Thread: Motor mount theory ?
10-31-2007, 10:16 PM #1
Motor mount theory ?
Guys im dropping in a Pirahna solid mount in my ski but as expected, it is slightly shorter than the OEM one. I know that the best and most accurate way is to use the SD Pump/Engine allignment tool. (I will eventually do it 110 % the right way, when I can $$$) My motor has never been removed from the hull. The only reason im changing the front mount is because I noticed that my front mount was cross threaded from the factory and the bolt that holds it was not completely in.
I went ahead and got ahold of the 2 different size shims available, .3mm and 1.3mm.
Here is my theory.
Because I didnt have any problems with my stock mount as far as allignment or atleast that I could notice. I'm trying to get the Pirahna mount to be level with the OE set up.
- I placed the OE setup (1.3mm shim and stock mount) on a table and then I put the Pirahna mount next to it (no shim-just mount)
- Ithen put a small level across the top of the OE set up and noticed I had a significant gap between the two.
-I added a 1.3mm shim to the Pirahna mount. (i still had a gap)
-I added a second 1.3mm shim and 2 .3mm shims...
After adding two 1.3mm and two .3mm shims the level is as centered as possible.
Can this even be measued the way I am doing it with a level ? Or am I just totally off.
here is the Oe setup
This is what the level looked like before the shims
Here is where the level ended up after the shims
10-31-2007, 10:27 PM #2
- Join Date
- Dec 2006
- Crest Hill, IL
My undestanding, but I could be wrong, is that you have to factor in the weight of the motor on both units...When installed, the weight of the motor will reduce the height of the OEM unit slightly. This will bring it to the same height as the solid mount...
I could be wrong...I will be doing this same mod this winter, so this is a question I was gonna ask...
10-31-2007, 10:31 PM #3
10-31-2007, 11:00 PM #4
No problems yet on my ski and just did the swap, no shimming. I have the Piranha mount.
10-31-2007, 11:02 PM #5
10-31-2007, 11:21 PM #6
I was down this road before and boy did it spark some debate. during a refit I worked with the sound mount team. On subs everything is mounted on a sound mount which strangely enough looks a lot like the fromt motor mounts.
Anyway I contacted some ole buddies and we were discussing this matter.
How about this for a conclusion. We are all wrong!!!!!!
on some sensitive equipment that required outside vendors ALL mounts were replaced at the same time. This was due to the compression wear on other mounts which is unpredictable, every batch of material is slightly different, and there is no way to properly shim the old to the new because the wear is not even. And these mounts were NEVER replaced in dry dock to due to the hull flex, the amount of deisel fuel, missile/warhead or ballast load out and so on. I wonder how much hull flex a full tank of gas would introduce?
Perhaps it is a differrent scale but I wonder how similar factors would play into the alignment of engine when out of the water and placed on a trailer or work stand.
Just a thought or two
Last edited by bruinsrme; 10-31-2007 at 11:24 PM.
10-31-2007, 11:21 PM #7
This makes perfect sense. And im sure it will compress. But will it compress that much? Unfortunately I cannot do the whole allignment process right now. What would you guys recomend I do ? I know its not the same, but Im compressing both mounts with my hands and it doesnt seem like the OE mount gives that much ?
10-31-2007, 11:25 PM #8
i personally dont believe the OEM mount compresses.. have never seen one squish or settle from motor weight... even if it did, it's not a mm i dont think.
when i switched from OEM to Rotax mount, my alignment changed... a drastic dangerous change? no... but it did change, and i did re-align.
225 lbs of motor or so distributed over 3 mounts... i dont particuarly visualize the mount compressing from that little weight
10-31-2007, 11:33 PM #9
10-31-2007, 11:36 PM #10
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