broke a cam
- Thread starter wingtip80
- Start date
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HotRodKid
mmmmm ... turbo
i have to ask: the sprocket is sstill bolted to a small amount of camshaft? or all the bolts are gone and you have ONLY the sprocket ?
Yes there is still about 1/4 inch of cam on the sprocket.
SuperHO
Mental Patient
well...that's a first...
pitaSHO
Why do I do this to myself?
ycode90
SHO Member
I've used one like a bat and never had one break. Good job .
sdpatt
Sr. SHO Engr.
Excess tension on the timning belt can place a shearing load on the intake camshafts. That tension would have to be very significant to fatigue the cast iron of the camshaft, but anything is possible. It could also be that there was a casting flaw in the samshaft itself. A closer inspection of teh break could provide some clues. How about some close-up pictures of the two break surfaces?
Along this subject, I have heard a hollow, rotating, moaning sound from the front end of my engine for years. I was told by SHO-knowin' folk that it was the oil pump and that was the sound that some of them made. I was never able to locate the source of the noise. After replacing the timing belt tensioner on the same day as my connecting rod bearing swap before Labor Day, I heard the noise to be much louder. I had added a bit more tension to the belt than provided by the spring during the tension setting process. With a mechanic's stethescope, I identified the source of the noise as the front (#1?) bearing of the rear bank intake camshaft. This bearing supports most of the load due to tension on the belt at the sprocket. I reset the timing belt tension by the procedure I have posted in the Forum and found the sound to be much less apparent. Too much tension on the timing belt is not a good thing.
Along this subject, I have heard a hollow, rotating, moaning sound from the front end of my engine for years. I was told by SHO-knowin' folk that it was the oil pump and that was the sound that some of them made. I was never able to locate the source of the noise. After replacing the timing belt tensioner on the same day as my connecting rod bearing swap before Labor Day, I heard the noise to be much louder. I had added a bit more tension to the belt than provided by the spring during the tension setting process. With a mechanic's stethescope, I identified the source of the noise as the front (#1?) bearing of the rear bank intake camshaft. This bearing supports most of the load due to tension on the belt at the sprocket. I reset the timing belt tension by the procedure I have posted in the Forum and found the sound to be much less apparent. Too much tension on the timing belt is not a good thing.
Scott, interesting analysis, and perhaps provides some reasoning behind Yamaha's move to a hydraulic tensioning system in the later 3.2 engine. I remember when my SHO was new, mine and many other new ones had a loud noise from the timing belt (low pitched whine IIRC) that went away after a few thousand miles, dealers were told to tell us it was normal.
I believe the material of construction is chilled cast iron, for hardness/wear resistance (but as a consequence more brittle than nodular or other grades).
Original poster - these were stock camshafts? Sure you didn't drop a valve or seize a lifter bucket or something that might have shock loaded the cam drive system? I can not see the pictures, but if you send me one end of the failed part I can do some failure analysis for you (or take some high mag pics with oblique lighting).
Generally, it would seem that if you are retensioning a used belt it might be possible to overtension it by following the factory procedure.
zak
I believe the material of construction is chilled cast iron, for hardness/wear resistance (but as a consequence more brittle than nodular or other grades).
Original poster - these were stock camshafts? Sure you didn't drop a valve or seize a lifter bucket or something that might have shock loaded the cam drive system? I can not see the pictures, but if you send me one end of the failed part I can do some failure analysis for you (or take some high mag pics with oblique lighting).
Generally, it would seem that if you are retensioning a used belt it might be possible to overtension it by following the factory procedure.
zak
Maybe NESHO can weld it :wave:
St Louis SHO
Rollin' coal
Oh noes! You haz cam cancer....
HotRodKid
mmmmm ... turbo
it appears to have wound up cracked somehow at some point, then eventually failed totaly, hence the 2 different shades to the broken surface. I wonder if at some point in its life it was the victim of a short tempered mechanic or something of the sort.
very interesting
and also, to fix the broken link above:
very interesting
and also, to fix the broken link above:
HotRodKid
mmmmm ... turbo
wait, whats with the double seals ? one left on the head and one left on the sprocket?
nevermind ... dust cap and oil seal.... to early to be trying to picture the single time i ver actually touched a cam seal on a sho
nevermind ... dust cap and oil seal.... to early to be trying to picture the single time i ver actually touched a cam seal on a sho
Last edited:
220ksho
Gen I bias
Does not look like rotational shear to me. Timing belt too tight, IMO.
Hopefully, your engine was still a non-interference one.
Good topic. Good info SDPATT. I'm going out to check mine right now.
Hopefully, your engine was still a non-interference one.
Good topic. Good info SDPATT. I'm going out to check mine right now.
I agree - it appears that it has been mostly cracked for some time, with just about 20% holding it on for a while.
It is not a torsional failure, so I would not expect any stuck valve type of issues.
I'm guessing somebody either dropped or smacked that cam at some point in the past and it got a little crack, which grew to cover most of the cross-section over time and stayed that way for a while - quite possibly aggravated by too tight of timing belt at some point in history.
sdpatt
Sr. SHO Engr.
The break appears to be brittle fracture and the location at the sprocket side of the #1 bearing leads to the supposition that the belt loading was a contributor in the failure. There does not apperar to be any rotational or twisting shear as would indicate the cams could not be turned and the sprocket just twisted off. The different shading of the area at 8:00-9:00 o'clock in the top image implies that there may have been a flaw in the casting or previous high load when the cam was static. Was this engine not used for a long period of time?
93rev2sev
SHO Member
Replace em BOTH! They both saw the load.
gmorrell
Never been a noob...
In 21 years of screwing around with these cars, I have never seen a break like that.
Put some JB Weld on that and buff it out, it'll be fine.
Seriously tho, send it to Zak, he loves to analyze broken parts.
Put some JB Weld on that and buff it out, it'll be fine.
Seriously tho, send it to Zak, he loves to analyze broken parts.
hawkeye18
Sorta cares
Just when you think you've seen everything... 
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