Boat project using a 4 cylinder bike engine...!

Boat project using a 4 cylinder motorcycle engine
Can anyone help here?

I’m looking for a 4 cylinder chain drive bike,it must be water cooled, electric start in excellent running order.

It doesn’t matter what it looks like or whether it’s an accident damaged bike or stolen recovered as I want to use it as a donor for a boat project I’m working on.
Why a bike engine?, they are powerful, compact well engineered with a smooth, well balanced operation and they sound great, perfect for my classic 60’s boat which is styled after the fabulous Riva Aquarama…
Looking good is one thing but it’s gotta sound and perform right too

I intend to strip out the engine, part of the frame, chain drive and electrics plus a few other parts so, a bike it will be no more!
I’ve overcome the obstacles on the final drive configueration, using a bevel gearbox to change the drive angle to verticle and subsequently linking that gearbox to the bottom end of an outboard, that way I get Foward neutral and reverse… now I just need the heart ie an engine!

My problem is that I have a height restriction in my engine bay so, the bike engine must be a maximum of 48cm+/- from it’s lowest point to it’s top in its normal operating position!

I would imagine that a 400cc or 600cc would carry the smallest 4 cylinder block but perhaps some expert knowledge can advise me here or maybe point me in the right direction.
I realise this will probably involve half the motorcycle commumity trapesing around a yard with a tape measure so I’ll thank all in advance for any help.

Please feel free to call me or pass my number on if you have any questions or hopefully if you have some much needed advice

Marcus Kendrick
07957 686095

Hi Mark, welcome to the site. I’d like to point out a potential flaw in your plan, if the boat is to be used at sea bike engines are not engineered to use sea water as a coolant and would rot the engine very very quickly.

Bike engines in kit cars tend to overcome the height problem by either:
(a) cutting down the sump or going dry sump,
(b) cutting down the airbox,
© ditching the airbox,
(d) cutting a hole in the bonnet and having the carbs/throttle bodies sticking out the top.

My experience of this is restricted to Fireblade (929) and ZX9R C/E engines, and the old CBR1000 engine.

Don’t know if that will help at all, but something to think about maybe?

Hey there K,

It’ll be freshwater use only, but I’ll be marinising the engine all the same…


Now that’s a possibility, cutting down the sump by 1 or 2cms may help but as a last resort, if I can get close on the overall dimensions, so far the units I’ve checked out have been 54cms+ so just too big to start with…

Coming out thru the top is not an option as it would destroy the all imprtant lines of the back

Thought about a Bergman but can’t use a shaft drive and if I recall they are only a twin…


If it’s a Riva Aqua type/sized boat I doubt you’ll get the horsepower you need from ANY bike engine.
Riva always fitted twin engines and the least powerful featured 2X185 HP.
The Aqua Special (and a few others) had 2X350 HP. They used a lot of Chrysler and a few Cadillac motors.
If you go ahead with it cooling will not be difficult to sort out but I doubt the end result will be worth the effort.
If it is a small wooden boat it might be OK ish.:wink:

Tee hee… I would soooooo love it to be the right size which is 29’ and very luxurious, they were powered by twin 5.7 ltr chryslers! take a peep here…

Mine is smaller (the story of my life! ha ha) at 14’6 the ony real comparrison is the styling ie similar polished wood decking and the all important barrel back shape of the stern…


You could save yourself a lot of work by forgetting about using an outboard leg…Mount the engine lenghtways in the hull and run a shaft straight out to a prop. Use a standard marine drive coupling to hook it up and off you go…more or less:w00t:

Thought about that option, even made the prop shaft I dismissed it tho as the coupling would be largish but in theory would work the real problem is the gearbox, I must have Forward neutral and Reverse, the smallest marine box cost a chunk and weighs a ton and is as big as a small engine in its own right which wouldn’t work.

The outboard bottom leg incorporates the gearbox and a water pump via an impeller so I can use a raw freshwater feed.

The one I’m using is a yamaha 25/35 hp, perfect size for the boat. I’ve had a rectangular plate welded around the cavitation plate which fixes securley to the underside wit a perfect seal, the whole set up including the bevel gearbox weighs about a third of a marine box… the 2 boxes are linked via a 25mm keyed rigid coupling…

The other shaft of the bevel bearbox will take a second sprocket and works with an anti-clockwise rotation, the same as the direction of the bike engine


VFR motor is supposedly very compact…

Any ideal of the VFR’s engine hieght?


oops I mean ‘idea’… duh (keyboard issues ! sheeesh)

Forgot about reverse…
You can buy a reverse gearbox from made by Comet (not that one) for about 250 Usd. The forward drive is direct from you existing drive and you only engage the Comet when you need to go astern. They only handle 20 HP or so but that’s more than enough in reverse…

Don’t know, sorry :frowning:
If i had a tape measure i’d pop outside and guess…

I toyed with the idea of using a comet gearbox well, a FNR gearbox from an ATV married up to a comet centrifugal clutch, they have the advantage of uniform bores and parts but, had to dismissed that idea as it would have invovled a permanent coupling to keep it aligned i.e. welding the sprocket to the end of the propshaft, in the end it was too much fabrication and I’d be stuck should I need to replace seals or sprockets/mounting plates on the propshaft and , already got the bearings and shaft on that option but just couldn’t make it work …

Good thinking tho!