Are the lifter tops all sitting at the same height, that is level with the engine block casting when they are off the cam lobe? Or have they actually dropped below the top if the lifter bore? Or do you mean the cup in the lifters where the pushrod sits has sunk inside the lifter itself?
Unless the cam has broken and part of it has moved, I can’t see how a whole lifter can sink? Lifters can collapse internally I guess, I assume they were new?
Are the pushrods all the same length?
Have the rocker studs pulled free of the head?
Thought of another, are the valves on 5 and 6 actually closed?

Edited by user Wednesday, 17 March 2021 9:42:53 PM(UTC)  | Reason: Not specified

It's only 5 and 6 all pushrods are the same and all new lifters oil pressure was 20 psi at 700 rpm and I primed all the lifters with oil for days also all new valve springs

Is the cam a new billet or re-grind?

Just spent the day pulling the cam out it's broken at 5 and 6 not sure why no con rods have come loose or any other damage lifters are ok push rods are all ok valves are seated and ok It was a brand new crow cam had new cam bearing installed with it everything in the engine was brand new and well lubricated on instalment dose anyone no why this would happen or would it have been faulty to start with just waiting from the company I got it from to answer back as I don't want it happning again with another new cam I wouldn't think 750 revs being tuned and 20psi oil pressure would do it

Yeah had heaps of cam lube on it we also had to tune a new holly 350 carb that was being installed and set the timing before we could run the cam in but didn't get to that stage as it sounded like someone running along a corrugated fence with a screw driver before it was shut down we got as far as the timing had just been set it was idling great sounded great then the cam let go.

Yeah, that stuff does need to be done, I suppose you just have to mimimise the time taken as far as possible. It must have been faulty by the sound of it

The broken cam was really the only answer to your symptoms as described. All the other stuff was just in case your description was misunderstood. I’ve always stressed about breaking in flat tappet cams on new engines. Last time I did one I bed the rings in using an old cam and lifters, then put the new cam in after a week. The 377 SBC I’m building now will be a hydraulic roller to avoid it altogether, but I’m also going to get that engine run in on an engine dyno.

Maybe it's just fatigue a old Holden red Cam could be 57yo now.

I have been warning people about that they must be using the correct oil for the flat bottom now for years but they just do not listen, they have come to me and said what is this problem from, lifters and Cam lobes totally rooted.
They claim Oh they use the best oil brand and full syn. No! that is not the point at all. the flat bottom lifter needs Zinc etc and only because of ADR laws was such taken out of oil because they claim Zinc can eat out the cat converter.

I like flat bottom lifters batter than roller lifters and all the problems that can come about from that crap. Them Gen 3 plastic lifter buckets are shit ! make sure you replace them with good quality ones, the early ones are crap, but if you are changing a Cam do not use the old buckets at all toss them, or you will destroy the roller lifter.

Roller lifters were a big problem with Mobil 1 full synthetic that I seen, then they changed that oil later down the track to improve it, I believe that the first type oil did not roll the lifters roller and this put pressure on one side of the roller bearing inside of it and wore the roller on the outside on one spot just a touch and the bastard will not spin, now not all the lifters suffered from such, as the ones that rolled a bit I believe were fine, it's the same with some Truck gearboxes the needle bearings have to move and roll around a bit or they will not last regardless.

Oil mainly only stops the metal from touching and dirt and fuel and water etc are what causes the grit between the oil and that is what causes the wearing out regardless, so a mate had a New KTM 380 2 Stroke and after a long week end I dropped my engine oil on the dirt and so did 4 others the same but the KTM bloke said no he uses the best synthetic so it needs not to be changed he claimed, well we got him to drain it and when we came back to his bike their was a pyramid of metal like you would not believe sitting on the dirt, I said their you go ! your bike does not have a oil filter so regardless what metal is in your oil is going to cut loose inside, I used auto transmission oil in my 2 strokes it's cheap and I just drop the basted freely every 2ed month or so.

I have heard turbo bearings rattle or even a stupid winding noise going up and down, such as that can be cured with using different oils that will make a rattling type or a noisy winding noise turbo, just disappear directly.

A roller or ball bearing when you think about it, if it does not spin you are wearing only a % of the bearing, but if she rolls you use all of the bearing surface.