Tuesday, November 21, 2006

VW - Loose Barrels

Loose Barrels

I am in the middle of building a 1776cc with 90.5mm barrels. I heard from a friend that the barrels are only supposed to move .5mm max around inside the case. Mine however move around at least 1mm. Will this cause any problems? What could I do besides have the case machined to accept bigger barrels and make an 1835?

You may have a problem. 92mm jugs are made from the same castings as 90.5mm cylinders . . . their skirt and head diameters are the same, only the bore diameter is different. That means your spigot bores are already opened up for 92's . . . and may have been opened up too far.

The spigot hole for cast-iron cylinders in a magnesium alloy crankcase must be kept fairly tight due to the difference in their coefficient of expansion. The normal allowance is about one-thousandth of an inch (0.001") of play for each inch of bore, rounded up, to a maximum of about one and a half thousandth (0.0015"). Since the nominal diameter of the spigot-skirt of a 90.5mm cylinder is 3.785", the nominal spigot-bore diameter would be about 3.790". But those are ‘nominal’ figures. There is considerable variation between the various manufacturers and even within them, with one batch of jugs being a thou up or down from the last batch. Whoever opened up your crank case should have miked your jugs and set their tools accordingly.

Having spigot-holes that are too tight results in hard starting, scuffed pistons and in the worst case, a thrown rod. When the spigot-holes are too loose the cylinders shuffle on the case making it impossible to maintain proper tension on the cylinder-head studs. As they loosen up you lose compression, start losing a lot of oil from the spigot-bores and generally end up with a doggy, drippy, unreliable oil-pumper.

Some lo-buck rebuilders of big-bore engines start with a used crankcase, open up the spigot bores to an enormous 3.825" or thereabouts, slather thick layers of blue RTV on the jugs, slap the engine together and cross their fingers. In most cases the thing survives the warranty period but not much longer.

Since your message did not cite specific dimensions I suggest you start there . . . blueprint what you’ve got and figure out if its usable. You can push the figures a bit . . . maybe three thou too fat . . . but any clearance more than .008" or thereabouts is going to produce the problems mentioned above . . . the bigger the gap, the bigger the problems and the sooner you’ll see them.

A properly built Volkswagen engine is capable of delivering twenty years of reliable service. It’s worth doing the job right.

(Revision Notes: This topic has generated a constant stream of mail, most arguing for greater radial clearance, citing the fact that engines for drag racing often use 0.005" or more of clearance per inch of bore and win lots of prizes.

Which happens to be a completely different subject.

Go find a stock crankcase & cylinder. Measure them. You will find the radial allowance is between 0.004" and 0.010". If you measure a lot of them you’ll find that 0.006" of radial clearance is a fair average.

Over the years I’ve noticed the dimensional tolerance on Brazilian crankcases and replacement cylinders is quite a bit more than it was on German cases & jugs. But that’s of little significance when opening up a case to accept larger cylinders since I always machine the case to match whatever set of jugs I’m using.

You are the Mechanic-in-Charge, not me. You may build your engine any way you wish. I build mine to last.)

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