Bent camshaft detection

[bjde0b]

I am asking another one of my stupid weekly questions. I picked up my heads and block from the machine shop yesterday. The heads look great, paid more than I wanted but they look really good.

The machinist has been doing engine work for 40 years and has had the same shop for 30, so I know he is experienced. I had him install camshaft bearings for me. I forgot to take mine in for him, but he said that he had one laying around that he was able to put in without any problems. When I got home I tried to put my camshaft in and I was able to slide it all the way in, but it was tight. When I tried turning int, it turns freely for about 115 degrees but is pretty tight the rest of the way.

Could this be from the bearings or is it the cam, I don't have a dial indicator to test for it being bent, but I think that the tightness over a partial turn would indicate it is bent? I would appreciate various thoughts.

[lee_ford]

Two things to consider: Bent Cam and/or Cam Bearings not lined up straight.

Remember simple science...
A straight shaft in a straight hole turns easy.
A straight shaft in a bent hole (that it fits in) turns easy.
A bent shaft in a straight hole (that it fits in) turns easy.
But... A bent shaft in a bent hole (that it fits in) turns only part ways.

You got the cam in, right?
So, it fits into this last one, bent cam AND cam bearings not perfectly lined up.

Remember, we are talking thousandths here, so do not give the guy any lip. YOUR cam is bent.

You may need to buy a new cam. I would take the cam down to the shop and ask him to check it. If your cam is within tolerance, he needs to hone the bearings in the block a tad more or he may want to polish the bearing surfaces on your cam to get it trued up. I would bow to his judgment on this.

DO NOT, I repeat, DO NOT just install it and hope for the best. It can cause major damage to both the Cam and bearings long before either one wears enough to fit right.

[BobbyFord]

My bet is the guy buggered a bearing on the install...
Especially if it's the same cam you had before.

[Johnm]

I dont know how much this means to you,but....im taking an Engineering Materials Science class in college right now and the amount of stress it would take to bend the cam and keep it bent is a LOT. Even the bast make mistakes sometimes.
seen here the straight line on the left is when the metal is "elastic" and will return back to its normal shape after a stress is applied and released. where the line goes up and down rapidly is called the yield strength. after this point the metal keeps the stress strain that has been applyed.

[fordman]

i am betting on bed install of the bearings. i do know they have to go in a certain order to get them in. and keep your tool. is it hanging on a bearing jornal? can you see how far in its gettign and where it might be hitting at?

[DuckRyder]

Camshafts are pretty brittle, I really don't think that you could bend one without it breaking.

[bjde0b]

Thank you for the discussion. I was planning on taking the block and cam to him on to see what he thinks. The camshaft is hard, but it can still bend. With the reciprocating stresses that occur, it is a possibility at elevated temperatures.

Johnm - glad to see you are working on an engineering degree. I have a couple of Mechanical degrees myself., and the only reason I am saying that is to prerequisite my next statement. When looking at the stress-strain curves, you have to be sure that you are looking at the same metal at the temperature of operation. The second is there are a number of alloys and treatments - which I am sure you are learning a lot about in your Materials class. You have to be sure that your curves are matching for the treatment that the metal has been through.

I work with rotating equipment for an Oil and Gas company, and the internals of engines are very similar to reciprocating compressors. When we are purchasing some of our multi-million dollar pieces equipment, we have a PMI done (positive material identification) that will give us the exact stress strain curve from the same material that is in our equipment. The same is done with failures - a sample is taken to the lab from the failed piece.

[lee_ford]

Let me better explain…

...YOUR cam is bent. ..

The intake and exhaust lobes on the very end of the cam are both in the same quadrant of the cam. This continued offset of pressure from the valve springs over time will cause an uneven wear of the end cam bearing surface. While the cam may not be BENT, it may be worn in a way that when measured the cam bearing surface on the end will be off compared to the bearing surfaces in the middle. This bending from valve spring pressure would more than likely spring back each time the cam turns. After all it is a very slight bend, but the wear would real. Remember we are talking thousandths here.

So I guess instead of BENT, I should have said unevenly worn. This could be the ONLY reason I can see that it would bind when your cam in installed and not with the cam the mechanic has. I bet your mechanic will get it right now that he will have both block AND cam.

I might still opt for a new cam.

[averagef250]

The machinist did not have the cam bearing installer tight inside the bearing when he tapped it in. The loose tool banged up the leading edge of one or more of the bearings and the lobe(s) is dragging.

The reason for the 115 degrees or so is the edges of the lobes are not typically machined. They are rough cast and vary in width.

I've done this myself. The fix for me at the time was a couple swipes with a fine round file to knock the raised lip off the edge of the bearing. On a performance engine I would replace the cam bearings.

Can't say I've ever seen a gas engine camshaft with detectable bearing journal wear.

[lee_ford]

I've done this myself. The fix for me at the time was a couple swipes with a fine round file to knock the raised lip off the edge of the bearing.

Nothing like the voice of experience.

As for cam wear, I was looking for some explanation and I have seen a worn out cam. It happened when the owner ignored the low oil “IDIOT” light until his engine started smoking like a freight train. His cam when we got it out had a kind of blueish tint to it. Of course the crank, rods and piston rings looked a little tempered too. His answer was, “I put STP in it.”

[averagef250]

It's easier to do than you might think. Cam bearing installers have a very small lip to push on the bearing with. If the installer isn't fully tight (you think it is, but actually isn't) the edges that are intended to push the bearing in will deform it.

I would expect the cam lobes to show wear long before the journals did. The journals don't have much load on them in your average OE engine. Many modern engines don't even run cam bearings anymore.

[bjde0b]

I took my block and cam by the machine shop and the guy laughed at me, not in a mean way because he knows I can take it. I put a little oil on it and spun it and told me to go put my engine together because the camshaft was good.

[70shortwide]

did you assemble it with lots of lube? everyone has a favorite lube for assembly. someone here will have an educated opinion on what works well. definately lube it with something though!

[bjde0b]

He said I had the cam too far in the hole. When I go home I installed the cam thrust plate and pushed the cam against it while turning. It turned pretty easy. There is a little tightness but not anything prohibiting the shaft from turning. I haven't started putting it together, but I will take advice on assembly lube if someone wants to give it.

[averagef250]

On a stock lift type cam lubriplate is great. If you have something more radical use the lube that comes with the cam or buy some extra from comp.