Head gasket - reuse the old head or not

I have an Isuzu inline single cam 4 banger. A head gasket replacement is
in the near future since I seem to have a bit of coolant getting into at
least one cylinder. The engine has 187K on it. It has been well maintained
by myself since I acquired it around 100K and runs strong with very little
oil usage, and has never overheated while I have owned it.

I also have a completely rebuilt cylinder head sitting around that I got
for next to nothing.

Should I reinstall the old head (after making sure it is straight) or go
ahead and install the new one since I have it? I would think this would
be an excellent opportunity to do so, but I have heard that installing a
new head on an old block will give me problems with excessive blowby -
but is this only if the head was milled down and I don't use a shim to
restore the correct clearance/compression?

Unfortunately the papers for the new head were lost so I don't know how
far down it was machined. Maybe I can measure this relative to the
old head somehow?

"Ryan Underwood" > wrote in message
news
> I have an Isuzu inline single cam 4 banger. A head gasket replacement is
> in the near future since I seem to have a bit of coolant getting into at
> least one cylinder. The engine has 187K on it. It has been well maintained
> by myself since I acquired it around 100K and runs strong with very little
> oil usage, and has never overheated while I have owned it.
>
> I also have a completely rebuilt cylinder head sitting around that I got
> for next to nothing.
>
> Should I reinstall the old head (after making sure it is straight) or go
> ahead and install the new one since I have it? I would think this would
> be an excellent opportunity to do so, but I have heard that installing a
> new head on an old block will give me problems with excessive blowby -
> but is this only if the head was milled down and I don't use a shim to
> restore the correct clearance/compression?

Sort of...Sometimes when you grind the valves you increase the combustion
chamber volume
which offsets, to a degree, a mild truing cut on the head. You can CC the
combustion chambers
if you really want to know.

The real situation is that if your rings are worn a bit, you might not see
the effect with an older
head with leaky valves, etc AS MUCH AS you may see it when you install a
head that is working
as it should. At 187,000 miles, I would not be surprised if blowby
increased....even if you just
'sharpened' the valves and did not machine the head surface at all.

"Ryan Underwood" > wrote in message
news
> I have an Isuzu inline single cam 4 banger. A head gasket replacement is
> in the near future since I seem to have a bit of coolant getting into at
> least one cylinder. The engine has 187K on it. It has been well maintained
> by myself since I acquired it around 100K and runs strong with very little
> oil usage, and has never overheated while I have owned it.
>
> I also have a completely rebuilt cylinder head sitting around that I got
> for next to nothing.
>
> Should I reinstall the old head (after making sure it is straight) or go
> ahead and install the new one since I have it? I would think this would
> be an excellent opportunity to do so, but I have heard that installing a
> new head on an old block will give me problems with excessive blowby -
> but is this only if the head was milled down and I don't use a shim to
> restore the correct clearance/compression?
>
> Unfortunately the papers for the new head were lost so I don't know how
> far down it was machined. Maybe I can measure this relative to the
> old head somehow?
Ryan, in older engines with poorer rings as well as other components,
when one did a "valve job" w/o installing new rings, it would introduce new
& higher compression (and pressure on the rings) and cause blow-by and oil
consumption. But with modern engines, if oil usage is not a problem now,
then either re-doing your head or using a newly rebuilt newer one (which I'd
do one of regardless, if I went so far as to replace a head gasket)should
not cause old-timey harm, like oil usage. When I say older, I'm talking
about pre-1970--times before moly rings and other improvements which
increased piston seal and decreased cylinder wear.
HTH & good luck. s