Broken Cam

[CJ81BU]

Ok, All you engine guru's. I have a 81 Malibu with a small block 406. It is mainly a bracket car and runs in the mid 10's on the motor. At the last race after I completed a run, while idling on the return road the motor shutoff. Well after towing back to the pits, I noticed the engine was not getting spark. I removed the distributor cap and cranked the engine. The engine turned freely but the rotor did not turn. I then removed the valve cover and bumped the engine over the first 3 cylinders valves moved but the last 1 didnt move at all.

Any ideas why. I even broke out on the run.

Thanks, Chris

[Pete L]

Is the block filled? There have been cases where the cam gets hardened because the filler. It has a tendancy to create a higher core temp. That temp is transfered to the cam and a hardened cam = a brittle cam, hense breakage.

[CNC BLOCKS NORTHEAST]

Check and see what core you have as we use all P-55 cores or the K-15 cores as these are the performance cores that have more carbon which makes the stonger and the only other thing that breaks cams is valve float.

And on most of the stock blocks that we machine for performance applications we blue print the cam tunnels as we have found that the cam bearing bores are all over the place and its hard for cam to survive in that environment as they are flexing more then then they are designed for.

This is a pic of Dart block that we are Blue print boring the cam tunnel for roller bearings.

[CJ81BU]

Well guys I took the engine out last night, no bent pushrods etc. , I did a leakdown and saw less than five percent on any cylinder. The number 7 cylinder which was on the had a very slight exhaust leak- it still had only 4% leakage. No metal was in the oil. I removed the cam and it was broke in front of the number 4 cam bearing journal. The cam bearings look good and the cam rotates fine it came out easily by hand. I took the broken piece out the back.

Now for what caused this problem the. The number eight intake lifter was stuck in the bore. I had to pry it out. It had a scuff mark on the side of the roller trunion. Once I freed the lifter I was able to install and remove the lifer with some drag. The cam was originally a crane roller that ultradyne had reground. It is also a reduced base circle cam. As for the filled block question it is a bow-tie block that is filled to the bottom of the freeze plugs with hard block. It set for 2 months filled before machining.

So do you all think the froze lifter caused the cam to break ?

Do you think I am ok to cleanup the lifter bore, put a new lifter in and change the cam ?

I have spare parts and have a race this weekend, just hate to tear the motor apart any more than neccessary.

Thanks for the replys

Chris

[CNC BLOCKS NORTHEAST]

check the lifter to bore clearance. sounds like something is off.

[Chris Uratchko]

This is not unusual to break a cam right before the number 8 exhaust lobe, if it is a small base circle cam. It's actually been happening frequenly as of late, and has me thinking cam cores. I know of 4 in the last year, yours makes 5.

Has nothing to do with the block. Purely camshaft related.

[CJ81BU]

Sorry after checking the lifter and cam the scuff on the lifter came from hitting the lobe in front of it after the break. The scuff on the lifter was copper colored just like the cam lobe sides.

Any more ideas or just a weak core ?