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MG MGA - A different crankshaft question
|Continuing with my engine teardown, I measured the crankshaft rod journals and they seem to be turned .050" under. Theres some white paint on a crank counterweight that shows ".050", so I assume that's in reference to the rod journals. That journal diameter is .010" less than the recommended minimum, and the parts suppliers only have rod bearing sets to .040" under. I have no idea when some PO may have rebuilt this engine. Were .050" under rod bearing sets available at one time? |
|Never seen them listed although that doesn't mean that they didn't exist. May also be that someone found a suitable bearing from another make/model, or had some specials made.|
|George, don't assume anything on an engine measurement, get a micrometer and check it against the spec for yourself or have a competent machinest check it. Who knows what the numbers mean until you check all the internal dimensions of the engine. If indeed your crank has been turned .050" undersize I think I'd try to souce another crank before I went any further with the rebuild.|
|Hi Bill. I have measured the crank, using both a micrometer and a digital caliper. All four journals are within a couple of tenths of each other, .050" under the nominal size. I'm just surprised at that, since it doesn't seem that bearings are available, at least not at this time. I guess I won't be using it, I have a spare engine to take apart now....Grrrrr!|
Is this the same engine that the timing marks did not line up on?
|G T Foster|
|Gerry, yes, the same. Does that give you any hints? John Twist thinks that maybe a non-stock cam is installed. But I have this condition on TWO engines, both BP15 production builds. The cam is in good condition and I was going to re-use it but to be safe I guess I'll get a new stock cam and just stop worrying about it!|
|George, -- I have the Crane Cams "Tune-a-cam" tool kit. With this you can measure the cam lift, duration, and intake/exhaust timing. This allows you to determine what type of cam you have in hand, as well as the health of the cam. As always, do the diagnostic work first, before throwing money at it.|
|Now my curiosity is aroused. .050" is such an unusual size to turn a crank there must have been a reason. Mr. Heisenfeldt might have hit on it. An old hot rodder trick to "stroke" and engine on the cheap was to find a shorter rod from another type engine and offset grind the crank. Have you checked the stroke dimension of the throws yet?|
|Interesting thought Bill. No, I haven't taken any other measurements yet. The pistons are MG for sure, I'll have to take a close look at the rods. I'm not sure how one would check the crankshaft throw dim, can you talk me through it?|
|Thanks Barney, I was thinking of doing something like that. We might be mixing up threads here, I still can't explain my timing gear "displacement", which might be due to a non-stock cam as you suggest. But it still nags at me that I have that condition on TWO BP15 engines. I also have a "funny" looking MOWOG piston I want to show you at the meeting on Monday.|
Just check the distance the piston moves from being at the bottom to then being at the top i.e. the stroke. That is if the rods and pistons are fitted. If not, measure from the centerline of a rod journal to the centerline of the crankshaft main bearing journal and double that (easier said than done).
|M F Anderson|
|George, a quick check would be to drop the crank in the block on its main bearings and then measure from the edge of the pan rail to a rod journal, then rotate the crank 180 and read the distance again. The difference will be the stroke.
|OK Bill, thanks, that I can do. Mick, the rods and pistons are removed, but I can put one back to take a measurement, thanks.|
Going back to your original question about the timing marks, could it be that these engines of yours have been fitted with the stock camshaft for a Z-type Magnette? The cam timings on the Magnette cam are 11 degrees retarded compared with the MGA cam (the lift is slightly less too). Maybe the PO moved the timing gears forward a cog to get back to the nominal MGA timing set-up?
I have done the exact oposite on my Magnette which is fitted with an MGA cam, although I used a vernier timing gear to do the adjustment.
Just a thought.
|Andy, thanks for your input. That's the best explanation I've heard for my timing gear dilemma. I'm going to get a stock MGA cam, and then I'll compare it with the cam pulled from my engine.|
This thread was discussed between 12/08/2008 and 18/08/2008
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