Adjusting engine mountings

[RAI]

Despite being correctly adjusted on the original new engine installation, I have just found that my Python CV drive shaft is now out of alignment. It needs the rear engine mountings lowered about 2-3 mm to realign it. Is there anything against simply doing this? (Yanmar 4JH4E with Yanmar mountings, ZF30 gearbox).

 

[LittleShip]

Despite being correctly adjusted on the original new engine installation, I have just found that my Python CV drive shaft is now out of alignment. It needs the rear engine mountings lowered about 2-3 mm to realign it. Is there anything against simply doing this? (Yanmar 4JH4E with Yanmar mountings, ZF30 gearbox).

Your engine mounts should have been checked a little after your new engine was fitted, they nearly always "settle" after installation.

If lowering the rear mounts will bring it all into line again there should be no problem with doing so.

Remember that you should be looking for it to be within a tolerence of 1/1000inch.

Good luck

Tom

 

[vyv_cox]

The point of having a constant velocity coupling is that you don't need to align it at all - the CV is able to operate at large angles of misalignment. You don't say how you have measured the 2-3 mm misalignment but you don't need to do anything to change it.

 

[RAI]

Hi Vyv, The Python drive needs alignment to keep the angle on each of the two CV joints about the same and not more than a certain angle for a given maximum power/rpm.
I think the forward engine mounts have settled more than the aft mounts, resulting in the forward CV at near maximum angle and the aft CV at near zero angle. (Zero angle is also bad news). So by lowering the rear of the engine I can get the two angles back to about equal.
I am worried there may be some other side effects from changing the rear mounting height, stress on the forward mounts? assymmetry of load on the four mounts?

 

[pohopetch]

Hi Rai - We have the same engine and gearbox as you do. While a Ujoint (Universal joint) requires both ends to be at the same angle (and between 1/2 and 3 1/2 degrees off centre) this is not true of a CV joint. The specific advantage of a CV joint is that it does not have the alignment restrictions of a Ujoint. I'm not familiar with the Python unit, only the Aquadrive. Is the Python definitely a true CV joint? 2 to 3mm seems to be exactly the sort of mis-alignment the CV joint should render immaterial.

Because of the geometry of our setup we were able to us e Ujoint (a shortened Land Rover drive shaft in fact) at about 20% of the cost of the Aquadrive unit. Has done 3000 hours of engine time with just a greasing every 6 months. Earlier this year we had a 7m log wedged under the boat that bent the p-bracket 50mm to one side. The prop and shaft was undamaged and we did another 500 miles under motor until we could get hauled out (British Columbia). Bashed the steel p-bracket straight with a big hammer after heating it near red hot and everything was sweet again. The ujoint and dripless seal in the shaft log absorbed all the misalignment and the engine was not adjusted at all. This is the beauty of decoupling the engine from the prop shaft with either a CV or UJoint.

Cheers
Peter

 

[vyv_cox]

OK, I understand your dilemma. I cannot think of any reason why you cannot do as you suggest, as presumably this will bring the engine back to its original position. If you adjusted it by extremely large and unrealistic amounts I guess it would be possible to reach a situation where the majority of the load was carried by one pair of mounts but for the small figure you are suggesting I cannot see it mattering at all.

If the current status is the result of compression set in the mountings, would it not be better to increase the height of the forward mounts instead?

 

[RAI]

Is the Python definitely a true CV joint?

Yes, it is very similar to the Aquadrive. The 3 mm I am talking about is vertical on the rear engine mounts. The transmission flange will come down more than that. I think that solves the alignment problem, making both CVs turn the thrust line through about the same angle. My propshaft is about 12 degrees down, the engine at about 2 degrees stern down, so I need both CVs taking about 5 degrees each. That's about on the limit for the drive I have.

Vyv, brilliant, why didn't I think of that? But looking at it again, there is not much thread left for "up" on the forward mounts and I would prefer to lower the engine from a CG point of view. I guess I will just try it and see if I get any unhappy sounds.

 

[vyv_cox]

I guess I will just try it and see if I get any unhappy sounds.

I can't see what is so unusual about the Python Drive. My Aquadrive fitting instructions have no warning about not having the coupling straight, although I have heard it said that some angle is necessary for correct ball lubrication. When I fitted my Aquadrive the engine remained exactly as it was with a normal coupling, i.e. in pretty much perfect alignment. It's been in for something like 10 years now, 1800 engine hours, still performing well.

 

[RAI]

The Python drive's installation text is quite specific that zero angle on a CV implies inadequate lubrication and shortened life. It explains the way to establish nearly equal angles on each CV and gives a table of not-to-exceed angles vs. rpm. Maybe the Aquadrive is better made and more tolerant. I have 1200 hours on mine.