Rebuilding my 91H - Couple questions for those for those that have done it

[DCflyer]

So I disassembled, cleaned and am in the process of rebuilding my 91H for my 700 and ran into a couple nagging issues. I watched Finless's videos and looked up some other tips online but got stuck there so was hoping you guys could shed some of your experience on this.

I installed new bearings in the motor from RC bearings. The smaller bearing is a sealed ceramic bearing. I heated the motor up to 300 degrees for about 20 minutes and inserted the bearings with the crankcase, installed the fan hub and screwed the nut on tight and pushed/pulled from either end to secure both bearings.

- Now here's the snag. I have a new fan hub I installed with the thrust washer in between the top bearing and the hub. If I tighten the crankcase nut down fully(very tight), the crank/fan assembly becomes fairly hard to spin. The bearings are seated squarely - the crank rotates very freely. Is this normal and will loosen up as it wears in again? Or no?

Should I be installing the hub just to the point of where there is no up and down play between the crank and hub and then loctiting the nut at that point? If I do this I can get the crank assembly to spin very easily (with no play in the crank) but then I have about a 2-3mm gap underneath the hub, between the motor.

In Finless's videos he cranks that nut down very very tight with loctite but I can't tell how easily he can spin the fan afterwards.

[nynextel88]

If you have a glow plug installed in the head or a pipe on the engine you will not be able to spin the fan/hub by hand very easily because of the compression. Although if this is happening just with the crank shaft and bearings seated (no piston or rod installed) then i'd say you have an issue with either how the crank shaft is seated inside both the bearings or they did not go in 100% straight.

I wouldn't run it with any gap in between the fan hub and engine case, it should be seated flush with the washer and the washer should be flush with the front bearing. And you definitely want to crank down on the crank shaft nut pretty good with some loctite. Only other advice I can add is that if you are certain the crank and bearings are installed correctly is that maybe one of the new bearings is possibly bad, and the pressure of the fan hub against the front bearing is not allowing it to move freely.

[DCflyer]

Thanks for the reply - I think my large bearing isn't seated fully. I HATE dealing with bearings - its always what holds up everything it seems when I work on things.

There is a small gap it looks like between the bearing and motor housing where it looks like it could recess a little farther. I will try heating the motor up tonight and pressing it down farther, but I don't know why it won't seat.

I had the motor at 300 degrees in the oven for 20 min, took it out and seated the small bearing, then pulled the large bearing up into the motor with the crankand quickly installed the fan and hub - then screwed down the nut in order to pull that bearing up into the case. I don't know what else I can do as far as getting it in straight. If it doesn't go in straight by that method I sure as heck am not going to do any better by eyeing it.

I was about ready to chuck the motor off my balcony last night.

[nynextel88]

I usually heat the oven up to 450 and place engine case in it for 10 min. While thats cooking I place bearings in freezer for 10 minutes as well, this together should seat the bearings pretty well.

[marc8090]

300 is a little low. 350 is better. When you get it hot enough the rear bearing will fall out with only a gentle tap of the crankcase on a block of wood. I place the room temperature new rear bearing on the crank and just drop it in. No need to press anything in or install the fan hub. Just place the engine (crankcase, crankshaft, and bearings) on a rag with the exposed end of the crank facing somewhat down and let it cool. Changed countless bearings this way with zero issues. With only the crank and bearings in the case adding the fan hub should not cause it to spin any tighter than without the hub installed.

[DCflyer]

Thanks for the tips fellas - broke the motor back down last night. Large bearing seated perfectly this time. It was off by about 1mm causing the connecting rod to rub against the backplate once the backplate screws were fully tightened down. This was certainly what was causing binding in the hub not being able to freely spin as well.

Everything is back together now and rotating smoothly.