Engine head bolts-ARE YOU KIDDING?

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.

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

|.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.

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

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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.

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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. Acorn vs. Hex Nuts
Tom

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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.

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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....

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

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.

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.