Posted From: 126.96.36.199
|Posted on Monday, 29 September, 2014 - 07:53 am: |
The camshaft is about the most highly stressed bit in an engine and it's a marvel that they last as long as they do.
One cam profile is pretty much the same as all the others.
The bit that wears is the apex or lobe top so to check measure with micrometer from apex to base circle and check the manual within 2 thou is acceptable.
An advantage with push rod engines is the cam is lower than the valves so loads of oil can be present without flooding the valve stems.
The camshaft journals because the lobes have to clear the brgs the brgs and journals are large which makes them seem to last forever and in most pushed engines never need changing.
An added complication is the eccentrics for the brake pumps and on other engines the fuel pump. The brake pump eccentrics should be flat a groove means wear and less pump stroke and maybe no brakes.
There is a way to repair but the cam will have to be removed and the engine must be removed to get the cam out so all in all a nice New cam is probably best.
If the cam lobe can't be directly measured them use a dti on a tappets method. The base circle rarely wears.
most times cam wear is obvious by sight.
Lifters. The tappit should with oil slide slowly through the tappit guide block under its own weight a little bit of a light finger push is ok. It must not be tight neither should fall through fast with oil.
The face of the tappit rides on the cam, this is highly loaded and it took years for the materials guys to get it right.
if the face of the tappit has straight lines then it has not been turning in the guide block. Of the lines come out by polishing with 400 grit then the tappit is ok and the outside needs a polish. If the lines a swirly the tappit has been revolving. If the swirls polish out then good if not as with the straight lines the tappit is scrap. Polishing a thou off the face is ok. The face should be flat not concave if it's concave it's scrap.
Hydraulic lifters are reliable and very simple and the only thing the goes wrong is they can gum up.
To clean, some like RR come apart others are sealed. To clean sealed ones compress gently and slowly in a vice to squirt the oil out and soak in paraffin. The paraffin will be drawn in, then squeeze again. The crimped ring can be removed and discarded. The push rod will hold the plunger in.
Sometimes the spring under the plunger can get tired.
If a new cam is fitted then the tappets must be replaced. No supplier will guarantee a cam only.
The cam kit will come with a break in oil which is smeared on the lobes. This oil is loaded with zddp zinc. The amount compared to the amount of engine oil is small and will be quickly washed away by the engine oil.
the initial start up is what worries the cam guys. So start engine idle for five minutes and take in easy for the first mile or so, after that drove as normal. Depending on what else has been renewed.
If a new cam kit is faulty the fault often shows up very quick often before the bonnets shut. I had a new cam last 5 minutes before it ate its self. The cam had be reprofiled and not re case hardened.
If the head has been skimmed check that Hydraulic lifters are in their operating range. And if the engine is a Rover V8 then check even if just fitting a cam kit.
(Message approved by david_gore)