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Crankshaft Keyway Failer

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Crankshaft Keyway Failer

Postby Goose Chase » Sun Aug 25, 2013 12:53 pm

Crankshaft Keyway
I have just had the key way fail on the end crankshaft where the damper goes on just before the gear box – it was done when accelerating and the drive train past this point i.e. gear box just stopped. Have located parts and sorted out the problem. The old keyway disintegrated and was spread around the crank shaft. This has been cleaned and new damper and other part (not quite sure what to call it) have been installed and we are now going. I have been told the problem will happen again because these motors in original configuration where 185 hp, the new motors with the larger bore must be considerably higher hp and it seams like the weak link in the drive chain.
The suggestion was to drill and bolt this but I just can’t see how that would not make it weaker. Anyone had any experience here?
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Re: Crankshaft Keyway Failer

Postby Don Ayers » Mon Aug 26, 2013 6:23 am


I have heard of people machining the other side of the crank to accept an additional key i.e. one key on each side of the crank. Clearly you have to machine the dampener as well.

I'm not an engine expert but this operation might not be able to be done unless the crank is out of the engine.

Was the key a brass key or steel, or stainless??
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Re: Crankshaft Keyway Failer

Postby Eric T. » Mon Aug 26, 2013 11:37 am


I've seen this issue on a few Chris Craft 283 Configurations on these motors.

There are a lot of details to sort on these engines to get them to be reliable (most important) and have power (second).

The problem is twofold... one, as you've mentioned, is the additional horsepower, the other is that often, when the long block (monoblocko) is substituted with a larger 350 cu in. (5.7L) engine.. the crankshaft is now a cast steel crankshaft rather than a forged crank... they are weaker in general... all the 283 CC engines used a forged crank.

I've built quite a few of these pair that I'm running this week on Lake Como, has increased output to 310 HP each and no issues... though all has been renewed inside and out.

I used a high quality crankshaft and, as Don mentioned, using two keys... the machine work must be precise and the there must be a slight interference fit that requires the drive-cup that holds the damper to be pressed onto the crank using the proper tool.

Often, the drive cup is worn out OR, worse yet, it was machined for a loose fit because the assembler did not understand the drive.

Often, these engines have poor carburetion and ignition systems where the idle is set too high to compensate for the engines wanting to die when put into gear....This will lead to increased load as the the transmission is "banged" into gear... stress is tremendous and can lead to the drive cup and key to be stripped.

If the springs in the damper plate are old or weak... this can also lead to issues. Plus... if the input shaft bearing in the transmission has any "play" in it.. this can also cause issues with the keyway.

If you have a cast crankshaft... I HIGHLY recommend substitution with a forged crankshaft and two keys cut into the crank. Scat makes high quality forged crankshafts and can cut the crank for 2 keys, 180 degrees separated.

What exactly do you have for a power plant and what has been renewed in the restoration of the engine and transmission.

I believe this is what you are talking about??
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Re: Crankshaft Keyway Failer

Postby Goose Chase » Tue Aug 27, 2013 3:44 pm

Thanks for your reply and help.

I have noted down the following points and will have them attended on the “Original Motors” which are about to be completely refurbished.
1) Crankshaft forged
2) Two key Ways
3) Drive-cup that holds the damper to be pressed onto the crank using the proper tool (with 2 key ways cut in Drive-cup).
4) Springs in the damper plate new
5) Input shaft bearing in the transmission should not have any "play" in it.

The problem I had was on the “New Motors” which went in this season. I will have these “New Motors” sorted out and see what sort of crank they have and what would be the cost of swapping over to forged Crankshaft if they are not already and the cost of putting the 2 x keyway.

The Original motors have done around 1,500/1,700 hours after re-installation 2006 season until end of season 2012, total of 7 seasons and around an average of around 200/250 hours per season.

I also hammer the motors, they did very well for 7 seasons, the new Motors are definitely more powerful then the originals and so I guess the Key Way issue!

We did several measured runs and achieved 69 kilmoters per hour / 42.9 miles per hour / 37.25 knots – my boat is Goose Chase Aquarama Hull 49 and I have owned the boat since 1995– so it’s a series 1 – we were 5 up and full fuel and assorted gear.
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Re: Crankshaft Keyway Failer

Postby Dvanness » Sun Sep 22, 2013 7:27 am

It seems that there is a nomenclature problem . What is being called a keyway failure is in fact a sheared key! If my assumption is correct than the key clearly did its job protection the drive train and a forged crankshaft would have no value. Regarding the number and size of key, remember that high output big blocks use the same size key in flywheel forward configurations, There are many 383 /283 converts used in cruisers producing much more power and doing more work!! using same size key and cast cranks
I gather the first set of engines ran about 2000 hours each .... no key failure
The drive damper is a shrink fit when installed properly and the addition of a bolt would have no effect in the shear mode.
I suggest that engine alignment could be the culprit. The additional loads from an improperly aligned are often not taken into consideration. I recently saw a boat that top RPM was about 2800 RPM proper alignment brought it to 4200.... no other changes,,,, The increase in shear load is obvious
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