Cam breakage poll
by Greg Solow
I have never experienced any camshaft breakage, but in a reprint of an article from the English magazine Autocar from April 8, 1955 excessive camshaft bending in the area of the oil pump drive gear is mentioned. And this was with early light springs but continuous use over 5,000 rpm. to cure the problems the front part only of the camshaft was increased in diameter by 1/8". Remember, the span of support in the front of the engine is twice as large as in the rear half. The drive gear has replaced the bearing that would be expected in this location.
I believe that the shaft diameter should not be reduced below its original as cast diameter for this reason. If a cam grind has so much lift or duration that the shaft diameter has to be reduced to accommodate a small base circle, then the lobes need to be welded up or a new cam blank with large enough lobes should be used. The new cam blank should be checked to make sure the front part of the shaft is 1/8" larger than the rear part. it should also be the same minimum size as the original shaft.
In the original design, according the article in "The Autocar" at 6,000 rpm the camshaft was flexing .020" at the #1 exhaust lobe and only .008" at the #3 exhaust lobe. Again, this was in the early development of the TR-2, before the valve stems were uprated to 3/8" diameter and before the valve springs were stiffened. The TR engineers found increasing the diameter of the whole length of the camshaft did not cure this uneven deflection, this was only cured by enlarging the front part of the camshaft and leaving the rear part untouched.
This uneven deflection could account for camshaft breakage in the area around the oil pump drive gear.
The article with this information is in a "brooklands books" collection "Triumph TR-2 & TR-3 1952 - 1960" The article is entitled "A Triumph of Development The story of the TR-2" by John Rabson, A.M.I.Mech.E.
By Larry Youg (Designs camshafts for racing TRs)
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.
By Hardy Prentice
I agree completely with Greg. I strongly suggest the use of blanks or billet cam blanks and not regrinding cams to a smaller diameter.
I never had a problem with the Kas cams, including the one that he ground
for me on the last known blank from the Triumph parts system.
I had two cam grinders brush off the information about the two diameters,
and lost three cams over the years.
The most frustrating one broke in front of the gear in T3 at the old
Laguna; if only the rotor had stopped pointing to the #1 plug, the crank trigger
might have made it to the top of the hill on the last lap of the last
season race, as the front two lobes still worked; it cost the divisional title.
The worst was one that broke in two places, and allowed the front part of
the cam to fall down into the crank.
I installed and closely watched the cam bearings, and polished the radii on
both sides of the gear, and magged the cam at least once a year.
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