To see the first article on this restoration please click the following link Rebuilding Forensic's Spectra 20 Part 1


Welcome to article two on the Spectra 20. In article one we water tested the Spectra leading us to discover that the engine had some potentially catastrophic problems. This led to the conclusion that the engine needed to be rebuilt. With the engine disassembled and underway at the machine shop, it's time to get going on the jet drive and some of the cosmetic upgrades to the boat.


The Spectra is equipped with an older Dominator 12S jet drive. Dominator jets have seen many transformations over the years making them unique to certain time eras. Unlike Berkeley jets, not all Dominator parts are interchangeable. The jet drive in the Spectra is an early model version which could use some help in the performance department.


We removed the jet and began disassembly. Once disassembled, it appeared that the jet was in fair condition. The jet was equipped with a bronze "A" impeller. The impeller was a little dinged up but could be repaired for the project. The jet had natural wear. The impeller to wear ring clearance was .034 thousands more than the desired clearance. Excessive clearance between the wear ring and impeller will allow water to pass around the outside of the impeller compared to going thru the impeller. This causes cavitation and affects the efficiency of the jet drive.


This early model Dominator jet is equipped with a small diameter split bowl. These bowls have a significantly smaller outlet size compared to Berkeley or late model Dominator jet. This bowl also requires a special nozzle housing that is obsolete with very few upgrade options.


At some point in time, someone installed a Place Diverter on the boat. Unfortunately, the Diverter that was installed was for a Berkeley jet. The original Dominator nozzle housing requires a special Place Diverter that is no longer available. In order to install the Berkeley style Place Diverter on the older Dominator bowl the installer modified a Berkeley nozzle housing and installed it on the pump. The end result was a very awkward water transition from the bowl to the nozzle housing. Any disruption in the water flow will hurt performance.


The stock Dominator bowl features a nozzle outlet that is roughly 4" ID. Compare that to a Berkeley bowl which features a nozzle outlet that is roughly 6 1/8" ID and it is easy to see why a Berkeley bowl would easily outperform the older Dominator bowl. With all that said, there wasn't a lot of reason to keep the older Dominator bowl. This older 12S pump uses the same bolt pattern as a Berkeley (10" on center), so we optioned to replace the bowl with a good used Berkeley split bowl.


I stripped the pump parts and freshened them up with a nice gloss white powdercoat.


The bronze impeller was detailed and machined matched to the new wear ring for maximum performance. We optioned for a shouldered bronze wear ring for the build. We also had to remove the step off of the back of the impeller.


Dominator jets use a male index on the backside of their impellers to integrate into their factory bowls. With the change over to the Berkeley bowl, the step needs to be removed. Once removed, depending on the pump shaft and wear ring combination, you could have a significant gap on the back side of the impeller.


In our scenario, I optioned to fabricate and install a stuffer plate into our new bowl.

A stuffer plate will tighten up the clearance between the back of the impeller and the bowl.
(No Stuffer Plate)

This insures a perfect transition for the water leaving the impeller into the bowl. In order to properly measure for a stuffer plate, we assemble the jet without the bowl. I installed a gasket on the bowl flange of the suction piece when measuring the impeller height. Next we measure the inside depth of the bowl. With our two dimensions we figure out the difference.

(Stuffer Plate installed)

I made this stuffer plate out of 3/16" material. This gave use a .020 gap between the stuffer plate and the back of the impeller. You do not want the stuffer plate too tight to the impeller. With the stuffer in place, we can finish assembling the jet.



We have opted to add a Place Diverter droop snoot to our project. This droop features a rideplate that can be added with no machine work. That is a big plus for someone who does not want to remove and machine their intake for a shoe and rideplate combo. The droop snoot lowers the trust point of the outgoing water 1.5" compared to the stock nozzle housing. It also adds 8 degrees of positive to our nozzle. While 8 degrees of positive in the nozzle is quite a bit for a big boat like this, I still like the idea of lowering the trust point of outgoing water. To correct the aggressive droop angle, I will install a four degree nozzle wedge. This will allow us to take advantage of that lower thrust point of water while not affecting the boat's normal cruising drivability. Too much up nozzle can very easily create a mid range porpoise problem.


While installing the jet drive we have also installed a set of billet swim steps and billet trim tabs to the boat. We used the Dana Marine LT500 billet trim tabs.


These tabs feature all billet aluminum construction which makes them very strong and the candy blue color goes great with our new swim steps. With the transom buttoned up, we're eagerly awaiting our engine parts to come back from Clay Smith.


Thanks for following our build and be to sure check out round three for engine assembly.

Bob Jeanblanc

Marine Industries West