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+--- Thread: Broken Crankshaft (/showthread.php?tid=9297)
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Broken Crankshaft - Colin Ayre45 - 11-04-2024
On my way to the VSCC Light car event in Llandrindod Wells my journey came to a sudden and noisy end, followed by recovery to home.
On stripping the engine I found a broken crankshaft but thankfully no other damage. Clearly a new crank is required, but what size should I use?
The car is a 1925 Sports and the crankcase is unique ( well I have yet to find another the same), and has at present an 1 1/8 inch crank. So, do I replace like for like, or upgrade to 1 5/16 crank, and if so why. I am interested to hear any views on the matter.
Colin
RE: Broken Crankshaft - John Mason - 11-04-2024
Hi, I don’t know if your crankcase will take a 1 5/16ths cranked. Others more informed than me will be able to tell you. The 1 5/16ths crank is stronger and modern ones even stronger made from better steel.
John Mason.
RE: Broken Crankshaft - Alan - 11-04-2024
A 1 5/16in crank can be made to fit but really a new smaller crank will do you fine.
RE: Broken Crankshaft - David Cochrane - 11-04-2024
If you switch to 1-5/16" you will have to change the flywheel, the con-rods and the rear main bearing & housing. The flywheels are getting hard to find. Unless you're going racing with it, there's no point in fitting a larger crank.
RE: Broken Crankshaft - AustinWood - 12-04-2024
We fitted an 1-1/8" Phoenix crank to our 1929 car as a precaution against breakage. The car still has its original engine which we didn't want to risk.
RE: Broken Crankshaft - Chris KC - 12-04-2024
It's more than likely a modern 1 1/8" crank is stronger than the original was when new, and it's certainly 99 years younger! Unless you are using it for all-out racing that would be my choice. Fatigue is the real enemy and any new crank shouldn't have any.
RE: Broken Crankshaft - Chris Garner - 12-04-2024
Interesting Colin...and bad luck! In my experience breakages of 1 1/8" cranks are rare. I firmly believe the metal of this type was either of better quality or manufacturing technique than the later larger diameter cranks. Look at the revolutions the cranks were taken up to in the early Seven racers. Like many others here, I would also replace it with one the same diameter.
RE: Broken Crankshaft - Chris KC - 12-04-2024
(12-04-2024, 09:22 AM)Chris Garner Wrote: Interesting Colin...and bad luck! In my experience breakages of 1 1/8" cranks are rare. I firmly believe the metal of this type was either of better quality or manufacturing technique than the later larger diameter cranks. Look at the revolutions the cranks were taken up to in the early Seven racers. Like many others here, I would also replace it with one the same diameter.
I was going to say "bad luck" Chris - but then if it's broken without taking out the rest of the engine it's arguably very good luck!
RE: Broken Crankshaft - Ruairidh Dunford - 12-04-2024
Both 1 1/8” cranks I have broken have gone through number 3 journal. One took nothing with it, the other damaged the front lip.
RE: Broken Crankshaft - Parazine - 15-04-2024
Fitting a 1 5/16" Phoenix crank to an early (1926) mag crankcase is covered on Page 10 of the "Restoring a Top Hat" thread, with photos.
To summarise,
1) drill rear main oil return hole
2) relieve centre camshaft bearing housing to provide sufficient clearance
3) relieve front lip to provide clearance. Crankshaft may have to be relieved as well, so do this before it's balanced.
All the rear main mounting hardware from a 1 5/16" engine is used as well as the rods (obviously!) and the flywheel. 1 5/16" rear mounted starter flywheels are becoming hard to find but a later, 4 speed type can be machined to suit.
Some people have found that the big ends foul the inside of the crankcase, I've done a few of these and not had any problems but it's worth checking.