Engine and jet pump rebuild

The crankshaft is back from Preben Jetski Engineering after having its rebuild with four new main bearings and two new connecting rod kits, the newly re-bored cylinder is back and the carburettors have had their overhaul, so we are ready for the engine and jet pump rebuild.

ENGINE

Bottom End

First of all it is very important to make sure that the crankcases are clean inside and out, especially if you have had some kind of engine failure, because if there is debris present when you start it up you will quite quickly destroy the engine. A good way of cleaning the crankcases is in a parts washer with a brush.

Once this has been done, dry them thoroughly and have a final check around for dirt or cracks. The easiest way to build a 2-stroke watercraft engine is upside down, this allows for the connecting rods to fall through the upper crankcase holes to aid assembly. Turn the upper crankcase upside down and rest it across two large blocks.

Now, take your reconditioned or replacement crankshaft and fit the crank oils seals using a small amount of grease around the inside seal lips where they touch the crankshaft. This will stop them sticking to the crank when first starting the engine. Lift the crank into your upside down crankcase, making sure that the seals fit into the grooves and all of the locating dowels/pegs slot into all of the crankshaft bearings . . . failure to do this properly may have a huge effect on engine life and in some cases once the crankcases are bolted together the crank won’t even turn.

Once the crank is in the crankcase, turn the crank by hand to make sure it turns freely. It is now time to put sealer on the crankcase faces. This needs to be a high-grade gasket sealer that doesn’t break down under contact with solvent. It is evenly spread on the crankcase faces that are to come into contact with each other. Now bolt the crankcase halves together using the crankcase bolts with Loctite on the threads, some of which hold the engine mount plates onto the underside of the crankcases. There is a sequence in which to torque them up (refer to workshop manual for specs).

Turn the engine up the correct way and, with a small amount of grease, screw the coupler onto the rear of the crankshaft. You can now use this to turn the crank to make sure it runs freely, but don’t let the connecting rods knock against the crankcase. Fit the stator onto the engine, line up the timing marks and with Loctite on the screws tighten them up. We are fitting a new starter motor and Bendix to our engine so that we know it is going to be “as new” once we have finished it, this was kindly supplied by Aquatorque.

Now fit the flywheel, making sure the woodruff key is located in the slot on the end of the crankshaft and with Loctite on the thread of the flywheel bolt and torque up to 65 ft lbs. With a new gasket in place fit the flywheel cover not forgetting to mount the cable ties for the negative lead for the battery.

The reed valve assemblies and reed spacer are now ready to fit but first we need to modify a standard gasket to fit. This is very easy to do (as pictured) when using the spacer as a template and a sharp knife. We chose to change our standard metal reed valve for Carbon Tech reeds. These are again, easy to change but you must remember to use loctite on the screws otherwise serious engine damage will occur if the come out get chewed up in the crankshaft and pistons. Once the reed valves are fitted and the intake manifold bolted on that’s the bottom end finished.

Top End

It is very important to make sure that the components for the top end are going to fit together well, without compromising reliability, and because of this it is critical to check the end ring gap of the piston ring in the cylinder otherwise engine temperatures may expand the ring and cause a seizure. Once this has been checked we then assembled the Wiseco Pistons. It’s a good idea to put a light coating of 2-stroke oil on the rings and piston pins so that you don’t have dry metal surfaces running against each other. When fitting the piston rings, take a look at them end on to check what shape they are. Superjet Wiseco Rings are tapered and will only fit one way up so its important to pay attention to the fact that there is a little man at Wiseco who writes “top” on the piston ring so that you get it up the correct way. Now fit the piston onto the small end bearing making sure that the arrow is pointing to the exhaust (you don’t need the OEM thrust washers) and fit the circlips.

Lightly oil the cylinder walls, again with 2-stroke oil and fit the base gasket. The next bit can be a bit tricky if you have not done it before so you may need to get a friend to help. Squeeze the piston rings with your thumb and forefinger and gently lower the cylinder over the piston, slowly slide the cylinder down over both pistons until it locates onto the crankcase dowels. Fit the cylinder to crankcase bolts using Loctite and torque up. Now it is time to fit the cylinder head. We have chosen to fit an Airtime Products Head that uses an O-ring instead of a head gasket. These are easy to fit and just require a small smear of silicone sealer in the o-ring groove before putting the O-ring into the groove and fitting the head. The bolts again will need Loctite to stop them rattling out and torque down. A BR8ES long reach plug from NGK is fitted ready to ignite the compressed air/fuel mixture in the cylinder. The ATP head has two water outlets which we plumbed one into the exhaust, and the other out of the side of the ski through a UMI 90 degree bypass. The stock water injector on the upper chamber is no longer needed so is blocked off using a bit of pipe, stainless screw and cable ties. The standard exhaust is then fitted back onto the engine to make it complete, using an ATP head to exhaust bracket.

JET PUMP

We decided very early on that we wanted to make this into a nice recreational ski to play on lakes and in the surf, so top-end speed was not of huge importance to us. Because of this we chose to fit a Solas YB-CD-13/17, ATP Steering nozzle, ATP Pump cone and to finish it off, a WSM pump housing with plastic liner.

Firstly a check of the bearings and seals in the jet pump was carried out. To check the seals, the end cap (or pump cone as some people call it) is removed. If there is no water present then the seals are OK. The bearings can be checked by turning the drive shaft by hand, to feel for any lumpiness or free play. Both bearings and seals are OK on our Superjet. Next it was time to check the impeller housing. This is a stainless steel liner inside an aluminium housing and it is very common for skis over three years old to have corrosion between the two surfaces, which cause blisters. This had happened to ours so we have fitted a WSM housing with plastic liner, a good idea as they are far less expensive and the plastic liner is a replaceable part. When fitting an ATP Steering Nozzle we had to do a bit of tinkering, but nothing worse than 10 minutes with a half round file to give clearance for the steering cable alongside the pump venturi.  Once the jet pump is fully assembled, it is ready to go back onto the hull.

Intermediate bearing and shaft

Superjets use what we call an intermediate bearing and shaft assembly. Basically, this is a short shaft that connects the main driveshaft to the back of the engine through a bearing housing. When we stripped our Superjet down we found that the bearing needed replacing and, whilst it was apart, we also fitted new seals. A quick call to Lou at N&C Jetski and the genuine parts were with us within a matter of days. It is important that when taking this assembly apart that it is laid out in order on the bench so that it’s easier to put back together. Once it is back together there is a measurement that has to be taken from the front of the housing to the end of the shaft. It should measure between 19.5mm and 20.5mm.

With the three main components now rebuilt (engine, jet pump and intermediate bearing assembly) we can look forward to putting the ski back together and water testing it next.