How Can An Exhaust Cam Snap?

[Rolls]

http://www.skylinesaustralia.com/forums/topic/354952-car-stalled-and-wont-start-turns-over-has-spark-and-fuel-ideas/

So my car snapped its exhaust cam but appears to have oil and the bearings look fine, what are possible causes for this?

[t_revz]

pretty sure there was a thread about this maybe 6 or so months ago. think if i remember correctly most people agreed it was to do with incorrect torquing. cant remember other ideas suggested...

[GTRsean]

+1

Very sensitive to the way they are removed and installed. Slow and steady by undoing all the bolts a little at a time is the safe way, same when doing them up. It's due to the length of the cams and which lobes are compressing their respective valve springs part way along the cam.

If you put on and do up the caps unevenly or one at a time, fracturing/weakening and or breakage may occur.

[DVS JEZ]

Or if the cam has never been out than check thencam belt

Hasnt slipped causing the exhaust valves to contact pistons then snapping the cam.

Couldnt u hear the engine had no compression when cranking?

[Duncan]

mine snapped due to an oil pressure problem. you would expect lots of other things to fail first but that wasn't the case this time.

[Rolls]

Or if the cam has never been out than check thencam belt

Hasnt slipped causing the exhaust valves to contact pistons then snapping the cam.

Couldnt u hear the engine had no compression when cranking?

Cranked like a normal engine as the valves are all shut so it would have had compression even if we tested it. Belt is fine, tight as f**k and absolutely no way it has slipped.

Doesn't look like an oil supply issue either as there was plenty of oil and the two bearings I checked (next to the snap) look brand new.

Edited March 7, 2011 by Rolls

[tm_r33]

Yer my mate snapped one. As said above it was due to incorrect instsllation procedure. He tried to start tightening from the center but because the valves were holding the two ends of the shaft up it must have started to bend and then it snapped when he applied pressure on the next bolts.

Silly mistake really but it was his first major engine work. Was years ago now.

If you have not had the camshafts out could maybe suggest that timing belt was over tention or it slipped a tooth. Surely u would of heard valve to piston noise if any contact occured tho.

[Rolls]

how would over tensioning the timing belt cause this?

[tm_r33]

Cant over tensioning the timing belt load up the front of the shaft. the extra tension is trying to pull the front of the camshaft downwards. And even the smallest amount of deviation to the cam rotation will cause u a problem

[Rolls]

Could a seized lifter also cause the cam to snap? Been speaking to a few people and a mate whos rebuilt 30+ motors reckons he has seen it all the time where if someone rebuilds a head and puts the lifters back in out of order etc it can happen.

I have a spare cam so might take all the lifters out, see what the deal is, then compression test it with the other cam, who knows might be lucky and all the valves are sealing, only take 10 minutes to try vs taking the head off, nothing to lose by trying it.

Be easier to take the whole engine out than to take the head off.

[tm_r33]

Yer posibly it could seize. Is ur engine an rb20? They is hydrolic lifter so would be rare for anything to cease up I would of thought but ur mate might well be right. But yer if you strip a head down its important to make sure any parts that are being re used go back where they came from.

[tm_r33]

Lol pulling head off is a pain in the ass in 33 gtst. I spose its gunna be the same across most models lol

[Rolls]

r32 with a rb25det

[tm_r33]

Well yer 25 iss hydrulic lifter also and imo not fun to pull the head off lol

[PM-R33]

I have my timing belt and all associated gear off at the moment due to doing some maintenance. God I hate doing it so much. I've done it about 10 times since owning the car and it never gets easier, always end up having no feeling in my fingers for a week.

[Bl4cK32]

I'd wanna at least have a look (camera in spark plug hole) at the top of the piston to be sure. Why bother fixing it all to then pull the car off the road to fix again...

[Rolls]

We will stick a boroscope in there as well, but it takes 15 minutes to change the cam and it takes what 5+ hours to remove the head then gotta do gaskets and all the old hoses that will no doubt snap. I have nothing to lose by putting another cam in and compression testing, takes no time at all, I don't want to pull the head off and find it is all ok in there, and if I pull the head off and its stuffed I need to get another motor anyway.

lysdexia over on the 31 forums said he snapped a cam in a 25 and got off with no bent valves or pistons some how, turned it over hundreds of times as well, cause if the cam snaps wont all the springs try and close all the valves and just put the cam in the position with them all as closed as possible?

[scotty nm35]

I doubt it, they have most likely hit and it takes bugger all to bend the valve. A compression test would show this I guess, but I would stick the camera down there if you have access to one.

[snozzle]

Have a set of pliers ready incase the valves are bent.

[Bl4cK32]

cause if the cam snaps wont all the springs try and close all the valves and just put the cam in the position with them all as closed as possible?

it wont though because there is always at least one cylinder with open valves, so my money would be on some valve / piston contact in the cylinders that are at the rear of the motor (the side that had no drive to turn the camshaft)

Edited March 8, 2011 by Bl4cK32