During a recent discussion with a few overseas Mini brethren - who were experiencing various engine-related problems - something cropped up that I have always taken as granted (in the sense of ' a standard for the application'); the matter of consistency in measurements. Now we're not talking about the sort of measurements you make with a ruler, since discrepancies of a small nature don't tend to matter a whole lot. No, we're talking about the sort of measurements made with fine measuring instruments such as micrometers or vernier calipers.

Now, the discussion that was going on was with reference to the best clearance to run between the bore and piston for a given piston type for race or street purposes. I hade quoted figures that I generally use for both situations for forged and for cast piston types. One of the guys concerned had experienced less than satisfactory results with my general recommendations as his engine smoked like a steam engine and blew a good 30% of the oil contents of the sump out of his various engine breathers into his catch-tank. Stripping the engine down revealed no telling problems. Another guy had experienced a lack of willingness for the engine to pull high rpm, struggling to make it past 7,000 rpm when it should have easily made it past 8,500 rpm. Stripping this unit down revealed the pistons to have an extremely polished finish almost the whole way round the piston crown wall above the top piston ring and shined sides down to the second ring above the wrist pins. In this instance, the pistons had been acting like disc brakes on the bore walls. So what was the problem?

After much head scratching, the reason suddenly dawned on me. Invoking the question 'what temperature were the block bore and piston skirt measurements done at?' initially was met by a seriously pregnant silent pause. Then the question 'why?' was voiced almost in chorus. Simple that - because the vast majority of subject matter on this planet will expand or contract with temperature change. As obvious as this may seem once it is mentioned, and is a recognised fact when under operating conditions (i.e. when the engine is running), it generally passes most folk by at the preparation stage. Largely because to most folk it's irrelevant because they don't get involved with such detail, they just trust their chosen engineering shop to get it right. But for the serious, DIY racer it's fairly crucial. So for scribes to quote specific clearances and measurements in engine building editorials is a bit dangerous without a reference point, i.e. to say you need to run a certain piston to bore clearance without giving a working temperature can cause problems.

I have established a working measurement consistency cycle with my local machine shop, but to illustrate this issue I had a block very recently bored by them to a specific size quoted by me. Leaving it in my workshop overnight, which is not heated at all and further intensified by the recent Siberian weather I've been experiencing locally, I then checked the bore size in the morning. It was 0.00075" under size - that's very nearly 20% of the total piston to bore clearance! Taking the block back to the machine shop and leaving it there until the afternoon, I then first re-checked my measuring equipment against there's - finding it spot on - and then re-checked the bore size. It was back to the correct, as finished by the machine shop and requested by me size. Checking the temperature of the block in my workshop gave a reading of 37 degrees Fahrenheit - not too far off freezing point! The temperature given in the machine shop was 64 degrees Fahrenheit - a considerable difference and not that far short of nearly twice the temperature. The general ambient working temperature for materials measurement is 68 degrees Fahrenheit, and what many of the measurements quoted in workshop manuals are quoted at. A consequent brief investigation shows not much variation from 60 to 68 degrees F, but getting on for very noticeable below 58 and above 74.

In conclusion then any tolerances or fine measurements quoted for machining work needs to have a base temperature stated as a reference point unless you are totally reliant on the machine shop/engine specialist to get it right in the absence of suitable measuring equipment in your tool box. And this applies world wide - not just to those in far-flung places, but all over the UK too.