> > Has anyone every bought forged pistons for their LBC?? If so what make
> > and how much?? That is a major part of the cost (about $5-400) whereas
> > the standard TR 6 pistons (which fit if you 40 thou overbore) cost less than
> > half this.
Forged vs. cast pistons are an interesting study. Forged pistons, because
of the manufacturing technique (literally, bang on 'em with a giant lump
of metal), run larger bore clearances than cast pistons, but are stronger.
So for race motors or others where strength is more important than oil
consumption, forged pistons win; for typical street applications, cast
pistons are strong enough up to about 10:1 CRs, and have far less blow-by
and associated oil burning.
There's a compromise piston manufactured by Hepolite, under the PowerMax
brand name. Their technique, called squeeze-casting, uses elements of both
forged and cast pistons: the pistons are cast, then the mold is struck
under pressure. This gives something like 80% of the additional strength
of a forged piston while reducing the need for the larger bore clearances
(and I've never read a really good explanation of *why* forged pistons
require additional clearance), making them a great street high-performance
choice.
> I put Venoulia forged pistons in the motor of the 69 MGB. When I priced
> off the shelf sets I found that forged were only on the order of 15-20%
> more than uprated pistons from the usual vendors. I also liked the fact
> that since they are made to order, one can specify the bore and compression
> desired. Keep an eye on what connecting rod types (specifically the pin) are
> required for the pistons. As I recall, the straight rods are slightly
> preferable over the offset ones from different years.
Don't know about Spitfidget motors; on the MGB, it's the other way 'round.
The 18G[B...F] con rods are the rods of choice for 5-main engines, as
they have the angled big end connector. This puts stress loadings through
the entire bolt rather than just through the threads. The angled con
rods will work in 18V blocks, but you may have to do a little filing to
get clearance. Also, they only go one way 'round, and I can't recall now
which that is... but I've got a picture at home, taken of the angled rods
in my 18V race block. I'm pretty sure that it's with the rod toward the
cam and the cap toward the distributor.
For extra yuks, you can get Teflon-coated buttons that fill the space at
the sides of the pistons where you insert your circlips. If you're on a
budget, you can modify VW piston circlips to fit your MGB piston; you'll
need to bend the lopcking loops inward just a bit, but they work. Note
that all piston sets should come with the correct circlips, but if you've
bought a set of good used .030" oversize pistons from a friend for a case
of Newcastle and he doesn't have the circlips any more, you can use a
set of Beetle clips if the guy at All-Parts takes them out of a VW kit, hands
them to you, and tells his assistant, "Send these VW pistons back to the
manufacturer, they left out the circlips." It pays to spend a few thousand
dollars on parts over the course of many months. :-)
Oh, right. At the little end of the con rod, there are two different
techniques for holding the piston onto the wrist pin. Early (3-main, 18G
and, um, GA?) engines have a little nut on the con rod that you clamp
down on the wrist pin, then the pin floats in the piston. Later Bs,
all the 5-mains as far as I remember, use a ful-floating wrist pin that
you install by heating the piston to expand the hole (hi Mark!), then
slipping the wrist pin through all three pieces. To keep the ends of
these pieces of hardened steel from sliding out and ripping huge chunks
out of your cylinder bore, there are circlips that fit into grooves in
the wrist pin hole. The wrist pin is a precision fit in each piston
and should not be swapped between pistons.
On 3-main MGB motors, there used to be a special set of con rods that were
offset in such a way that there were effectively unique rods for each bore
position. The distance between the #1 and #2 rods is less than the distance
between the #2 and #3 rods (though of course the journals are evenly
spaced). Can't remember why they did this, though I think it was to even
out high-rev imbalances on the 3-main crank. This was a racing only trick
back in the days of the Abingdon Special Tuning shop.
The 18V rods make nice paperweights. The crank isn't much better; when
they commonized the MGB's engine with the "vertical" (hence 18V designation)
B series in late '71-early '72, they went with a cheaper specification for
the crankshaft material as well (EN16B instead of EN19B as were the early
M.G. cranks; Mini Cooper cranks are of EN40B alloy, a significantly harder
alloy if I read the specs correctly). My 18V engine has in fact worn out
the crankshaft fairly early in its life -- this is the one we had to grind
down to .040" undersize on the rod journals. The one good piece about the
18V is the block, as it already has the relief passages cut into the deck
to help exhaust gases flow out the valves. I'll probably keep the block
but replace the crank; I don't like the idea of turning a ground-down
crank at 7000 RPM for very long anyway.
> A related question... What is the best way to check valve/piston clearance?
Pull the head, put modeling clay (Plasticine) on the top of the piston(s),
then install the head, retorque it, turn the engine over by hand a few
times (with the plugs out to save your arm), then pull the head and see
how thick the clay is where the valves pressed into it.
> Really needs to be
> done when the head is all set up and torqued down but then there is no access.
That's what makes the clay such a neat trick: you measure with the head
torqued into place so that the gasket is compressed and everything.
> I'm interested in changing to the higher lift roller rockers that are
> available but do not want to mess with the (balanced) lower end. Certainly
> would like to determine the clearance before ordering.
Be sure to allow a certain amount for connecting-rod stretch. From the
inertia and heat, the con-rods actually get longer on the upstroke, which
can reduce valve clearance. Unless you're already running wild compression
and a very high-lift cam (I've *got* to call Piper for the specs on my
cam for the engine-building article), you'll be pleased to note that the
MGB engine has a pretty heavy safety tolerance built in. (Oh, nevermind,
isn't this your super-cool alloy head we're talking about?)
--Scott
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