I replaced the front motor mount and looked at the spindly grade 8 bolt supposed to simply clamp the mount to the plate above it and the large washer below. the plate hole above it had worn from a loose bolt and washer.
(I'm restoring to good rideabilty a 1989 flhtp beat up by driving it badly by cops for 100,000km- but Motor and trans are excellent).

A 1/2" bolt slid with play throught the works with only a little filing of the bottom washer, and i added a locknut and tightened it up gently.
Anyone know why HD used the spindly Grade 8 bolt with a washer - the idea is to fasten it to the rubber mount, right? So is there any reason not to use a bigger bolt if it fits and is a grade 5? Clamps up fine, bike is good and mount is breaking in with vibes going away pretty quick. I asked others that bought this mount on ebay, supposed japanese source of original mount, and they are happy with it as opposed to the new Hd which they say was too hard.
Anyone know why the Grade 8 bolt was used in the 1/2" hole? It is not a flexible bolt, thats for sure and is lost in the 1/2" hole.

thanks, bw

 

HD couldn't afford the 1/2 bolt so they went with a 3/8 grade 8 ! I always wondered about this I've done many rubber mounts and I always replaced the 3/8 with a 1/2 in

 

I don't have a clue as why they used those bolts but they did it for many many years. It never made any sense to me.

 

thea 1/2"grade 5 is not as strong as the smaller grade 8.. the bigger bolt is ok but never use anything but grade 8 on drivetrain

 

That bolt doesn't do anything except keep the engine in place, it is not subject to an loads of consequence. The stabilizer link nearby takes the lateral loads that might otherwise get to it, which is much more important.

 

The play in the hole allows for the variences in manufacturing, allowing the engine/drivetrain to be properly ligned up using the stabilizer links.

 

^^^ Yup. Production decision. Same reason that the body mounts on my CJ5 have large holes filled with 3/8" bolts: Alignment.

 

I looked at the alignment with the new front motor mount plus both new stabiliser turnbuckles set to the old ones dimensions to start.
The back wheel was clearly off enough to see with the 1/2" bolt in the front mount hole..
Started did a really slow careful alignment, I first made the back wheel vertical to the frame ( after I leveled bike frame) and it was only out a little. Same level on the front discs too. good.
then i looked at the wheel alignment - aargh. clearly off to the left , as a mechanic friend riding behind me had said. following the manual, i found I had to pull the whole thing over a couple of complete turns (new tie rod) to bring the edges of all the wheels in line using string and 10' extruded aluminum with bike upright.
Now the hole in the rubber mount is not on centre at all, and I can see why they used a thinner Grade 8 bolt- am having a friend check it tomorrow but the wheels are are now aligned, and now the little grade 8 bolt goes through ok on the non-centred mount collar, making sense now to me. thanks.

 

Definitely an alignment situation.
Earlier models did have a larger bolt.

 

www.glide-pro.com has a video where he shows how to align Bagger wheels with the heavier 1/2" front motor mount bolt he provides with his swingarm/front mount kit. The way he does it the increased diameter of that front mount bolt is not an issue.