Zahid writes...
>My 1980 Spitfire 1500 still has blowby after the rebuild. It could be
>because I lapped the valves instead of machining it. Usually cars have
>some sort of crankcase venting that burns the secondary mess through
>the carburettor, one comes from the valve cover and the other through
>the bottom of the engine block and feed through the carb. But mine
>has only the valve cover venting to relieve the backpressure. If I
>put the oil cap on, the pressure through the crankcase causes oil to
>spew out of every nook and cranny including the dadgum dipstick hole
>placed next to the exhaust causing a smoky mess at high rpm.
>You usually figure out your rpm is high when people start honking behind
>because the smoke is causing them to gag. One of the main causes of this
>backpressure is that I have Webers and the large venturi is, as far
>as the engine is concerned, uncool. I do have the valve cover vented
>to the carburettor but it is not enough to relieve the pressure.
>So my question is, how do get rid of this backpressure and how come
>the bottom of the block does not have some form of venting? The pistons
>do create some pressure in the crankcase because of the downward motion
>that must be vented. Is this an oversight? somehow I don't think so. I
>would like to keep the Webers and the oil cap on because it is creating
>an unsightly mess. The car has no emissions rig attached.
Zahid--sounds like to me we have the same problem. I rebuilt a 1500 cc Midget
engine and did
grind valves and seats, so I don't think that is a factor. At first
reinstallation, I got
oil out everywhere, especially dipstick tube. I replaced (twice) the oil pan
gasket before
getting it right (used an rtv gasket compound put on very liberally with no
gasket, torque
bolts very little until rtv sets up, then retighten). I also had a problem
with the shield
which is mounted inside the oil pan at the rear. It apparently shields the
rear seal from
direct splash, and it got bent a bit during time on garage floor. It
interferred with the
block in such a way as to prevent proper seal of oil pan to block, but not
enough to notice.
Anyway, by straightening that shield, the pan went on/off much easier, and we
finally got it
sealed. I haven't had any problem with oil coming out of valve cover. Mine
has an open vent
(no vacuum connection to the Weber--I tried that and it didnt seem to help me).
I had oil
blowing (being thrown?) out distributor mounting also, which seems to have been
eliminated with more careful installation with another new gasket. I still
keep a 3/8 in.
wooden dowel plugged in the dipstick tube, and with it in place, I don't get
any oil out
anywhere! With the dipstick in place, I get some oil thrown out, which runs
down the edge of
the pan and drips onto the exhaust (James Bond had a smoke screen button in his
lbc, too).
My dipstick doesn't have any sort of gasket under the lid--I made one from
innertube, which
stopped (nearly) the leak. The gasket swelled, though, and I had to go back to
the old
reliable dowel.
I don't think it is blow-by in my case--just oil being thrown everwhere inside
the engine,
and splashing out of the dipstick and anywhere else not well sealed. I thought
a long time
about your theory of piston going down, compressing air in block, but decided
it won't fly.
(There is always a piston going up balancing the piston going down.) The only
pressurization
angle is if there is blow-by thru the rings. Since my engine had 6 mil piston
clearance and
sticky rings prior to rebuild, and *perfect* (?) fit plus new rings after
rebuild, and since
the amount of leakage was no less after rebuild (before I got all the leaks
sealed good) I
conclude that the blow-by theory doesn't explain my problems. The splash-out
theory appears
to be a better explanation. Try putting your finger over the dipstick tube (or
put a
pressure gage on it) to test your theory.
In short, I don't think the problem is in need of a vacuum solution, but a
sealing solution.
It does still bother me a bit that the dipstick needs such a good seal, but it
seems to work
with fine for me with the dowel plug (I didn't even consider this qualified for
entry into
the bodge contest.) I plan to make a gasket for the dipstick out of the same
rtv compound I
used for the oil pan. I'm confident that will stop the leakage. Right now,
with the dowel,
it is as leak-free as any MG I ever drove. Will others with this engine please
describe the
gasket in their dipsticks..cork, rubber, nothing,...?
Good luck, and patience (as I kept telling my 17 yr-old son, the primary
driver and
assistant mechanic in this project, "It's only a *machine*, surely we will
eventually figure
it out!")
Ray
-------------------------------------
Name: Ray W. James
Texas Transportation Institute
Civil Engineering Department, Texas A&M University
E-mail: Ray James <rwj4123@sigma.tamu.edu>
Date: 10/04/94
Time: 13:29:20
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