Engine head bolts-ARE YOU KIDDING?

Prasinos · Oct 30, 2009

Ok so I have a 80cc engine kit that I bought from ZoomBicycles, It was the cheapest kit I could find and that's all i really look for when I buy a kit. The first time I took off the head of the engine I noticed that 2 of the M8 studs came out of the crankcase rather than the head bolts coming off of the studs. I do a lot of tinkering with my engines and the second time I took of my head I stripped one of the holes in my crankcase. Thank god for Heli-coils i repaired that and ran my engine again. About a Month later i decided to add a thinner gasket below the cylinder, and guess what, another hole stripped in my crankcase.
Ok that cant be a coincidence. I inspected the studs further and realized that the threads are different on each side. The side made to go into the engine is an M8 coarse thread, and the side made to accept the head bolts is a M8 fine thread. But the most ridiculous part is that the head bolts are made for M8 coarse threads. The Head bolts were basically forced onto the studs cutting new threads into them, so you are forced to either cross thread the head bolts onto the studs every time, or to remove the studs from the crankcase.

I know these engines aren't made to be the most quality pieces of engineering but the wrong threads?! I don't even see how it would save money to do this.

Honestly, i want to ask for my money back but Ive had the engine for 2 months now, so all I can do is complain to you guys.

I ordered some M8 coarse threaded rod to replace the studs, in the meantime avoid ZoomBicycles or make sure that you got the right studs/bolts with your kit when you get it.

BarelyAWake · Oct 30, 2009

blarg - that's a bummer, now I'm scared to pull the head off my other engine

|.o

I figure yer "upgrade" will fix that problem though, hope it works out for ya. I haven't noticed this problem before - I wonder if I just put 'em back in the same? *shrug* Thanks for the heads up tho - I'll pay more attention next time.

Venice Motor Bikes · Oct 30, 2009

Knowing what I know about the "quality" of these kits; It doesn't suprise me a bit!!

Bikeguy Joe · Oct 30, 2009

Not taking side here, but it's not just zoomcycle. It could be any vendor you buy from. It is the luck of the draw with the chinabuilt beauty.

2door · Oct 30, 2009

Prasinos,
Not questioning you, well, maybe I am, but are you sure you got that right? I've looked at a number of these engines and see the opposite: Fine threads, actually 8mm X1 in the crankcase and 8mm X 1.25 on the top or nut end. I was going to replace the studs with bolts but was unable to locate an 8mm X 1 bolt long enough. When they get to that length they go to a 1.25 thread. I could find shorter bolts in a 1 thread but not longer ones. I even tried a place in Denver that specialize in metric fasteners.

While we're talking about this another thing I discovered while 'tinkering' was that some of the acorn nuts were bottoming out on the studs before they were tight against the cylinder head. That wouldn't normally present a problem unless the stud was already screwed all the way into the crankcase. Then you'd have a loose head which I suspect is the reason for some of the head gasket failures I read about here. I replace all the acorn nuts with shouldered (a wide surface) hex nuts to eliminate that possibility. I leave the head off, double nut the stud and torque it into the crankcase to the recommended 12 foot pounds. Then I remove the double nuts, install the head and gasket and torque the nuts to about 10 ft. pounds. Seems to work for me.
Here's a link to my original thread regarding this issue. http://motorbicycling.com/f3/acorn-vs-hex-nuts-6846.html
Tom

Spunout · Oct 30, 2009

Bikeguy Joe said:
Not taking side here, but it's not just zoomcycle. It could be any vendor you buy from. It is the luck of the draw with the chinabuilt beauty.

yeah thats true.

anyway prasinos, have you tried speaking with them regarding this issue, or just assumed nothing would happen because you're past the 30-day warranty? you'd be surprised what few kind, well-worded and problem-specific emails/calls can do.
good luck.

Prasinos · Oct 31, 2009

well its possible that i rotated the studs around when re installing... all i know is that when i re taped the stripped holes with heli coils i used coarse threads, so im going to replace all the studs with coarse threaded rod and lock-tite them into the crankcase.

i still don't see why they would have a stud with different threads on either end

Im pretty good at being a squeaky wheel when it comes to things like this, but ive already re tapped 2 of the holes with heli coils so i know it wont happen to those again. Also, Ive had issues with an engine from zoom before, it seized up after 5 miles because a peice of the cylinder wall cleaved off. To replace an engine they make you pay the shipping to return it. They never answer their phone so It took me about 5 emails to get them to send me the replacement parts. They did, so i cant fault them, but i just don't want to deal with all that trouble right now.

Its kind of hypocritical, but i just ordered another engine from zoom yesterday, il let you know what the stud orientation is for that one. I came to terms with the quality of these engines a long time ago, but i just felt that this took the cake.

And Joe is probably right, i just wish there was some sort of disclaimer on these sites. For instance, i build bikes for a lot of people, i normally build 50cc kits so that people can ride legally. However, when i recieved my first 50cc kit it had a 67.5cc sticker on the side. I contacted Zoom and they said that they ran out of 50cc badges so they just used that one. Now that just seems ass backwards to me, rather than put a badge on your engine that PROVES you are breaking the law, why not just leave it off all together?

but for 109 bucks shipping included its tough to resist....

lovehamr · Oct 31, 2009

Seems like Dax would be the better deal even paying the shipping. Haven't heard any of these nightmares out of that shop.

