1958 Evinrude Bigtwin 35/1969 Evinrude Ski Twin 33 powerhead swap

[Trackbolt]

Good morning,
I am considering swapping a 1969 Ski Twin 33 Model # 33903A powerhead onto my 1958 Bigtwin 35 Model# 25936. I have a decent 33 motor in my collection. The 33 hp block looks identical and the mounting bolt pattern on the base and the exhaust housing is identical. Both motors use the same water pump, gear case and both have the single water tube. My goal is to retain classic hood. I have had the 35 since new and it has been in regular summer service throughout most of that time. Thanks in advance.

 

[racerone]

May have to swap some parts / brackets to mount the hood.

 

[jimmbo]

I think the Heads are different, not just because of the Compression Relief on the 58, but because the Cowl attaches to a Bracket on the Head, might also apply to the Crankcase.

 

[Trackbolt]

May have to swap some parts / brackets to mount the hood.

Thanks. That was my intention. I was planning to use the 33’s flywheel and ring gear while keeping the 35’s starter and starter brackets. I’ll do some test fitting before the actual swap. Not planning to transfer the compression release but will transfer the 35’s recoil starter.

I think the Heads are different, not just because of the Compression Relief on the 58, but because the Cowl attaches to a Bracket on the Head, might also apply to the Crankcase.

I think the Heads are different, not just because of the Compression Relief on the 58, but because the Cowl attaches to a Bracket on the Head, might also apply to the Crankcase.

Thanks! Yes. In addition to the compression release there are castings on each side of the head to accommodate both Evinrude and Johnson cowls. I will need to transfer the 58’s head to the 33.

 

[jimmbo]

I know the Pistons are different between the 58 motor and the 69. Might just be the Tin Plating used on the newer Motor, but...
I would put some Plasterzine on the Top of the Pistons and check the Clearances.
You are planning to swap the Heads, and the Recoil, so you might as well transfer the Compression Relief Linkage
As for the Flywheel, It and the Ring Gear have different Part Numbers between the years, so does the Crankshaft. Differences on the Ring Gears could be difference in Tooth Count, or Height. Flywheel and Crankshaft, could be Taper Differences, or location of the Flywheel Key, which could affect the Ignition System.

 

[Trackbolt]

I know the Pistons are different between the 58 motor and the 69. Might just be the Tin Plating used on the newer Motor, but...
I would put some Plasterzine on the Top of the Pistons and check the Clearances.
You are planning to swap the Heads, and the Recoil, so you might as well transfer the Compression Relief Linkage
As for the Flywheel, It and the Ring Gear have different Part Numbers between the years, so does the Crankshaft. Differences on the Ring Gears could be difference in Tooth Count, or Height. Flywheel and Crankshaft, could be Taper Differences, or location of the Flywheel Key, which could affect the Ignition System.

I was planning on keeping the 69’s flywheel. Crankshaft and taper are larger. I seem to have much to consider here. Thanks

 

[jimmbo]

A lot of people think that cause the Displacement, Bore and Stroke, even the Rated Rpm and Rpm Ranges are the same, that the 57, and some of the 58-59 35hps, and the later 33hp, are the same Identical Motor. In 58 and 59, there were two completely different 35hps motors by OMC. with not a lot of interchangeability.

 

[Trackbolt]

A lot of people think that cause the Displacement, Bore and Stroke, even the Rated Rpm and Rpm Ranges are the same, that the 57, and some of the 58-59 35hps, and the later 33hp, are the same Identical Motor. In 58 and 59, there were two completely different 35hps motors by OMC. with not a lot of interchangeability.

I know the Lark in 1958 and 1959 were very different from the Bigtwin. Mine is a 25936. At the end of the day my 1958 is worn out. My father bought it new and it saw regular April-October use from day one up till about 10 years ago. Getting noisy in the bottom end. Always run 24-1 fuel but nothing lasts forever. But thanks for the help. I’ll continue slowly and make a decision one way or the other at some point.