Richard71 Posted April 24, 2018 Report Share Posted April 24, 2018 Hello All, An engine block with serial number prefix "MG" has came into my hands, I believe this to be from a saloon 2.5Pi. My question is, are there any differences advantageous or otherwise between these blocks and TR5/6 blocks. Regards, Richard. Quote Link to post Share on other sites
jerrytr5 Posted April 24, 2018 Report Share Posted April 24, 2018 It's the same as the CP block. Jerry Quote Link to post Share on other sites
lightningburns Posted April 25, 2018 Report Share Posted April 25, 2018 (edited) Hi Richard, I think you will find that the MG block is the factory Mk2 2.5PI block, so has the recessed bores and extra strengthing rib on the block, so not the same as earlier CP (pre CP75001) but is interchangeable, it would be the same as CR. Triumph thought there was an advantage in doing this but in reality who knows. I have an MG block fitted which appears to be an exchange block, so dealers were swapping them out back in the day. Regards, John Edited April 25, 2018 by lightningburns Quote Link to post Share on other sites
astontr6 Posted April 25, 2018 Report Share Posted April 25, 2018 Hi Richard, I think you will find that the MG block is the factory Mk2 2.5PI block, so has the recessed bores and extra strengthing rib on the block, so not the same as earlier CP (pre CP75001) but is interchangeable, it would be the same as CR. Triumph thought there was an advantage in doing this but in reality who knows. I have an MG block fitted which appears to be an exchange block, so dealers were swapping them out back in the day. Regards, John The CR block is more ridged in use, there were problems with the early 2,5 blocks with twisting / warping and valve seat distortion in the cylinder heads hence the change of head gasket and casting design. In my view a better block. This was done to cut down the number of warranty claims due to loss of compression. Bruce. Quote Link to post Share on other sites
TRTOM2498PI Posted April 25, 2018 Report Share Posted April 25, 2018 (edited) I concur that the later blocks have the rib down the length of the block, (exhaust/PI/Carb side), which provides additional strength, and interchangeable, as per above. Cheers. Edited April 25, 2018 by TRTOM2498PI Quote Link to post Share on other sites
Richard71 Posted April 25, 2018 Author Report Share Posted April 25, 2018 Thanks to all for the info. Is this strengthening rib internal or external? I cant look at the block at the moment, dropped it in at the machine shop. Richard. Quote Link to post Share on other sites
TRTOM2498PI Posted April 25, 2018 Report Share Posted April 25, 2018 (edited) Hi Richard, It is visible externally. Cheers. Edited April 25, 2018 by TRTOM2498PI Quote Link to post Share on other sites
jerrytr5 Posted April 25, 2018 Report Share Posted April 25, 2018 I disagree with some of the above. Definitely not recessed, can't say about strengthening rib. I built an 'MG' series block up using all the components from a 'CP' engine that had ice damage, with no issues. It certainly appeared identical to me. Jerry Quote Link to post Share on other sites
saffrontr Posted April 25, 2018 Report Share Posted April 25, 2018 The recessed blocks on the 2.5PI were from MG50001 onwards i.e. at the same time as they were introduced on the TR6 at CC75001 / CP75001 cheers Derek Quote Link to post Share on other sites
Richard71 Posted April 26, 2018 Author Report Share Posted April 26, 2018 The recessed blocks on the 2.5PI were from MG50001 onwards i.e. at the same time as they were introduced on the TR6 at CC75001 / CP75001 cheers Derek The block I have is definitely recessed, though I never took note of the serial number. Richard. Quote Link to post Share on other sites
astontr6 Posted April 26, 2018 Report Share Posted April 26, 2018 The block I have is definitely recessed, though I never took note of the serial number. Richard. Make sure you use the later head gasket for the recessed block, it was designed to give better sealing between the bores! Bruce Quote Link to post Share on other sites
matt george Posted April 26, 2018 Report Share Posted April 26, 2018 My TR6 is CC78553… on the pic below you can just about see the strengthening rib on the block. My 2000 saloon has an MM code engine from a 2500TC and that also has the rib. Matt Quote Link to post Share on other sites
Richard71 Posted April 26, 2018 Author Report Share Posted April 26, 2018 Thanks Matt, I can see the rib clearly now. On another note concerning the later, improved engine blocks.....if the block was skimmed to accommodate zero piston-deck clearance, would it be safe to assume the recess would have to be re-cut accordingly, does anyone know what the standard depth of the recess should be? Richard. Quote Link to post Share on other sites
matt george Posted April 27, 2018 Report Share Posted April 27, 2018 Thanks Matt, I can see the rib clearly now. On another note concerning the later, improved engine blocks.....if the block was skimmed to accommodate zero piston-deck clearance, would it be safe to assume the recess would have to be re-cut accordingly, does anyone know what the standard depth of the recess should be? Richard. Richard, I don't know what the standard depth of the recess is, but yes, my block was decked and then had the recesses re-cut. Matt Quote Link to post Share on other sites
Nick Jones Posted April 27, 2018 Report Share Posted April 27, 2018 I forget the actual numbers now (though there is a thread somewhere on one of the Triumph forums) but it is usually possible to have a very light block skim without needing to re-cut the recesses. I managed this when I built my PI engine. That was an MG prefix block from 1972 with the rib along the bottom of the manifold side. Conversely, if you are decking for a specific purpose (to zero deck your pistons for example) then you will certainly have to either re-cut the recesses or deck far enough to completely remove the recesses for a flat gasket and skim the pistons a few thou as well. I did the latter when I built my Vitesse engine with a late block because I wanted 0.005" pop-up and don't much like the recessed gaskets. Still had to skim the pistons a little, which at least meant they all had identical deck height. Nick Quote Link to post Share on other sites
astontr6 Posted April 27, 2018 Report Share Posted April 27, 2018 I forget the actual numbers now (though there is a thread somewhere on one of the Triumph forums) but it is usually possible to have a very light block skim without needing to re-cut the recesses. I managed this when I built my PI engine. That was an MG prefix block from 1972 with the rib along the bottom of the manifold side. Conversely, if you are decking for a specific purpose (to zero deck your pistons for example) then you will certainly have to either re-cut the recesses or deck far enough to completely remove the recesses for a flat gasket and skim the pistons a few thou as well. I did the latter when I built my Vitesse engine with a late block because I wanted 0.005" pop-up and don't much like the recessed gaskets. Still had to skim the pistons a little, which at least meant they all had identical deck height. Nick Hi Nick, the recessed block head gasket had a fire ring design based upon formula 1 design for their application and gave much better sealing than the original design of head gasket which often blow between bores because of a lack of rigidity of the block! Bruce. Quote Link to post Share on other sites
john.r.davies Posted April 27, 2018 Report Share Posted April 27, 2018 (edited) Came across this idea in a video from Janspeed, the well-known Mini specialists. Place a length of fuse wire between two close bores. Presumably it acted like a full fire ring. See: http://gerardsgarage.com/Garage/tech.htm. Video 4, one minute in. John Edited April 27, 2018 by john.r.davies Quote Link to post Share on other sites
Nick Jones Posted April 28, 2018 Report Share Posted April 28, 2018 Hi Nick, the recessed block head gasket had a fire ring design based upon formula 1 design for their application and gave much better sealing than the original design of head gasket which often blow between bores because of a lack of rigidity of the block! Bruce. No doubt that is the intention. In fact I've never had any problem with the flat gasket on the Mk2 version of the 6 cylinder engine (in spite of working them quite hard). The Mk1 version and the 4 cylinder engines are a different story but they only have weedy 3/8" studs and don't get clamped down properly. Nick Quote Link to post Share on other sites
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