Techbiker · Oct 31, 2009

I second Dax. Duane doesn't mind answering any of my questions in a timely fashion if I have any problems with the kit. I also think that Dax is one of the best places in terms of quality and return policy.

Scotchmo · Oct 31, 2009

Prasinos said:
i still don't see why they would have a stud with different threads on either end

When threading studs into softer materials such as cast aluminum, a coarse thread is the best design practice. It should be locked in tightly in order to insure that it stays in place when removing the nut from the other end. The other end of the stud that accepts the steel nut may be fine thread in order to have the desired clamping force and vibration resistance.

A coarse thread screwing into the aluminum case and a fine thread on the other end makes perfect sense. Your choice of coarse thread helicoils in the case and coarse thread head bolts (nuts) should work OK but may need some kind of locking feature to insure that the nuts stay tight at the recommended torque.

If the studs that came with your engine had the fine thread ends in the case and the coarse thread ends for the nuts, then they were definitely installed backwards.

2door · Oct 31, 2009

Scotchmo said:
If the studs that came with your engine had the fine thread ends in the case and the coarse thread ends for the nuts, then they were definitely installed backwards.

I'm in no way arguing with you Scotchmo because what you're saying is good engineering practice. Nevertheless, out of five engines, three from one supplier and two from others, I found the opposite. Fine threads in the casting, course threads at the cylinder head end. I have access to engines from one supplier and I'm going to research this a little. I'll let you know what I find.
Tom

Scotchmo · Oct 31, 2009

2door said:
I'm in no way arguing with you Scotchmo because what you're saying is good engineering practice. Nevertheless, out of five engines, three from one supplier and two from others, I found the opposite. Fine threads in the casting, course threads at the cylinder head end. I have access to engines from one supplier and I'm going to research this a little. I'll let you know what I find.
Tom

I was making the assumption based on the correct design practice. For verification, I just pulled one of the head bolts (acorn nuts) on my bike and checked it. My metric thread gauge is back at the shop but the nut appears to be an M8x1 fine thread. I tried a coarse thread (M8x1.25) nut on the stud and it will not fit. I did not unscrew the stud to see if the other end was coarse. So I’m still not certain that the case has coarse threads. The head end of the studs on my engine are correct (fine threads), at least one of them anyway. That does not mean that all engines are the same. Because of the lack of quality control on these engines, some could be wrong. It would not surprise me if some factories put all of the studs in backward and did not have a clue.

2door · Oct 31, 2009

Scotchmo,
I'm in agreement on that. If you've ever seen those funny videos of the assembly plants for our engines nothing would surprise you. The Chinese guy hammering on the crankshaft end breaks me up every time I see it.
Tom

Prasinos · Oct 31, 2009

well i can tell you my head bolts are definitely coarse threads, so either the wrong studs were used or the wrong bolts.

Scotchmo · Nov 1, 2009

I did a little on-line research. Sick Bike Parts sells both M8x1 and M8x1.25 cylinder studs. They do not sell the proper M8x1-M8x1.25 combination stud. It appears that these China engines are made with either type of thread in the engine case. The BEST way is to have M8x1.25 threads in the case and M8x1 threads on the head end of the stud. But this would mean a specialty stud and not merely a piece of thread stock.

For simplicity and to save a little money, I could see why some factories might use a singe thread pitch stud. But if they are already using the more expensive combination stud, why would they put it in backwards? I just thought of a reason (I’m just speculating); the “assembly plant” orders parts from different suppliers. The engine cases sometimes arrive with the M8x1 thread and other suppliers give them the M8x1.25 case. The combination stud fits either type of engine case but only if they install it backwards in the M8x1 case.

If you have an engine case with M8x1.25 threads, your best option is to use a combination stud.

If you have a case with M8x1 threads, you might want to use a single thread M8x1 stud like Sick Bike Parts sells. Fine threads in the soft aluminum case are not as desirable as the coarse threads.

Helicoils can be used to repair stripped threads. If so, I would use M8x1.25 Helicoils and the combination stud. If you insist, and you already have the single thread M8x1 studs, you could use M8x1 Helicoils which would still be more durable than the original aluminum threads.

Any of the studs or thread pitches will work OK on these low powered, short lived motors. Unless you are determined to make the motor as good as possible, it may not be worth the effort to change something that already works well enough. Good quality hardware is often more important than the thread pitch used on the engine.

M8x1.25 case/M8x1 head – best possible combination

M8x1 case and head – more likely to strip threads from case.

M8x1.25 case and head – less clamping force on head gasket for a given bolt torque. More likely to loosen from vibration.

M8x1 case/M8x1.25 head – worst combination

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Engine head bolts-ARE YOU KIDDING?

Prasinos

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BarelyAWake

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Venice Motor Bikes

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Spunout

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Prasinos

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Scotchmo

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