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Engine block cylinder bore recesses


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I am looking ahead now to best practice for engine refurb. I currently have a CP recessed block and am wondering whether to have the block skimmed to remove recesses or not. I am having the cylinder head skimmed, ported refurbed etc and I asked for the compression to be 9.5:1 as original CP heads were--it's not a CP head that I've got hence the skimming, anyway if the block is skimmed to remove the recesses what sort of compression would I end up with - is it a waste removing the recesses? I'm just chucking this out there as I've read all sorts of ideas and just trying to get some wisdom from you guys. I have both types of original nos Payen head gaskets so I'm not too concerned which way to go, also are the ARP head bolts worth using or just stick to original bolts? 

Kind Regards

Michael

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To raise compression, it's more usual to skim the head.     This because the pistons come up to, roughly the block deck height, and will 'pop-up' into the chamber if the block is skimmed.     This will raise compression, but risk valve contact.

"Decking the block" is a process that ensure that all the pistons do rise to the same height, by skimming the block to the height of the lowest, and then skimming the other pistons to make them the same.     Not to raise compression.   And it's expensive, compared to a simple head skim.

So why do you want to skim the block?      And if you want to calculate the change in CR from a block skim, may I refer you to my article on the subject?  See: http://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/     The formulae are for head skims but easily adapted.

John

Edited by john.r.davies
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I thought all the CPs had flat blocks. 

The CRs certainly has the recessed blocks but didn’t think the CPs did.

If you skim the block the recesses will diminish so they will need re machining to accommodate the fire ring on the head gasket or machine them out completely and use a CP gasket.

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Some late CP series have recessed blocks. I would never think of skimming out the recess. As John suggests, it will lead to other problems if the pistons rise above the top of the skimmed block.

The recessed block works fine with the correct gasket. If you wish to raise compression, skim the head would be my advice.

When building a 2500S saloon engine recently for my GT6, I wanted to raise the compression ratio from 8.5:1 up to 9 or 9.25:1. Skimming 35 thou from the head achieved this and it runs beautifully, with a recessed block.

Nigel

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Thanks for the advice. Yes I knew how to raise compression by skimming cylinder head that's why I've asked for mine to be skimmed. The block is a late CP block yes and was bought by myself believing to be a CR or USA engine even having seen the engine number stamp photographed on sellers advert! However when I got around to cleaning all the old oil off the block with a good degrease the number which looked like CR when dirty in the photo turned out to  be CP no mistaking-- bizarre but true and hasn't been messed about with or restamped as it still has original pistons in from when it left Triumphs factory all stamped A and standard size. Anyway thanks again and I'm now better advised to know what to do. I guess I will be selling a set of rare original Payen gaskets on Ebay over the winter as I won't be needing them lol. 

Best Wishes

Michael

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14 hours ago, michaeldavis39 said:

I am looking ahead now to best practice for engine refurb. I currently have a CP recessed block and am wondering whether to have the block skimmed to remove recesses or not. I am having the cylinder head skimmed, ported refurbed etc and I asked for the compression to be 9.5:1 as original CP heads were--it's not a CP head that I've got hence the skimming, anyway if the block is skimmed to remove the recesses what sort of compression would I end up with - is it a waste removing the recesses? I'm just chucking this out there as I've read all sorts of ideas and just trying to get some wisdom from you guys. I have both types of original nos Payen head gaskets so I'm not too concerned which way to go, also are the ARP head bolts worth using or just stick to original bolts? 

Kind Regards

Michael

You would be very foolish to do away with the recesses as they were put their to use the later Payen or Coopers( OEM) design gasket to stop compression leakage which was based on formula 1 fire ring design with the strengthen block to give far better sealing as against the original design. BL had a lot of warranty claims for this problem. During the development of the later design I was an apprentice toolmaker and helped with the modification of the original tooling. The later design of gasket will not seal properly with a flat block that's why the recesses (counter bores) were put there!!!! In my view you could argue that the old design flat blocks should be counter bored to use the later design gasket.

Bruce.

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