camshaft broken

[ftsoft]

One of my camshafts was broken in half about 2/3 of the way down the shaft. The thing is, not only did it not damage anything, the car ran pretty well with it like that. Any way, I was lucky to find a used camshaft at RD.

So only one valve in one cylinder was not working, sounds reasonable, the spring pressure would push the valve closed, out of the way of the piston. Or am I missing something?

Actually, they were all working. I'm not sure why, unless the camshaft broke when the engine was dismantled, which seems doubtful. Looking at the break, everything fit together so tight that I'm not sure the camshaft knew that it was broken.

Frank

[tim littlewood]

I have not got any further yet with my investigations, but all valves under the damaged cam appear to have free movement, these were original cams that had been ground to L14 spec so maybe they just cant cope with the extra stress, the engine has done very few miles since it's rebuild

[rgh0]

What other mods were done with the L14 cams, the valves, springs and buckets all need replacing to avoid spring bind or other interference issues with the higher lift and if this not done correctly could cause a failure.

Also I would certainly normally modify the cam to accept fitting of the long sprocket bolts that run past the first bearing if fitting this profile cam also. You correct in assuming the standard cams struggle to handle the extra loads from the higher lift profile.

regards
Rohan

[tim littlewood]

it appears from all the info I have that all the other parts were upgraded to accept the new profile cam, so its possible that the weak point are the original cams, or is there any reason why the original cams should not be able to deal with the extra strain?. Would new cams be any batter than the originals
Tim

[rgh0]

If the original cams were not modified to take the long sprocket bolts then they are likely to fail. New cams are the same and need long sprocket bolts. Some racers change to steel billet cams which are stronger but they introduce their own problems in terms of galling between the cam and follower.

regards
Rohan

[Mr.Gale]

I have not got any further yet with my investigations, but all valves under the damaged cam appear to have free movement, these were original cams that had been ground to L14 spec so maybe they just cant cope with the extra stress, the engine has done very few miles since it's rebuild

How do you regrind original cams to become high lift cams?
Gale

[types26/36]

How do you regrind original cams to become high lift cams?

......by changing the base circle.

[Mr.Gale]

How do you regrind original cams to become high lift cams?

......by changing the base circle.

Huh? I thought the camshaft lift is "the resultant net rise of the valve from its seat and the further the valve rises from its seat the more airflow can be realized". You can grind a cam all you want but unless you add metal to the lob it will not push the valve in any farther. But, what do I know?
Gale

[types26/36]

Found this that explains the process more eloquently then I could.

"Lift and duration are increased by grinding away the "base circle" of the cam. The lobe doesn't have material added to it, but the non-lobe part is ground away making the difference between highest part of the lobe and the lowest part of the base circle a greater distance. If you grind away enough of the base circle, you can flatten the top of the lobe and still increase the lift while changing the duration profile substantially."

[c42]

Hi Mr Gale

When the base circle radius is reduced and the tip of the camshaft is left relativly untouched the difference between the two radii becomes greater than on the standard camshaft, the clearance between the cam and the cam follower is then reset to the correct value; this gives you the increased lift/duration.

Hope this helps.

Regards
John

[Mr.Gale]

Thanks Brian and John, now I understand.
Gale

[tim littlewood]

hi, I thanks to all for comments so far.
I have just started rebuilding after the cam break and notice that there are 2 timing marks on my cam shaft pulley's.
How do I no which are the correct marks for timing the cams? and why are there 2 marks anyway?

Tim

[rgh0]

I presume the second set of marks were added for timing the reground cams. Typically longer duration cams have a lower MOP timing say 105 degrees ATDC versus 110 degrees for the standard cams. Have a look at the cam timing quoted in various catalogues and you sould be able to work out the mark that was made for your cam but it may be only an approximation as you are limited to the increments of one sprocket tooth.

In practice i would always individually time the cams on a non standard engine using offset dowels or vernier sprockets to be able to set the correct timing.

cheers
Rohan

[tim littlewood]

completed fitting the new cam, reshimming was a bit of a paint but at least the engine is running again. It appears that I have been lucky and that there isnt any major damage elswhere. The only issue is that the engine gives out a little puff of black smoke when you first rev the engine,could this be because the cams are not perfectly timed?

[tim littlewood]

No I did'nt paint this shim's

completed fitting the new cam, reshimming was a bit of a pain but at least the engine is running again. It appears that I have been lucky and that there isnt any major damage elswhere. The only issue is that the engine gives out a little puff of black smoke when you first rev the engine,could this be because the cams are not perfectly timed?



Tim