Was reading about the removal of one edge of the block around each bore to enable transition of top of piston area into valve area on the later and sports heads. It doesn't give details of depth you take away, but presumably even a mm or so would be effective.
Is this still something any of you do? Or is it seen as damaging blocks which might be re-used for LC where it wouldn't be useful?
(16-01-2019, 10:41 AM)JonE Wrote: Was reading about the removal of one edge of the block around each bore to enable transition of top of piston area into valve area on the later and sports heads. It doesn't give details of depth you take away, but presumably even a mm or so would be effective.
Is this still something any of you do? Or is it seen as damaging blocks which might be re-used for LC where it wouldn't be useful?
It's something I do, but I don't know how effective it is. All I know is that other people say my car goes extremely well, but it's never been on a dynamometer.
Robert Leigh
16-01-2019, 11:46 AM (This post was last modified: 16-01-2019, 11:51 AM by JonE.)
Both - thanks.
So from Terry's pic... that looks like slightly less than 45 degrees and seems to be slightly asymmetrical removal towards one valve type?
The Williams book makes it look huge... I'm presuming for standard Austin pistons a couple of mm depth. (I cant see the piston edges cos they are in the engine)
The problem with block relief is that you are removing material from the combustion chamber, inreasing its volume and therefore lowering the compression ratio. To maintain the compression ratio, you will need to skim the head, which narrows the gas path again from the valves to the piston.
This is the main "gotcha" for the sidevalve engine and limits the ultimate amount of improvement that can be made to gas flow vs compression ratio.
As I understand it, taking the sharp square edge off the bore on the valve side helps things somewhat, so there is no need to gouge a great big trough. Like many block and head modifications, more harm than good can come from removing too much metal.
(16-01-2019, 12:23 PM)Malcolm Parker Wrote: As I understand it, taking the sharp square edge off the bore on the valve side helps things somewhat, so there is no need to gouge a great big trough. Like many block and head modifications, more harm than good can come from removing too much metal.
That's kind of how I see it - small potential gain, if everything else is on song - vs risk of buggering up a good block. I have never used a block like this so don't know what I'm missing; on the other hand my car goes alright with parallel bores.
Well worth relieving the block on any sidevalve motor IMO. Some manufacturer's motors, e.g. late Ford V8 came from the factory with the block relieved. On an A7 motor I mark the block around the inside of the head gasket fire ring and that's the extent of the relief. It ends up looking just like the photo posted up by Terry -going too far down the bore would expose the top piston ring to excessive heat on the firing stroke.
That's all good news thanks.
I've been resisting putting a new Ricardo on this engine as the engine is old and untested... so it's actually a benefit to lose a bit of CR for longevity and smoothness. (For the same reason I was thinking to use a HC gasket which is supersize around the edges of the chamber, rather than a LC which is right tight up?)
Just out of curiosity, what happens when the block eventually needs a liner? Or indeed if it's already lined?
There are other ways to lose a bit of compression Jon - for example a shim under the block, or in the war they used to stack 2 head gaskets together (I'm told).
