Re-using old cam in a rebuild.

I know an old camshaft can be polished and re-used, but wouldn't the lobes have worn down, which would affect valve lift? 

Also, wouldn't the cam bearing clearance be too wide as well?
 

Thank you. 

Flat tappet ChevroletFordMopar?  Inspect, run the same lifters on the same lobe they came off of, because new lifters are made of cheese.  If the lifters are no longer slightly convex, throw it all away and buy a roller cam and lifters.

Anything from anybody built in the last 25 years has rollers, and damage will be fairly evident on careful inspection.

Btn74 said:

Also, wouldn't the cam bearing clearance be too wide as well?
 

Thank you. 

Everybody knows cam bearings are only bad if you check em!

Run it!

If it's a roller cam, yes.  Flat tappet, I wouldn't run it if I was broke and you offered me $1000.

Flat tappet cams are super-soft cast iron that are induction hardened on the lifter face and lobe surface, but it only goes about 0.005" deep.  When you install new lifters on a new cam and break it in, those surfaces polish and wear to each other.  Even if you meticulously keep the lifters organized to their original lobes, the new lifter bores in the other engine will cause them to spin differently and you risk killing things.  FAST.

Think of it this way.  Take two fresh sheets of 120-grit sandpaper and rub them together.  They'll eventually wear off the bigger pieces of grit until they're smooth.  They'll be fine for a long time that way.  Now replace one of the pieces of sandpaper with a brand new one.  It will tear through the old paper in a few strokes.  That's how flat tappet cams really are.

It's also important to know what kind of cam/engine/setup we're talking about.  A roller cam will lose zero lift over time.  A flat tappet cam can't lose more than 0.005" of lift without ripping into the soft iron below - known as wiping a cam lobe.

You can re-grind a cam, but what you're usually doing is grinding the whole lobe.  If you take off .050" off the peak and also 0.050" off the base circle, you still have the same lift.

I've never really heard of polishing a cam, other than one of the steps to re-grinding it.

If you end up polishing the cam journals (or resizing them) the aftermarket makes cam bearings that are sized to fit so you have the same oil clearance.  It's just like re-sizing a rod journal.  If you grind/polish a crank 0.010" undersize to re-surface it, you just buy bearings that are 0.010" over-thick to compensate.

In reply to Curtis73 (Forum Supporter) :

David Vizard found that spinning a flat tappet cam in a lathe and smoothing the lobes with sandpaper on a block of wood was beneficial for... some reason.  Eliminated patterns ground into the cam from irregularities in the grinding wheel that made it? 

Personally, I'd just install a roller cam unless budget was tight enough that it was worth the risk of losing lobes if things didn't get put back exactly where they came from.  Flattening lobes sends SO much metal through the engine, even in a 20 minute "break in", that you end up having to take the engine back apart and buy new pistons to replace the ones that now have iron chunks embedded in the skirts.  Worst case you also need to bore to the next bore size, if the gouges can't be honed out without wrecking piston clearance.

In reply to Btn74 :

Sorry, I should have said that it's a Nissan KA24E with flat tappets.  Nissan OEM stills makes new rockers for it, but not the cam.  
 

It looks like it'd be best to just run the original head and components, but do a valve job with new valve seals.  Or, look for aftermarket cam with roller rockers  

I really appreciate all the advice, everyone.

Thank you 

The KA24E should have cam followers, not tappets per se.

If they don't look nasty, you're probably good to go. They do not "rotate" like a typical V8 application does.

Oh, yeah, when the question of "reuse cam?" comes up, it's assumed to be a pushrod engine.

OHC engines rarely eat camshafts.  Oddly enough the ones that do, have rollers!

The spring loads are far, far lower on OHC engines so the camshafts last a good long while.

Don't worry about wear on the bearing journals, that almost never wears.  And when it does wear in an OHC engine, it's the bores in the head that wear, and there's usually no way to repair it since very very VERY few shops are set up to be able to align-bore a cam tunnel.

I got away with swapping a small block ford short block and installing the cam and lifters from the previous engine. This was a Competitive Cams 260H that was bought for the first engine in about 1989, had about 60,000 miles at the swap. 
 

Reading this, I am realizing how lucky I was. Obviously, every lifter went back on the same cam lobe. 

My Datsun is a flat tappet engine and I have reused cams at least three times over the years without issue.

I run used lifters and brand new lifters as well.

Note I am running the engine to 8200 rpm.

In reply to Pete. (l33t FS) :

I see.  So I shouldn't have to worry about cam lobe and cam journal wear.  It is an OHC I4 with hydraulic lifters built into the rocker arm end, and flat cam followers.  

