I've been reading your comments on this issue with considerable interest.
I have ruined lobes on several Isky555 regrinds over the years - always
blamed it on the lifters. I dug one out from under the work bench today and
examined it. The diameter of the fat half of the shaft has been cut down
about .050 in. The cam lift on a 555 is .298 in.
Do you think that is enough to be causing my problem of ruining lobes?
Bruce
----- Original Message -----
From: "Larry Young" <cartravel@pobox.com>
Cc: <fot@autox.team.net>
Sent: Sunday, October 04, 2009 3:03 PM
Subject: Re: [Fot] Survey: TR3-4 camshaft breakage
> My comments are in line with Greg Solow's. My principal concern is with
> the shaft diameter as it relates to the stiffness of the valve train,
> but it could also be related to breakage. Ideally, you want the valve
> train to be stiff to reduce the deflection and vibrations. I found the
> same article that Greg mentioned in the Brooklands reprints (for TR2 &
> 3) that explained that Triumph beefed up the front portion of the shaft
> to increase the stiffness. According to my measurements, you can't get
> more than 0.300 lift from a regrind without cutting into the core (the
> part between the lobes). If you want 0.370 lift from a regrind, you'll
> have to undercut the core about 0.200 inch. That reduce it's stiffness
> by more than 50%. If you grind on a new blank and keep it as large as
> possible, you will not have to make it as small, so it should be both
> stiffer and less likely to break.
>
> British Parts Northwest did carry blanks for a while, but I understand
> they no longer plan to stock them. I've just ordered a small batch from
> the UK. I've seen problems with both the drive gear and the bearing
> surfaces. My TR3 and Tony Drew's TR4 both have been running cams ground
> on new blanks without any problems.
>
> Perhaps in this discussion, it would be useful to know the lift of the
> cam and whether it was new or reground. You'd expect the front portion
> of the cam to be the part most likely to break.
> Larry
>
> Bob Kramer wrote:
>> Bob,
>>
>> Larry Young, who is designing camshafts for some of the FOT, likes to
>> use new blanks rather than regrind for his race camshafts. His
>> thinking is that during the regrind they also reduce the diamter of
>> the shaft to keep it smaller than the base of the lobes. He is worried
>> about your exact problem. Maybe he'll chime in.
>>
>> Bob K
>> Austin TX
>
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