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E: Engine Flywheel Fit


Well Known and Active Website User
VOC Member
Could I ask for opinions please on the following problem.
The fit of the crankpin into the flywheel should be , Pin at 1.002 and hole in wheel at 1.000 - giving a fitting pressure of approx 6 tons.
As can be seen from the photo`s the test bar at 1.001 is " not right "!!
The Go - No Go gauge in Brass ( smoother surface ) has various diameters of 1.001 , 1.003 and 1.006. As can be seen at 1.006 the gauge disappears. The Timing side is correct at 1.000 ( and a bit )
Several solutions come to mind one being to bore out the hole in the wheel and weld in a insert as per the Drawing. There is probably another 1/16 inch to be had on the outside Diameter of the insert. Using the large Diameter of the Nut recess as a register this can then be bored out to 1.000.
Solution two is to weld " strips " across from front to back ,finishing with a circular run front and back. Face off and bore out.
If you are not already asleep these next points are important
!. I / We will not buy a new oversize pin at 1.125 since we already have a perfectly good ,new Vincent Big End assembly
2 I / We will not buy a new wheel since - in the words of Mr Dunn " unless its a pile of ash everything is salvageable " This oversized hole is just an engineering challenge.
3 I / We have access to ( in order of priority ) Single malt , Coffee and Cake , A Bridgeport Milling Machine , a BIG Colchester Lathe , MIG , TIIG , Gas and arc welders
And a bucket of inspiration ( empty ).
Comments please . Thanks.IMG_2031.JPGIMG_2028.JPGIMG_2033.JPGIMG_2035.JPGIMG_2031.JPGIMG_2028.JPGIMG_2033.JPGIMG_2035.JPGIMG_2031.JPGIMG_2028.JPGIMG_2033.JPGIMG_2035.JPG


Well Known and Active Website User
VOC Member
The bore of the flywheel looks to be in poor condition with lots of pitting and potential distortion. Given this connection is relying on friction, you should consider resizing this bore to the correct surface finish and making an oversize pin.
It might be more cost effective to buy a new crank assembly from Terry Prince?

Bill Thomas

Well Known and Active Website User
VOC Member
Alignment after welding will be Tricky,
As I am about to find out !, Doing a long stroke 560 cc 102 mm,
I think Big Ed said, I should weld to two wheels together before boring ?,
Which I plan to do,
Just looking at it , Can't think why it was made SO weak,
The hole, Not wide enough or long enough to me,
Which is why the racers have used bigger Big ends and NO nuts !.
Good Luck.

Bob gave up on my Light flywheels !!, Said they were cracked and too weak !.
Last edited:


Well Known and Active Website User
VOC Member
Hello, Would not chance the bore in your wheels. Some years ago Bob Dunn work his magic on a set of wheels for me.Left over from my racing days the crank pin bore was opened up too large for any available pin. He bushed the holes with a top hat shaped bush the flange forming the spacer for the cages.It sounded pretty frightening but the set up is still in my solo and has done 50k miles. We shall miss him.


Well Known and Active Website User
VOC Member
I have stopped using the original flywheels for use because the material used in them is not resilient enough. It tends to deform and stay deformed. It also lacks the elasticity of the newer materials. Renewing a 70-year-old flywheel turns into an expensive way to build a flywheel that is unsuitable for racing, at least in the long run. I buy them from Coventry and they order them from Maughan.

Rebuilding the original will teach you a lot, which is of great benefit, also. The reliability of the flywheel is so important for what I do with the Vincent that I cannot afford to use the stock wheel.



Well Known and Active Website User
VOC Member
The problem is that the original poster (Powella) has given a list of things that he does not want to do. The problem that I see from the photographs is that the mainshafts and the hole for the crank pin are corroded. My advice would be oversized mainshafts and crank pin but he seems to have ruled that out!

Peter Holmes

Well Known and Active Website User
VOC Member
Does reclaiming with metal spraying work, I am not an engineer and no nothing about the process, but my ex deceased father-in-law used to work at Rolls Royce aircraft engines and used to enthuse about the process, along with spark erosion to remove busted taps etc, this was all about 50 years ago, I guess things have moved on a tadge.

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