Valve Stem Seal replacement

I have a line on a seemingly nice 1984 Honda 700 Nighthawk S, but the seller
says that the valve stem seals need replacing. He already has the parts.

Is this a relatively easy job that I could try to do myself, or should I
take it to the dealer for this repair? How much would a shop charge to do
this job?

Thanks.

 

I could do this job. Since you have to ask, you probably cannot do it.
A $tealer$hip mechanic would probably around $500.00 for labor, plus
parts.

The question you should ask yourself is, "Does this job really really
really need to be done?"

How bad is the oil smoking? Worn out valve guide oil seals will allow
oil from the top of the cylinder head to seep into the combustion
chambers and the engine will smoke for a minute or so after starting,
then it will stop smoking.

The oil that leaks onto the exhaust valves will burn off, because the
exhaust valves will get a lot hotter than the inlet valves.

The oil that leaks onto the inlet valves will eventually cook itself
onto the
valve heads. This may get so bad the inlet valves cannot close
completely and the inlet valves may burn.

The engine will lose compression and you'll hear it spitting back
through the carburetors when you first start the engine. The
compression loss will eventually be so bad the engine cannot suck in
enough fresh fuel air mixture because the rising piston pushes air
backwards through the intake tract on the compression stroke.

You can combat this oil accumulation on the valve heads by running
Chevron's Techron Concentrate through the gasoline on a regular basis.
You can probably get 12 ounces of Techron at Wal*Mart for $5.00.

Also, ask yourself if you mind adding a quart of oil every 300 to 500
miles because the engine smokes. 1000 miles between adding half a quart
of oil is not such a bad condition for an engine, and I wouldn't mind
adding oil every 500 miles to avoid tearing into an engine, if I could
buy the motorbike really cheap.

As far as compression goes, I wouldn't tear into an engine unless it
had less than 135 pounds of compression in the cylinders.

You'll need to buy a repair manual, like Chilton's or Clymer's or the
dreaded Haynes manuals with the nice drawings and inaccurate text.

Ordinarily a top end repair job requires removing the seat, gas tank,
airbox, carburetors, exhaust system, cylinder head cover, camshafts,
cam chain tensioners, and cylinder head. The valves and the valve
springs have to be removed to install the new valve guide seals.

With all that out of the motorbike, you'd be silly not to have the
pistons re-ringed and the valves lapped.

You could save yourself a lot of money by removing some of the parts
that you think you can successfully reinstall yourself, but some
$tealer$hip $hop mechanic$ won't touch a job where the motorcycle was
partially dissassembled.

And the $tealer$hip$ often won't work on a motorbike that is ten years
old and is discontinued. They want to do maintenance procedure$ on
newer machine$, because that's where the ea$y money is at for them.

But, there is another way to replace the valve guide seals without
removing the cylinder head. You remove the valve cover and the
camshafts and the tappets with the cylinder head still installed.
Remove the spark plugs from the cylinder head and run a long string
down into the cylinder you want to work on. Then you turn the
crankshaft until that cylinder comes up on top dead center. The string
is compressed by the piston. With the valves prevented from dropping
into the cylinder, you can then compress the valve springs and remove
the valve keepers with a magnet.

There are special valve spring compressors for car engines that attach
to the cylinder head so you can compress the valve springs with the
head still installed. If you have a good imagination, you may figure
out a tool for doing this to a motorcycle engine. You might make a
valve spring compressor out of a piece of PVC tubing with a big hole
drilled in it for fishing the valve keepers our of the valve spring
retainer cap.

Then you could use the hooked end of a machinist's scribe to remove the
old
valve guide oil seal. Cover the end of the valve with plastic tubing so
the lip of the new oil seal won't be damaged as you slide it down over
the valve stem. Then you can reinstall the valve springs and retainers.

With the camshafts out of the engine, all the valves will be closed so
you can turn the crankshaft all you want without hurting anything, just
turn the crankshaft carefully and keep track of the cam chain.

Before you put the camshafts back into the cylinder head, you will coat
the bearing surfaces with a thin layer of assembly lube to avoid
scoring the aluminum bearing surfaces when the engine first starts.

When it's time to reinstall the camshafts, you have to line up the
crankshaft
so it's at top dead center and align the cams so they are set up for
the number 1 cylinder to fire (all the valves on that cylinder will be
closed with the cams installed).

When you go to torque down the cam bearing caps, that has to be done
with all the bolts clean, dry and and oil free, and the holes that the
bolts go into have to be clean and oil free also, or you will probably
break a cam bearing trying to torque the oily bolt.

Breaking a cam bearing cap is a major disaster, since all the cam
bearing were installed in the head when it was manufactured and the
bearings and head were all line bored as a set.

Reinstalling the exhaust system can be a nightmare, too. Home mechanics
often strip out the exhaust pipe flange bolt holes, or they cross
thread a spark plug.

If all this mechanical talk terrifies you, maybe motorbikes are not
your cup of tea...

 

Skimmer, thank you for the very helpful post. I won't try to do it myself,
if I buy the bike.

 

Your best bet is to look for an independant mechanic with a good
reputation.
Some of them are very slow, but you can't expect to rush one of the
patient methodical old time mechanics with a lot of pressure to "get
'er done." Let a good man take his time to do the job right the first
time.

 

It sounds harder than doing it by the book.

 

Replacing valve guide oil seals without removing the head *is* doing it
by the book, if you have the right book.

But if cranking compression is low, you might as well do the whole top
end job.

 

Replacing valve guide oil seals without removing the head *is* doing it
by the book, if you have the right book.

But if cranking compression is low, you might as well do the whole top
end job.