Engine alignment, done properly
New mounts, new engine or new vibration — they all end at the same place: feeler gauges between the coupling faces. Here's the sequence from the Vetus manual, the tolerance rule, and the step everyone skips.
The Vetus adjustment sequence
Rough-align first
Set the engine roughly to the shaft line on the mount adjusting nuts — with the shaft coupling NOT yet bolted to the gearbox flange.
Equalise the front pair
Lift the front of the engine until both front feet just come free, then lower the adjusting nuts so both feet touch together — left and right must carry equal load.
Equalise the rear pair
Repeat at the gearbox end. Front and rear compression may differ — that's fine. Left and right at each end must not.
Fine-align in pairs
Bring the coupling faces together and adjust front or rear nuts — port and starboard by exactly the same number of turns — until the feeler gauge reads even all round.
No pre-tension
Mounts must sit square with no fore-and-aft distortion — a twisted mount transmits the vibration it was bought to stop. Use the baseplate slots for sideways correction.
Re-check afloat
Vetus' manual is explicit: verify the alignment again once the boat is launched. Hulls settle in the water — the hardstand reading is provisional.
The feeler-gauge rule
With the coupling faces brought together, slide the thickest feeler gauge that fits into the gap at four points around the flange. The accepted rule of thumb: no more than about 0.001″ of difference per inch of coupling diameter, to roughly 0.004″ overall — so a 4-inch coupling face tolerates a 0.004″ wedge and no more. Some gearbox manufacturers specify tighter; their manual wins.
Then the check everyone skips: rotate the shaft 180° and measure again. If the readings move, the problem isn't alignment — it's a bent shaft or an out-of-true coupling face, and no amount of mount adjustment will fix machined metal.
Smooth doesn't mean aligned. A misaligned driveline can feel fine at the helm while it quietly eats the gearbox output bearing, the cutlass bearing and the shaft seal. The symptoms — vibration, fuel burn, premature wear downstream — arrive on their own schedule.
Alignment FAQs
How accurate does engine alignment need to be?
The accepted workshop rule is a maximum of about 0.001 inch of gap difference at the coupling faces per inch of coupling diameter, to an overall maximum around 0.004 inch — measured with feeler gauges at four points around the flange. Some gearbox makers are stricter, so the gearbox manual wins where it differs. A flexible coupling does not excuse sloppy alignment; it absorbs running movement, not a built-in error.
Why does alignment have to be checked with the boat in the water?
Because hulls change shape when they float — supports on the hardstand load the hull differently from buoyancy. Vetus' own manual instructs that alignment be verified anew after launch, and good practice is to give a fibreglass hull at least half a day afloat to settle before the final check.
Do I align the shaft to the engine or the engine to the shaft?
Engine to shaft. The shaft's position is fixed by the stern gear; the engine is the adjustable component, which is exactly what the threaded mount studs are for — vertical via the adjusting nuts, sideways via the slotted baseplates. Allow for shaft droop (its sag under its own weight) before judging the coupling faces.
My boat vibrates but the old mounts 'look fine' — replace them?
Rubber ages invisibly: it hardens, sags and takes a set long before it looks damaged. Sagged mounts drop the engine out of alignment, and hardened rubber transmits vibration it once absorbed. If mounts are more than a decade old, or alignment won't hold between seasons, new mounts are usually the cheapest part of the fix — just align after fitting, then again afloat.
Mounts due before the alignment?
Aligning on dead mounts wastes the afternoon. Send the engine model or the load numbers and we'll have the right Vetus or Poly Flex mounts ready first.
Selections checked against the manufacturer's load windows before we quote.