how long it will last. Have no idea what's making the noise.

When I install the new engine on the bike I'm going to pull apart and rebuild the original. I already have a spare head and piston / bearings / seals ready to go.

One thing I will say. I've noticed on the engine on my bike that the bearings with the metal covers that come stock with these leak out their own bearing grease slowly over time (specifically the ones on the clutch shaft of course). This means that small particulates can also get into the bearings, which I have no doubt is happening.

I should be getting my driver's license back in a few weeks so I think once that happens I'm going to pull the "old" engine and put this one on the bike and see what happens. The only thing stopping me from doing that at the moment is I use my current motor/bike for essential purposes (I live 10ish minutes from town and cannot afford a Taxi every time I need to buy something) and it's proved itself reliable to me at this point.

Basically the only thing I didn't do to this engine is balance the crank, which I'm considering pulling it back apart to do. But I think I may just do it on the 'old' engine once I rebuilt it. I haven't quite figured out how I want to properly balance it. I've watched a lot of videos on that and have read a few threads but still haven't quite figured out how I want to go about it. I'm kind of anal with that kind of thing.
 
Pics or it didn't happen ...
🤣
I balanced mine with the half inch 15mm method, and it made a world of difference. Not perfect, but 80% better. I like the way the guy from Build Brake Fix did his. Seems like it was about as balanced as one could get it without a computer. I was impressed. Gonna try that next time. Any way you do it is going to be an improvement.
 
Here's a simple method I used. Bike revs up high, and fast really smoothly. Vibration is nil compared to stock, and actually feels like it runs better.

View: https://youtu.be/0Jm0JKCZJ0c
This here is the best way I've seen so far. Longer video, but absolutely worth it. Trying this myself next round.

View: https://youtu.be/KDCdSi4Zhwk
Here he goes more in depth on the different crank and rod types/weights. Breaks down the weights of everything like a good cheat sheet.

View: https://youtu.be/c2PImNzCufo
 
Yeah I think I'm going to pull the crank back out and do this. I'm going to wait until I get my actual driver's license back, just in case. My current engine has been making a strange noise lately that I don't like and want a backup available.
 
(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.

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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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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:
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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.
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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.
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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.

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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.
Get the green clutch pads on Amazon, they're about the best you can get right now I believe....
 
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