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Re: 2639 to 2912

To: Healey List <healeys@Autox.Team.Net>
Subject: Re: 2639 to 2912
From: Rick Swain <grain@auracom.com>
Date: Wed, 06 Jan 1904 00:41:49 -0400
I got several replies to my question about boring out a 100-6 block to
accept 3000 pistons. Some, but not all, of the replies went to the whole
list so you will no doubt have seen some of the replies I got. I thought
others might be interested so I'm sharing what I have learned.

Derek Job said it wasn't possible while Chris Dimmock suggested that not all
2.6 blocks were the same, that later ones could be bored to 2.9 but not
earlier ones. He thought that later ones have external ribbing and those
would be okay. I don't believe that's a way to tell because I have photos of
a 2 port head 100-6 (presumably an early block) and it has external ribbing.

The difference between 2.9 liter blocks and 2.6 ones appears to be that the
liners of the 2.9 blocks are larger than at least some of the 2.6 ones. If
you have a 2.6 block with smaller liners then when you overbore to 2.9 you
either come very close to the water jackets or bore right through into them.
The result is a useless block.

Don Anglesey, however, wrote the following:

"I converted my early BN4 to the 6 port head, When I had the engine
rebuilt I changed to the 3000 pistons because of the higher compression
ratio and early BN4 pistons are hard to come by.   My biggest problem
was that the earlier cars used a clamped type connecting rod so I had to
replace the rods as well.  The differences are minor but it can be done."
Don 57' BN4

Apparently overboring his particular 100-6 block didn't leave Don's cylinder
walls too thin.

Roland Wilhemy suggested replacing the liners:

"The simple answer is that a simple rebore is probably not going to
work.  It depends on some vagaries of the block's casting.  If you
want to keep your block consider relining with the larger cylinder
liners -- not cheap but more reliable than boring out what's there."

Michael Salter provided graphic evidence of what can happen when a 100-6
engine is bored to 3000 specs. He wrote:

"Hi Rick,
The attached pictures show the results of doing exactly what you are
considering. These are the remains of the engine in a 100/6 that I bought
with a "seized" engine. I can't imagine what sort of noise it made when it
blew. Some of the sections of cylinder wall are less than 0.050" thick."

Michael Salter

That's where I am right now. Unless someone can tell me how to determine
whether my block has liners that are thick enough to be overbored to 2912
ccs I'm not going to risk ruining it. I'll be happy with 2634 ccs.

Cheers

Rick 

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