Top end is normally piston, rings, wrist pin, wrist pin bearing and carburetor. Bottom end includes main bearings, crank seals and possibly rebuilding of the crankshaft assembly.
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Oh ok thank you! So when doing a rebuild I should look to replace those parts (like the pistons and carb) or at least clean them if they’re still in good condition? Sorry for bombarding you with questions xD
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You probably should try to find a mechanic to teach you this stuff if you are gonna do your own rebuilds. It’s probably not that hard, once you know what to do. I think they basically sell what you need for top end as a kit. I am not an engine smart guy, though. I just break them.
Never had to replace a carb. Usually take it apart, clean it thoroughly and replace the gaskets, seals and diaphragms (pumper carbs). Top end, parts get replaced and cylinder honed. Bottom end, bearings and seals get replaced with crank checked and only replaced if needed. Cranks are tricky as they are two pieces press fit together at the rod. Takes some specialty tools to do well and not tear up things and get them back together true. They make special jigs for this.
You could probably do the top end by yourself, but might suggest you send out the bottom end when it comes time. The guy I used for my last top end charged less that $300 for parts and labor. As it was a side job for him, it took about 2 weeks to get it back, but that didn’t matter as there was plenty of time between races to complete the work and break-in the new bits before I raced again. He also checked the bottom end for play and what not, saying it was good to go until the next top end service or more. Some builders will also break in the new parts on a dyno and set up the carb for best power. Of course this adds to the fee.
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This clears things up A TON. I can do the top end definitely, I just gotta find someone who can do a bottom end rebuild for a decent cost then.
Yeah I remember typing carb rebuild and I usually find how to clean the carb not replace it, similar to what you’re saying. Makes more sense now. How often did you do a top end rebuild and bottom end rebuild?
Mind you doing the top end will include honing the cylinder with a proper mandrel style hone and measuring it with a dial bore indicator that measures in .0001. You may or may not also need a wrist pin remover and a proper circlip installing tool. There’s more to it than just replacing parts.
Greg is right (no pun intended). Personally I could not justify the expense of buying all the extra tools to do the job right, which is why I had a someone else do it. If I had multiple motors and were running more often, I could see myself making that investment. Maybe down the road if my daughter takes up the sport, but for now the expense of the extra tools is more than the expense of having someone else do it.
As for bottom end, it depends. Generally I would think every 2 or 3 top end rebuilds. It depends on how often you are rebuilding top and how things are wearing. Checking the play in the crank bearings and connecting rod will determine when it is time.
Thats good that you guys brought that up. By the time I switch to x30 or rok, I’m planning on running probably a total of 10-15 races in a year, plus practice. Personally, I kind of wanted to rebuild at least the top end myself since I wanted to kind of get my hands dirty, but I’m planning on sending the bottom end to shop. Besides spare parts and tool kit (wrenches and bolts), what other tools do i need to rebuild top end?
In my above post I described that the cylinder will need to be honed and sized for a replacement piston, a wrist pin remover, a circlip installing tool (can be done without but it’s a pain), a torque wrench.
Ok thanks, and you recommend for mid season top end just replace the pistons and then do the rest of the top end at the end of the season along with the bottom end unless I really have to replace a top end part midseason?
Not exactly, at mid season do the top end completely, that includes honing the cylinder and replacing the piston, rings etc.
Then at the end season go through the entire engine top to bottom.
So I got a question about the top end of a X30.
Every thread I read, the rebuild schedule is based on the amount of hours. Some even do this based on the amount of fuel consumed.
Are there other things you could check to see if you need a rebuild? Compression testing for example?
I have a used X30 and I’m not really sure how many hours it had. I have like 6 hours on it now.
The sprocket ratio I ran last week was 8.5 vs to others with for example 8.2 to achieve the same speed on the straight. (Corner entry speed is about the same…)
Any thoughts?
A top end can never hurt. I’m in a region where I’m a little down on power so I do top ends much more frequently. You really can’t do them to often but you can absolutely wait to long
I’m not sure it gets done much anymore, but I know in the days of the KT100 it wasn’t uncommon to swap rings on the daily or weekend when running at a higher level. Very soft rings that would break in fast, but didn’t last. They’d swap out pistons still frequently, but less so than just the ring itself.
I’m at a piston every regional weekend. So cheap to ensure power is at the pointy end not to.
The GP that @NikG used at Medford had undetermined (lots) amount of hours on it, and he lost because the piston cracked and lost power. Still finished though, but we should’ve had it rebuilt prior to that weekend. 
Is there any way of testing this? Compression gauge?
A compression test is one way to quantify the condition of your top end yes, but I don’t know what the exact number or drop in number which would signify a need to do the top end. You’d need a baseline compression test when fresh and then track it and see when it starts slowing down.
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Anyone knows what psi an X30 should have?
Would be great to have a Squish & PSI chart for reference somewhere…
You can inspect the skirt of the piston for blowby. It’s somewhat subjective, but if there’s a lot of carbon past the ring then you can be sure there’s excess leakage.
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