I started out modifying outboards for my hydroplanes as a youth, maybe starting out when I was 12, then moved onto muscle car engines, then motorcycle engines, which were the most challenging since they already come in a high state of tune.
When I recently rebuilt my 4108 it was like turning on the wayback machine, they are pretty straight forward mechanical engines, but have lots of parts because they are totally mechanical motors.
Tips for a happy and successful rebuild
- As you remove parts take pictures.
- Label and bag all the parts, assess the condition of those parts as you do this, some will be reusable, some not.
- Take more pictures.
- If you have not taken apart this particular type of engine before, take notes to accompany your pictures.
- Buy a shop manual, the Perkins manual is an excellent manual, yes it's a bt wordy in parts but very thorough.
- Have a MAP gas torch handy, MAP gas torches provide a hotter flame than a propane
torch. Some bolts which have been in the motor for decades don't want to leave that casting. Heating
the casting boss into which the bolt is secured will allow the bolt to come out easily without stripping the bolt or the hole it came out of. This avoids a lot of costly rework later.
- If your going to do a total rebuild
buy quality parts, these are sleeved engines, in other words, you don't bore the existing cylinders, you buy a kit with new sleeves and pistons, bearing, seals
, valves, valve guides, etc.
- find a quality machine shop which is familiar with these motors, an average machine shop only knows chevy small blocks and popular motors. The shop I used is a one man shop who took extreme care to properly machine the engine. even though the motor gets re-sleeved, the new sleeves need to be final honed to match the pistons. In my case the machinist matched the individual cylinders to the individual pistons to get the tolerances just right.
- Take your time, patience will pay off.
- Have the injector pump
rebuilt by a reputable service
- Measure, measure, measure. when your bock and parts come back fromt he machine shop, don't assume it's all correct, measure all the parts for tolerance before assembling, especially the crankshaft to crankshaft bearing tolerances and piston to cylinder tolerances. I've found mistakes
more than once over the years which would have led to catastrophic failure upon first startup.
- Use torque wrench on everything, proper assembly pays off in the end.
- Even though this was the first 4108 I've rebuilt the thorough rebuild process paid off, the motor started and ran flawlessly on the first kick on my test stand. It's now in the boat
and runs nicely, the only thing I had to do was adjust the idle speed, which was off due to the injector pump
rebuild, if the idle is too high it can make starting hard since it tends to flood upon starting.