Miglia Engine Completed!

29 05 2013

This engine has made me realise more than any previous one why it costs so much to have an engine built by the top builders like SH, Swiftune and Vickers. The amount of thought that goes in to them in the design phase is immense, as is the work required. These guys have a massive head start on me having done literally hundreds of successful motors, so it would be daft of me to think I can challenge early on. But, same as me they all started somewhere so perhaps anything is possible..?

Upping the compression ratio is often a great way of getting a bit more power out of a mini engine. It’s a dangerous game to play though, as going to high can cause detonation, but too low and you aren’t releasing those potential horseys. The 1293cc Miglias have a minimum swept volume of 32cc to adhere to, which basically equates to a CR of 11:1. Going higher can give you decent gains but you have to have decent pistons and fuel. 11:1 is a good all round figure and still in the zone of reliability. Anyway, the unswept volume is basically the volume above the piston when at TDC, so the compression chamber, head gasket, piston dish, ring land and any deck height space. To measure all this I use a combination of lab equipment and David Vizard books. Just as designed, after quadruple checking everything, I found that I had an unswept volume of 33.8cc giving me a reasonable excess allowing the option of a mid-season emergency skim of the head or block if need be.

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So it was all ready for the final assembly. With the short engine upside down on the engine stand I lifted the gearbox on and tightened up the uprated 5/16 allen bolts making sure the seals were all correctly in place. Then on went the flywheel housing with a new clutch seal, and a double check of the primary gear endfloat. Then once everything was tightened up, on went the steel backplate, paddle clutch and steel flywheel, then the grey pressure plate, and finally the clutch cover with new thrust bearing.

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The head is believed to be done by Bryan Slark, but I have had several Slark heads over the years and this one has none of his usual markings – in fact no markings at all. That said it has all the usual features of one of Bryans creations and I’ve always found them to be very good. Previous owner Mike Jackson said it was a good head and I’ve no reason to believe any different. LCP supplied and fitted new colisbro guides and a fresh set of Piper springs to match the cam were purchased from Anderson Race Engines. I tested the old ones on the LCP valve spring dyno and they were fine, so will be kept as spares.  So on went the head – with a new gasket of course, and then the roller rockers. All torqued down and the tappets set.

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The LCB I’m using came from Bob Humphreys. Apparently it was made at great cost to a design by Peter Vickers possibly by Maniflow. It has longer primaries and is very big bore. I’m guessing it didn’t work any better than the off the shelf stuff available otherwise it wouldn’t have been sold, but it was quite cheap so we will give it a try. If it doesn’t work I’ll try a different one.

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