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Spit mk.4 1300 engine problem


Steve C
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I went to look at an engine one of our area members is in the process of assembling this morning, and would welcome any ideas or advice.

The engine has been through a machine shop, and has been bored 60 thou over, and the crank has allegedly had a 20 thou grind. The crank spins nicely with the mains torqued down, and the pistons slide freely in the bores. However when you start to nip up the big ends, the engine locks and is very difficult to turn. Cracked off the big ends, checked that all the right caps are with the right rods, everything drowned in clean oil, shells sitting correctly in their babbits. Big end caps are assembled so that standing at the front of the inverted engine, the stamped lettering is the right way up to read. Problem is not confined to any one crank journal, try nipping up any of them and the effect is the same. Unless I am missing something idiotic, the only inference I can draw is that the crank grind is less than claimed? I also checked the bearings, which were supplied by Moss, and the box is clearly labelled 020.

The owner did have some issues with the machine shop, who were stringing him along with completion dates (they were paid to build the engine, but kept fobbing him off with excuses, and he ended up retrieving it as a box of bits...nice). This makes me suspicious about the sidings. I have suggested he take it into a reputable machine shop with some decent precision measuring kit to check the sidings, but beyond this, I am stumped. All suggestions from more experienced hands appreciated.

 

Steve C

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Are the end caps back on where they came off? The wrong end cap in the wrong place can give you exactly this problem. I think the con rods are cast as one piece , drilled and cut which means one is slightly different from another and the ends must go back with their appropriate other half.

 

I'm concerned the crank spins with just the mains, it should turn fairly freely but not spin! Again I would expect the pistons to be pretty stiff in the bores.

 

Also 60 thou? My engine's been re-bored twice and it's up to 40 thou which they told me was the max, no more. So, a bit suspicious! :(

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Caps are with the correct rods, and have turned them round following a close look at the works manual diagram so that the lettering in the castings is now as described above. The engine was too tight to turn before this, but the problem persisted after they were rotated. Also noted some light scoring in the shells at the outer ends, suggesting something is way too tight. With the big end bolts slackened, the engine turns with a bar through the flywheel bolts for leverage, and there are no tight spots. As to the 60 thou over bore, I would agree this is on the outer limits, but it is what they have done.

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