How about valve guide? Do those wear very slowly in this application as well? 

 

When I had the head rebuilt on the 280k BMW M30B35 (SOHC I6) I dropped in my E28 a few years back, the machine shop just polished the old cam, threw new lifters in (known weak spot on the M10/M30), and said "go beat on it and have fun".

They did replace the valve guides (among other things).

SkinnyG said:

The KA24E should have cam followers, not tappets per se.

You're right. I see that it does have flat cam followers, not flat tappets

If I was going to the trouble of rebuilding an engine, I'd just do it all fresh. 

But I'm a "while you're in there" kind of a guy. For example, when I built my NA track rat, since I was redoing the brakes and suspension, the suspension got all new OEM bolts/nuts. 

In reply to z31maniac :

I agree with you on doing everything new and perfect while you're in there, using new OEM parts.  The only thing is that Nissan has discontinued a lot of the engine parts.  It's a 1996 Nissan Hardbody King Cab 4x4, which is one of my dream vehicles, and it must live on! 😂.  So, i'm trying to figure out how to do the best rebuild possible without access to new OEM parts. 
 

Thank you everyone for your insight and advice.  Answered all my questions and more.  

In reply to Btn74 :

If you have a worn cam, it's a wasted cam and needs to be reground/replaced, so you're right... don't worry about wear.... unless it's worn :)

Valve guides do wear as the lobe/follower/lifter places a bit of lateral wear on the valve tip.  Easy fix with new guides, but best to determine if they're needed.

In reply to Btn74 :

Are there aftermarket cams that might give a bit more oomph?

In reply to Curtis73 (Forum Supporter) :

So I'd need to pull the cam out, check it with calipers, and if it's  in spec, I'm good to reuse it?
 

I'd much prefer a new cam, but Nissan doesn't make them anymore   

In reply to z31maniac :

I'll research it. I'd ideally like to use the stock cam profile just to keep the engine management close to stock. 

In reply to Btn74 :

I haven't done many OHC engines.  The old-school way was visual and audible.  You hear a tap, you look at the cam, and say "yup, it's berkeleyed."

Unless you have more accurate calipers than mine, you won't be able to tell much.  99.9 times out of 100, if a cam is worn, you'll see it.  No calipers necessary.

In reply to Curtis73 (Forum Supporter) :

I have also never seen a cam that was worn that was not visibly worn.  The nose wears first, and depending on if the follower/tappet/roller/bucket is larger or smaller than the lobe width, the nose will either have a channel in it, or it will be wider/mushroomed out at the nose.

I mean, aside from having visibly worn through the hardening, or heavily pockmarked from wear if a roller.

Cannot stress enough that OHC engines (except for Ford CVH engines) are extremely light wearing for the camshafts.  They have larger diameter lobes because they don't have to slide through a tunnel, and they have much lighter valvesprings.

The CVH is the odd one out because... the cam slides through a tunnel, and it has pushrod looking lifters that act against high ratio rockers that may have been taken from a 351C.  They wore camshafts out regularly.  But when you saw it, you knew without having to dig out measuring tools more complex than the Mk1 Eyeball.

Cannot stress enough that OHC engines (except for Ford CVH engines) are extremely light wearing for the camshafts.  They have larger diameter lobes because they don't have to slide through a tunnel, and they have much lighter valvesprings.

Learning this, it now makes sense to me why the old Jeep pushrod 4.0L I6 has two springs per valve: To deal with the extra inertia of the pushrod.  

In reply to Btn74 :

I suspect a lot of it is how large/heavy the actual valves are.  There are plenty of OHV engines with single valvesprings, but to get the correct rate and travel requires more expensive materials.  Plus, there is a damper effect from the friction between the two springs, which had some benefits as well as wasting horsepower by converting energy into waste heat 

Btn74 said:

Cannot stress enough that OHC engines (except for Ford CVH engines) are extremely light wearing for the camshafts.  They have larger diameter lobes because they don't have to slide through a tunnel, and they have much lighter valvesprings.

Learning this, it now makes sense to me why the old Jeep pushrod 4.0L I6 has two springs per valve: To deal with the extra inertia of the pushrod.  

Yup.  Pushrods and rockers add a significant amount of inertia, but one of the benefits is that they don't occupy as much real estate in the head and lack the complexity of the additional timing chains/belts that go all the way to the top of the head.

OHV - simplicity, but can limit RPMs with the additional inertia.
OHC - complexity, but more flexibility