Flywheel ring gear question

Greetings all. My flywheel is the early 6-bolt type and the ring gear is pretty well beat up. At some point in its sordid past, the ring gear was replaced, so I figured that it was just a matter of removing the old one and replacing it with a new one. The problem is that it seems that all of the replacement ring gears are from the same manufacturer and the replacement I got is narrower (larger ID) than the one that was on the flywheel. The old one is marked “REPCOM” on one side and “270” on the other. The internet apparently has never heard of that. I need to find a ring gear that fits the machined diameter of the flywheel, which is around 308mm to the best of measuring ability. I do not want to replace the flywheel if at all possible. 6-bolt flywheels are like hens teeth and when one does surface, it is clear that the owner knows how scarce they are. So I am looking for ideas. One possibility is to have one custom made, but my Google-fu has not been up to the task of finding someone who will do it. Also, I don’t know the tooth pitch and profile which I assume would be needed for a custom gear. So I’m looking for brainstorm ideas. Thoughts?

Edit to add: The replacement gear that I have is CB132/10R which cross references to C4809. That seems to be the gear that all of the parts shops have.


Incrementally build up the old teeth by TIG welding. Clean up the excess on a lathe. Follow this by centering and securing to an indexing head configured for 132 divisions then mill with the appropriate cutter on a horizontal mill. That’s my first thought.

Thanks. that’s a thought. However I believe that the teeth are hardened (I can check) and welding would destroy the temper. Another thought is to build up the shelf on the flywheel with weld (I only need a couple of mm) and then turn that to size for the new gear. I think that would be easier than trying to build up and then hob gears.


Is my memory fuzzy, or did we decide that a 10 bolt flywheel (12 holes @ 30 deg) can be used on a 6 bolt crank (6 @ 60 deg)?

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Had the same problem the other day the Repco 270 were made by Repco in australia in the 1950’s i believe.
So unfortunately another Flywheel required.
I believe a 10 bolt will go straight on as holes all he same spacing


100% a ten bolt fits a 6 bolt crank. That is how my engine was when I stripped it down.


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That option would be prohibitively expensive. Far easier and cheaper to get a new flywheel.

If you are intent on keeping and using the original flywheel, you’ll have to find a shop that can fit it with the new larger ring gears. After the damaged ring gear is removed, the flywheel rim would have to be built up and then machined down to fit the new ring gear. However, that too would be an expensive job.

Thanks everyone. I knew a 10-bolt flywheel would fit the 6-bolt crank. I just want to preserve the 6-bolt flywheel if possible. Also even the 10-bolt flywheels are not cheap. Does anyone have any experience with the Fidanza flywheel that so many places sell? If I am going to go with a non-original flywheel, I might as well go with a good quality lightened new one rather than taking my chances on a used one for the same price.


My opinion: If you have a light car with a free-reving engine, you may find a light flywheel to your liking. If you have a heavy car you need a high-inertia flywheel to get the car moving from rest. If you are more inclined to use a rear axle gear in the 3.73-3.90 range, you’ll like a lightweight flywheel. If your preferred gearing is more like 3.07, you probably won’t like a light flywheel.

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Another question regarding the Fidanza flywheels: will their 133-tooth flywheel work with a stock 10-tooth 150 starter?

No you need 132 teeth

What is the diameter of the starter pinion?

Yes, that’s what I was afraid of.
My original flywheel is damaged and would require significant repair.
The cost of machining and fitting a new ring gear would far outweigh the cost of a new flywheel and high-torque starter.

Approx 1.15 in at the widest point.

YES it will work, i have a 132 Fidanza on my 150 for 4 years now with a 10 tooth pinion and there is no conflict .
All that happens is there is a fraction more room for the pinion to engage the teeth, I have never had a jammed starter with this set up.

The new flywheel I got from them has 133 teeth.

Let me check my paperwork and I’ll get back to you .

see a 133 tooth jaguar flywheel fitted to xK150 original bellhousing and starter motor cranking it over sounded like a gatling gun. maybe the mechanic elongated mounting holes in flywheel to alter mesh?
If you do try it do it motor out on the floor and check.
In 1977 doing my XK120 the mechanic noted the most important item before putting motor in is to check the starter motor work and it engages properly and to check the clutch works.

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Someone please check my math. The flywheel diameter is approximately 13". The circumference is then 40.8". A 133-tooth gear would have pitch of 0.307" and a 132-tooth gear would have a pitch of 0.309". So a difference in pitch of about 2 thou. That would seem insignificant to me.


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I eat my words from this morning. The Fidanza has 133 teeth, just counted them with a bit of effort.
I recall after I took possession from Fidanza Manufacturing that there was a 1 tooth difference, l contacted them and they said the same thing as spokeshave Tim has just said , the difference is negligible. I have had zero trouble with 133 tooth flywheel with 10 tooth pinion.
I have also just lately installed a high torque starter for weight and battery saving, and also to preserve my Lucas M45 starter, as 10 tooth pinions are extremely scarce.

As another note re teeth , my high torque starter has 11 teeth,[ eleven ] and works fine with a 133 tooth flywheel.