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Re: How can you tell what size the pistons are on a 1966 MGB

Subject: Re: How can you tell what size the pistons are on a 1966 MGB
From: (Rick Morrison)
Date: Mon, 20 Apr 1998 21:45:56 EDT
The piston diameter will be basically the bore diameter minus the piston
 I say basically, because the piston, depending on make and design, may
have more than one diameter, and/or may in fact be oval in shape.
 But for your and our purposes, we can accept the bore diameter as the
piston diameter, again less clearances.
 Without a parts number cross-reference, the numbers inside the pistons
won't help much.
 But most pistons have the oversize stamped on the piston crown (top),
usually something such as "20", "30",or "40" for +0.020, +0.030, or
+0.040 oversize pistons respectively.
  The problem is not so  much what is in there now, as what is going to
be required to bring clearances back to spec. And with that much skirt
clearance, you can count on a bore job.
 The machinist should measure the existing bore, checking for taper and
determine just how much wear has taken place, and then, based on the std.
bore diminision, determine how much overbore will be required.

Rick Morrison
74 Midget
On Sun, 19 Apr 1998 21:51:32 +0600 Dirk de Boer <>
>Here is technical question for all of you:
>How can you tell what size pistons one has? The manuals only give the 
>clearance in the cylinder. In my engine, the clearance is a little 
>0.020 so it's time for a rebore. I'm going to take it to the machine 
>next week but I'd like to know in advance what pistons I have. Who 
>what the DPO put in there. Anybody knows what the cylinder and piston
>diameters of the standard and oversize versions are? Can you tell from 
>numbers inside the piston?
>Dirk de Boer
>1966 MGB with an engine in bits

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