broken rear mount bolts - update

GoldenMotor.com

kallsop

Member
May 2, 2009
106
1
18
CT US
I broke a drill bit in the one broken rear stud trying to extract it. Anyway, I was still able to drill it and extract it but didn't trust the thread in the block any more. So I drilled it out and tapped it for a 1/4-20 helicoil. I wanted to raise the motor in the frame a little so bought the sickbikeparts front mount. For the rear mount, used 1/4-20X4 grade 8 fully threaded bolts. I'll be surprised if this new mount gives me any trouble, but you never know. Next time I'll use shorter rear bolts LOL. I took these pictures before putting the exhaust mount back on the bike:



 

DudeZXT

New Member
Jun 20, 2010
115
0
0
Lexington, KY
I had to use 2 of the mount extension blocks & 4" M6 bolts to reach the seat tube on my cruiser and have found that those bolts have broken in the engine block.

I got some 5/16" fuel line and plan to cut it into <1/8" thick pieces and place the pieces between the mount strap and the bolt heads, to reduce vibration and stress (which caused the original breakage). There's probably 5/8" of bolt threads that go into the engine...

Does this sound like a good idea or do you foresee a problem with that idea?
 

kallsop

Member
May 2, 2009
106
1
18
CT US
I'm no expert, but in my experience rubber mounting in the mount bracket or the bolt head doesn't help. Make sure the face of the mount is square to the frame and use the hardest bolts you can find. With a double rear spacer and needing 4" bolts, you will always get a lot of shear loading on the rear bolts where they enter the engine. Rear bolts breaking is such a common problem, I'm surprised no vendor is offering a fix. I don't mean just harder bolts, but something completely different kind of like the sikebikeparts front mount that I used but custom for the rear.
 

Kevlarr

New Member
Jul 22, 2009
1,628
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Mi
Actually I've used 2 5/16" nuts as spacers between the rear mount block and the engine so I could mount the front directly on the frame. Since I did that a year ago I've never had any problems with the rear mount. Honestly the rear gets the least amount of stress as long as you have a solid rear mount.
 

DudeZXT

New Member
Jun 20, 2010
115
0
0
Lexington, KY
I may just end up turning the head & piston head around and evening up the mounting distance between front & rear better, so the carb & K&N have enough room to go up front, while I try to custom fit a CAG exhaust/expansion pipe out the back.

Seems like a good idea to go for it all since I have to remove the engine anyway...

Anyone had any luck using a CAG pocket bike exhaust on these? From what I can find, it looks like it has a 24mm ID and, worst case, I can get a guy to weld the stock exhaust flange & 12" of pipe to the expansion chamber.

Figure these modifications could very well put me at over 40. Already using plug & wire upgrade, CNS carb with stock carb needle, Manic Mechanic billet intake, 36t on the back, no tensioner, and extremely close to that mark..