Asymmetric wear

My local cylinder boring shop has advised against reboring the iron barrel of my Villiers 1H (224cc) engine as it measures only two thou more diameter in one plane than another, and this is due to asymmetric wear. Do you think this sound advice? Aidy Long, email.

Assuming you aren’t confusing two terms here, wear and asymmetric thermal stress which can cause asymmetric distortion especially to cast iron barrels, then I assume your boring shop is probably referring to the variance of wear measurement travelling down the barrel (or up it). Cylinder wear occurs in the region of travel of the piston rings and very little or no wear occurs below this point.

All air-cooled engine barrels are subject to asymmetric stress, but the situation is worst with cast iron two-stroke barrels where they get honest at and around the exhaust port and then the combustion region, but is cooler travelling down the barrel below this point. Then, the two-stroke suffers a second thermal stress because it draws in cool fuel vapour through its inlet manifold/stub, moves it to the crankcase via inlet port/s, often cast into the barrel, then moves it from crankcase to combustion chamber by transfer ports, again often cast into the barrel. Thus hot is fighting cold causing distortion. And this may be what the boring shop is referring to.

However, two thou (0.002 inch) wear or distortion is marginal unless you are building a peak performance racing engine, so I’d follow the advice given and remember piston rings flex by tiny amounts in travel so will easily cope with such tiny variances they encounter during their travel up and down the bore.

Note. Aluminium two-stroke barrels don’t suffer so badly from asymmetric thermal stress as aluminium dissipates heat much faster than cast iron.

10L The Villiers 1H engine.