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how can I tell if the crank and bores have been ground out?

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how can I tell if the crank and bores have been ground out? #1
bradmgb76 Brad Roe TORQUAY, Devon, UK Sign in to contact	Topic Creator (OP) Oct 16, 2017 08:26 AM Joined 6 years ago 1 Posts

The piston rings have disintegrated on my '76 MGB so I've taken them all out. How do I tell if the shells are original size or need oversize replacements?
I am a complete virgin at the bottom end so all of this is very new (and kind of exciting really)

Many thanks in advance.

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Re: how can I tell if the crank and bores have been ground out? #2
riley1489 Bruce H Great White North, QC, Canada Sign in to contact 1953 Jaguar XK120 1959 Riley 1.5 "King George" 1973 MG MGB	Oct 16, 2017 08:29 AM Top Contributor Joined 12 years ago 26,297 Posts

If you do not have accurate measuring instruments, (Micrometer not vernier) or the experience to use these, involve a machinist shop that will check the crankshaft journals for wear. Only then will you know.

B

Life's most persistent and urgent question is, "What are you doing for others?"

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Re: how can I tell if the crank and bores have been ground out? #3
Speedracer Hap Waldrop Taylors, SC, USA Sign in to contact 1967 MG MGB Racecar "The Biscuit"	Oct 16, 2017 09:25 AM Top Contributor Joined 18 years ago 43,753 Posts

You can't if the engine is assembled, if apart by measuring the sizes, most oversized piston will be marked such on top, but may be covered with carbon, the bearings shells at the crank will marked of there undersize, .010, .020, etc,, if they have no size then stamped on them, then that is standard from the factory bearing as the factory standard bearing are not marked for size, and even if standard bearing was replaced at some point, it would have STD. stamped on the back of the bearing shell them.

Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Acme Speed Shop www.mgexp.com

MG/ Triumph Performance Street Engines - Cylinder Head Porting for street performance and race - DIY Engine Rebuild Kits With Free Tech Advice - VTO alloy wheels for British Sports Cars, and others

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Re: how can I tell if the crank and bores have been ground out? #4
ingoldsb Terry Ingoldsby Calgary, AB, Canada Sign in to contact 1971 MG MGB	Oct 16, 2017 10:12 AM Top Contributor Joined 20 years ago 20,145 Posts

We're all assuming you don't have the head off. If you have the head off, then you should be able to see if the pistons are marked with an oversize. And, of course, you could measure the bores. If you only have the oil pan off you *might* be able to measure the bore from the bottom. It would be tricky, but might be possible - I've never tried.

On a practical note, it probably doesn't matter because there will likely have been wear on the cylinders that will require new oversize pistons and boring to the next oversize. Of particular concern is "taper" - the difference between the diameter of a bore at the top and bottom of the cylinder. If you can catch your fingernail on the ridge at the top of the cylinder then there is likely enough taper to require boring.

Similarly, unless the crankshaft journals are in very good shape, the crank may require turning (and oversize bearings).

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: how can I tell if the crank and bores have been ground out? #5
malski Peter Malkin (RIP) Bright, N.E.Victoria, Australia Sign in to contact	Oct 16, 2017 04:52 PM Top Contributor Joined 14 years ago 2,054 Posts

In reply to # 3610904 by ingoldsb We're all assuming you don't have the head off. If you have the head off, then you should be able to see if the pistons are marked with an oversize. And, of course, you could measure the bores. If you only have the oil pan off you *might* be able to measure the bore from the bottom. It would be tricky, but might be possible - I've never tried.

On a practical note, it probably doesn't matter because there will likely have been wear on the cylinders that will require new oversize pistons and boring to the next oversize. Of particular concern is "taper" - the difference between the diameter of a bore at the top and bottom of the cylinder. If you can catch your fingernail on the ridge at the top of the cylinder then there is likely enough taper to require boring.

Similarly, unless the crankshaft journals are in very good shape, the crank may require turning (and oversize bearings).

Head is off as rings are removed.

Peter Malkin
NE Victoria
Australia
MGBGTV8 (1977)
MGB Roadster (1967) Supercharged
Triumph TR3A (1959)
MGA 1500 (under restoration)

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