Hi all,
I am not trying to be confrontational here but in all honesty I've never run an engine in yet, just taken care to build it with appropriate clearances, or in some cases larger than appropriate! :big:. Seriously though, I bet many people would be surprised how forgiving steam engines are. This is why I find it strange that people go to the nth degree trying to lap bores and pistons and running in tight big ends and main bearings when it's really not necessary unless you're competing in specific power output competition or something. However, most engines are either run on air a few times and put on a shelf or run at exhibitions a couple of times a year. An engine with a bit of clearance in its joints will run more smoothly and slowly and consume less air than a tight one.
I think sometimes the drawings and articles are partially to blame. Model Engineering drawings tend to give nominal sizes rather than anything toleranced which is ok as these things aren't intended for mass production and they assume the builder will machine parts in a logical order and machine them for fit rather than an actual size. Sometimes the build instructions just assume this though, they'll just tell you the method of manufacture and neglect to mention anything about fit or clearances on certain parts.
An example would be your crank webs between the bearings and the big end between the crank webs. You need a certain amount of end float in these as if your bore isn't perfectly square to the cylinder end face, piston rod perfectly square to piston, cross-head guide perfectly square to cylinder etc. it will cause a tight spot. If however, a small amount of clearance is built in, it can allow for the build up of the other geometric tolerances.
Attached is an extract from the an old British Standard BS4500 which gives a number of different types of fit and specifies the dimensions you would require to create that fit for your particular sizes. Well worth a look if you haven't before.
In my view, if an engine doesn't turn over freely by hand (and that doesn't mean there's excessive play anywhere) it's really too tight and has the wrong fit somewhere. That's not to say that running in with grinding paste and the like doesn't work, it may do if the right precautions are taken but it would just giving it the clearances that should have been there in the first place.
Looking at a car engine for example, if the correct clearances weren't there in the first place, the chances are it would quickly overheat and probably seize or destroy itself quickly. The only thing gets 'run in' is the piston rings, everywhere else there has to be room for a film of oil between the two parts otherwise it wouldn't do it's job.
It's a similar story when loctiting something ... it's no use trying to loctite something with a tight press fit and 2 smooth surfaces, there has to be somewhere for the loctite between the two surfaces otherwise how can it do anything? Another reason that a perfect mirror finish is not desirable on cylinder bores and pistons, better to have surfaces that can retain oil.
Just my 2p worth anyway.
Nick
