Exhaust Manifold, on Bintelli Fury 150 ( GY^ Engine )

[Shadowfire]

Fastening the exhaust to the exhaust stud bolts... yeah.

It is quite possible, when the engine is properly designed, to hold the exhaust flange to the engine with only appropriately torqued down nuts.

"Real" manufacturers, who hire real engineers, specify the material that the cylinder head is made of, specify the stud bolt thread tolerances, and specify the torque values so that the stud bolts are stretched into the plastic deformation point of tensile stress. They also ensure that production lines are set up so that these items are manufactured to their specifications before the product is shipped.

Most of the cheap Chinese crate scooters are not assembled by real vehicle manufacturers, they are assembly houses which have none of the engineering expertise in-house. As a result, bike quality will depend almost exclusively on the oversight of whoever is placing the order. When companies like Kymco and Sym have a scooter built in China, you'd better believe that they send over manufacturing engineers to train staff for as long as it takes for them to do the job correctly. If your scooter happens to be assembled at the same plant where a Taiwanese/Japanese firm has already run the gamut with getting the assembler up to snuff, your odds are substantially improved of having a bike that will last longer than 2,000 miles without needing major repairs.

In any event, at a low quality chinese manufacturer, compromises can be made which otherwise make ANY engine service an exercise in frustration; in many of these cases, you are using the service manual for the engine that was cloned, and that manual has torque values suitable for the design of the original manufacturer, and will just snap or strip out the same threaded part on the Chinese clone.

In any event, Bintelli has a reputation on the scooter forums for being one of the better Chinese scooters (disclaimer: I have never owned a Bintelli scooter, so have no personal advice about it, just hearsay), so it may just be a case of someone going crazy on the muffler flange nuts without knowing (or caring) what the nuts should be torqued to.

So, finally, now to the information that the OP is really looking for.

The good news, if you can call it that, is that the cylinder head is already unusable as is (you won't have an exhaust seal, so you'll be poisoning yourself whenever the engine is running and you aren't moving), so nothing you do to it can screw it up more. You can attempt to extract both the stud bolts and install new ones. For the stripped one, if you have enough unstripped threads adjacent to the cylinder wall, you can put two THIN nuts on, then use a thin wrench on the nut nearest the cylinder to attempt to get the stud out.

For the snapped stud, if there is still some protrusion of the stud above the surface of the cylinder head, I recommend using a Dremel with a cutoff wheel to cut a slot through the center of a bolt, which should allow you to use a flat bladed screwdriver to extract the bolt. Otherwise, you would need to drill out into the center of the bolt and use an easy-out, however that is a difficult operation for a typical mechanical novice to perform.

Finally, if you do succeed in replacing the stud bolts, MAKE SURE you replace the gasket.