Re: A raft of engine rebuild Q's

[Dan Bubb <jdbubb@ix.netcom.com>]

Two minor points:
After doing any work on a crank, such as knife edging, it is always
balanced as the last item. Knife edging will have minimal impact on
crank strength, since it doesn't remove metal in the primary load path.
A three angle valve job doesn't refer to angles on the valve itself. It
refers to the valve seat in the head and a relief cut on the inside and
outside of the seat. Actually, a three angle valve job is pretty
standard because the relief cuts are needed to get the seat at the right
diameter and the right width. It is a good idea to smooth the edge on
the valve on the inside of the seat since it helps airflow.
And a third point just for shits and giggles. Valve springs are cheap
and they are one of the highest stressed components in an engine.
Dan

Vee Dubb wrote:
> 
> Knife-edging the crank means machining off any non-critical material from
> the crank and leaving a relatively sharp edge where the counterweights on
> the crank used to be.  The "knife-edge" cuts through the oil reservoir
> better and reduces viscous drag on the crank as it passes through and
> reduces foaming of the oil.  Removing or reducing the counterbalances on the
> crank can leave it unbalanced, which will accelerate fatigue of the
> crankshaft (usually not a problem) and accelerate wear of the crankshaft
> main bearings.  The engine will rev faster.  Because of the accelerated wear
> problems, I would only suggest doing this if you have already done
> everything else to lighten the rotational mass (i.e. machined ALL extra
> material off of the flywheel).  Either of these will lead to slightly more
> driveline vibration, so have good motor mounts on the car.  For my 16v, I
> would consider knifing the crank pretty far down the list of performance
> mods just because of the longevity issues it raises.
> 
> Micropolishing reduces surface imperfections that could otherwise lead to
> cracks.  I don't know how routine or practical this is, so ask around.
> 
> The idea behind putting 3angles on a valve is essentially to retain a good
> sealing surface with the first two angles, and then to knock the sharp edge
> off of the circumference of the valve with the third angle.  Any sharp edge
> left on any part creates a place with lots of surface area and little
> volume.  Heat can enter along the ample surface, but can't diffuse quickly
> through the volume toward the cooler center of the valve (or piston or
> whatever).  So the edge is called a heat concentrator, and the temperature
> of the metal there will be higher than in the rest of the surfaces.  High
> heat concentration zones can cause detonation of the fuel, or more
> specifically, pre-detonation, a.k.a. knock.  To reduce knock in an engine,
> eliminate the sharp edges in the cylinder by polishing the cylinder head
> (you can do this yourself, just like porting / polishing the intake), and
> cutting that edge off the valves with a 3-angle valve job.  These steps
> aren't really necessary unless you are raising the compression ratio.  16v's
> use 10:1 compression with premium fuel and have the same 2-angle valves and
> cast combustion chamber surface as your engine will have.  Don't worry about
> these things unless you excede this 10:1, or wish to use lower grade fuel.
> 
> I've seen hand-lapping done, but I don't know why it's done.  My first
> impression would be that it basically can clean up the valves nicely, but
> you won't be able to change the angle (i.e. add a third angle) by this
> means.  Lapping is basically polishing.  The tools to do this are pretty
> basic, so at least check to see how much they would cost to do it yourself.
> 
> Reusing valves and springs is relatively safe.  Having them magnafulxed
> would be nice, otherwise just inspect them well.  If the valves are all
> straight, and if they show no pits from corrosions or burns, you can
> probably re-use them.
> 
> I'd replace all bearings (cam, crank, and rod).  That's kind of the point of
> an engine rebuild, isn't it?
> 
> Also, Deck height check should be dirt cheap and a usefull measurement
> towards checking compression ratio.  It's not really important unless you
> are planing/decking the head for higher compression.  Magnafluxing is a must
> (yes, to check for cracks).  No sense in investing in a bad block.  Honing
> the cylinder walls should also be cheap, and very do-it-yourselfable.  Some
> Autozone/PepBoys type shops will rent a cylinder hone.  Ask them how to use
> it, or ask the list.  Basic idea is to run the hone up and down in the
> cylinder to create a spiral up and down the walls with a 45° incline to it
> to provide a smoother surface than machining provides.  The sprial pattern
> helps oil distribution along the cylinder walls.
> 
> Basically, similar to other's suggestions, I'd do "everything" to the block,
> including magnafluxing, thorough cleaning (don't know about the freeze plugs
> on this engine in particular), new bearings, and honing.  Re-boring the
> cylinders will require new pistons, and it doesn't sound like you are up for
> that.  Shouldn't need to be done unless the walls of the cylinder are
> scratched for some reason, or if you just get bad compression (i.e. bad
> "dry" compression test, but good "wet" compression test).  Start with those
> basics.  Lap the valves and seats to get them nice and clean, and polish the
> inside of the combustion chamber if you are bored and have something to do
> it with.  Don't chew up the valve seats while trying to polish the chamber.
> 
> A tap will work as a thread chaser.  I don't know if anyone sells something
> other than a tap to do this, but I'm sure someone will take your money for
> something by that name if you ask them.  Don't know what taps you would
> need, but if you have the bolts that will go in the holes you can measure
> them easily enough.  Metric bolts only have one tap size for a given bolt
> diameter.  That's the diameter of the bolt, not the head of the bolt, but
> the two are related.  If you have no other way to measure it, go to Home
> Depot and buy a micrometer to measure the diameter.  Or you can borrow the
> micrometer out of the package there in the store (I'll admit to doing this
> only because I already own a micrometer).  You will probably find that it
> costs more to buy the tap set than it does to pay someone to chase the
> threads, so consider whether you would ever use the taps again (bearing in
> mind that much of our world isn't metric (unfortunately)).
> 
> Freeze plugs are made to let the water in the engine block expand when it
> freezes, pushing the plugs out instead of breaking the block.  Galley plugs
> are actually orifices (plugs with holes) that regulate the flow of oil in
> various passages.  Otherwise either plug could just be sealing up a hole
> that was needed to make a passage.  Don't know about removing them for
> cleaning.
> 
> Don't know about the paint first hand, but it's an iron block, so I would
> assume it should be painted.  High heat engine paint is available at auto
> supply stores (in lots of nifty colors if you are so inclined).  A definate
> do-it-yourself thing.
> 
> Any responses are welcome.  I'm no expert.
> 
> Sincerley;
> 
> Brian Haygood
> 
> --
> Email LIST problems to: scirocco-l-probs@scirocco.org.
> To unsubscibe send "unsubscribe scirocco-l" in the message to majordomo@scirocco.org

--
Email LIST problems to: scirocco-l-probs@scirocco.org.
To unsubscibe send "unsubscribe scirocco-l" in the message to majordomo@scirocco.org