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Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.
I had some machine work done recently which included polishing my crank journals for my Mazda B6 engine. When I tore down the engine I measured crank journal clearances at around 0.025mm (using platigauge), but I don't know the early history of the engine so I'm not sure what size bearings it had (can I tell from the thickness of the bearing?
I started assembling this past weekend and when I assembled the polished crank I measured clearances at around 0.050mm with STD ACL bearings. The Mazda service manual states that spec is 0.018-0.036mm and maximum 0.1mm. So I'm wondering if that's acceptable, or if I should get the crank re-ground to accept undersize bearings?
The other thing that I found is that my ring gaps (factory Mazda) are all slightly large at about .33mm for top (spec is 0.15-0.30), .43 for second (0.15-0.30). I've read/heard that ring gaps that are larger are generally preferred to smaller. But is it worth looking at different sized rings?
It's not a high power build or anything, just something of an improved stock build where I'm going slightly higher compression (by virtue of a conservative head shave of 20 thou and a 1mm (vs 1.5mm) head gasket), but down the line will run ITB's, cams and a more aggressive tune. Or something like that. But main aim here is to have a relatively bulletproof weekend driver; it's not a daily but it when it's driven it's generally driven pretty hard.
That's approximately 0.002", which should be fine - Japanese manufacturers tent to run clearances a little on the tight side. For future regference, OEM shells will usually be stamped "STD", or have code letters on them, oversized will be marked -0.25 or -0.010, or similar.
With the rings, it's common for the rings to be a little on the wide side, sometimes called "jobber" rings, because it's much cheaper and easier for simple rebuilding than having to resize each ring. There may also be a little bore wear, if just re-ringing, with 1 thou' of wear increasing the gap by just over 3 thou' - because of this it's a good idea to check the gap at the bottom of the bore, as sometimes you may have the clearance at the top of the bore, but not the bottom. I have seen several tests and reports that there's negligible difference until the gap reaches around 0.050", or 1.25mm - of course, the wider gaps will wear to that point more quickly, but still may take a few hundreds of thousand kms.