Yanmar 3GM30F Engine Mounts - front/back vs port/starboard

[Capt.Don]

Regarding replacing Yanmar 3GM30F engine mounts -- several threads discuss replacing the front with 100 (hardness) 124772-08341 and the rear with 75 (12820-08341). I have the 75's on port and 100's on starboard. Does anyone know if this is correct or they should be installed front/back?

Anyone have a set of 75's and 100's looking to sell?

Thanks,
Don

[Frankly]

Because my Autoprop seems to work on engine mounts (particularly the rear), my last replacement was 100s all around. Can tell no difference in engine vibration, life yet to be determined.

AIR the original 3GM Yanmar referenced 70/75s in the rear 100s in the front not port and stbd.


Frankly

[tkeithlu]

Port and starboard are forward and reverse for thrust and for moment. In forward gear the engine will compress one side while stretching the other, and will push on both. Reverse reverses that, and tries to suck the engine out the shaft log. You want mounts that will minimize engine movement (you aligned the shaft with the engine off) while absorbing as much vibration as possible. If the engine visibly moves when you engage the gears, you need stiffer/bigger mounts. Since the lifting of one side and the compressing of the other is the same, having different stiffnesses of mounts on the two side does not make sense. You want to balance squeezing and stretching.

So, same mounts port and staboard, your choice as to whether they need to be stiffer or softer. You are not dependent on Yanmar - plenty of companies sell engine mounts. Such as:

https://www.go2marine.com/DF-4408-4-...kaAiRhEALw_wcB

[wsmurdoch]

...and if your engine mounts are on a plane through the transmission output shaft, and if the port and starboard mounts have the same stiffness*, engine torque pushing down on one side and lifting up on the other should only cause the engine and transmission to rotate around the transmission shaft and make no change in the propeller shaft alignment.

* Notice I did not say forward and aft.

Is that right?

[Capt.Don]

I'm still a little confused. I can visualize this both ways. On one hand, the counter clockwise rotation of the engine (facing forward) puts more downward force on the port side. That would argue the 100's should be on the port side; however, I have the 75's (softer ones) on the port side, which doesn't make sense. On the other hand, putting the boat in forward will put more forward force on the front mounts, thus needing stiffer engine mounts in the front. That's consistent with the 100's in the front and the 75's in the back. The 3GM is a small engine. I can't believe the rotational forces are significant compared to the fore/aft force through the prop.....Perhaps Frankly has this right - 100's all around!

[Pete7]

Originally Posted by Capt.Don

I can't believe the rotational forces are significant compared to the fore/aft force through the prop.....Perhaps Frankly has this right - 100's all around!

My thoughts too, after all its not a car engine with hundreds of horse power. There will be some movement fore and aft as you go from forward into reverse, not much perhaps 1/4" but some.

Pete

[Wotname]

Another question which needs answering before deciding which mount goes where is - Where is the fore/aft centre of gravity of the engine in relation to the engine mount location?

Note - I don't know the answer!

[tkeithlu]

Quote: "I'm still a little confused. I can visualize this both ways. On one hand, the counter clockwise rotation of the engine (facing forward) puts more downward force on the port side. That would argue the 100's should be on the port side; however, I have the 75's (softer ones) on the port side, which doesn't make sense."

Capt. Don, when you visualize that rotation about the shaft, realize that you want the same stiffness on the "compress" side as on the "stretch" side. Don't think in terms of softer on one side than the other. Stiff on one side results in the engine rotating around the engine mount rather than the shaft.

As others have suggested, you may be best off simply using the same mounts on all four corners - there's really not a reason not to. Your goals are minimal engine movement and maximum vibration absorption. Those two suggest stiff and soft mounts, respectively. Plenty of commercial boats have the engine hard mounted, hang the vibration. Cars have rather soft mounts, but they have constant velocity shafts so engine movement isn't as important. You choose, and see if you llike the result through experience.

[Frankly]

Yanmar once had a two page description on how and why of their motor mounts, can't seem to find it now. The reason for the unusual configuration is the rubber is mainly loaded in shear which allows for a low resonant frequency. Shut her down and watch her shake like a dog as she spins down. AIR the table for different engines did have different rubber on port and stbd for some of the larger engines (this was all the smaller Yanmars), but on the GMs the harder mounts were in the front and the softer mounts in the rear.

Got to figure Yanmar knew what they were doing and don't think I ever heard of anyone substituting more conventional mounts that was happy with the results. These are pretty lightweight engines (as Diesels go) so maybe they felt a little "out of the box" was required.

It does hurt when you belly up to the parts counter but nobody ever said this was a cheap hobby.

Think about them 4 outboard center consoles when they pull up to the fuel dock, at least you get years out of the expense not just till the next trip.


Frankly

[sail sfbay]

Originally Posted by Capt.Don

Regarding replacing Yanmar 3GM30F engine mounts -- several threads discuss replacing the front with 100 (hardness) 124772-08341 and the rear with 75 (12820-08341). I have the 75's on port and 100's on starboard. Does anyone know if this is correct or they should be installed front/back? Anyone have a set of 75's and 100's looking to sell?.....
Don

For front mounts, this source recommends '50 shore' mounts (max 75kg load) Yanmar reference this mount as their '100' mount with part number 124772-08341. For rear mounts, we recommend our '40 shore' mounts (max 40kg load) Yanmar reference this mount as their '75' mount with part number 128270-08341.

https://www.parts4engines.com/yanmar...-engine-mount/

[sail sfbay]

Originally Posted by Capt.Don

Regarding replacing Yanmar 3GM30F engine mounts -- several threads discuss replacing the front with 100 (hardness) 124772-08341 and the rear with 75 (12820-08341). I have the 75's on port and 100's on starboard. Does anyone know if this is correct or they should be installed front/back?...........

Buy them here also:

Buy substitute Yanmar part from PYI: https://www.pyiinc.com/downloads/r-d...price-list.pdf

[Topanga]

I bought them from Partsforengines (same link as listed above). I was impressed that they shipped them out very quickly, mid-pandemic (October 2020) and it was an overseas shipment to me in the US (from UK). https://www.parts4engines.com/yanmar...-engine-mount/
Unfortunately once I replaced them, there was a 2-inch gap at the shaft coupling plates and it turned out to also have a bent shaft, strut and installed a new cutlass bearing and drip-free seal.

[Capt.Don]

Thanks everyone for the comments. There doesn't seem to be any reason why I have the 75's and 100's on the same side instead of front/back. Yet another boat mystery.

[tkeithlu]

Maybe the previous owner hadn't gotten his gyro compass compensated.

[Capt.Don]

Originally Posted by tkeithlu

Maybe the previous owner hadn't gotten his gyro compass compensated.

I'm wondering if these are original, installed by the factory. The boat's 21 years old, we've had for 11. I doubt the previous owner replaced the mounts.