This is just a page for me to track the parts I have and anything I learn about whether they are right for what I need or whether I need to change them…
The Donor:
MGB GT 1800, wire wheel
Reg = TUU 503F
Car Number = GHD4 / 146973 G
https://www.mg-cars.org.uk/MGB/mgbchassis.html indicates the car number falls somewhere in 1968.
https://www.mustard.co.uk/car-insurance/guides/what-year-is-my-car/ (and others) indicates F-reg cars were August ’67 to Jul ’68 so this looks like this car was built in ’68 and registered before the end of July.
The Engine:
Bought on eBay and described as for a Triumph 2.5 PI.
Engine Number = MG77166E
Cylinder Head part = 218225.
It also has “TR 2.500 PI +30 1974” marked on the side – presumably by whoever bored the cylinders by 0.03″/30 thousandsths.
https://en.wikipedia.org/wiki/Triumph_I6 confirms an MG prefix indicates an engine from a 2500 but non-specific about the fuel system whereas https://triumph2000register.co.uk/the-cars/technical-specifications/ classifies MG engine numbers as a 2.5 PI Mk2 (69-74), which fits.
The standard bore is 2.9″ diameter (3.7″ stroke), so if these are 0.03 over (does that mean at each edge making the total bore 2.96″? The cc goes up by about 100 making this now a 2.6l engine(?)
As for the cylinder head – “Any PI, or UK TR6 between 72 and about 74. Should be 3.400″ thick. Also used with domed pistoned 2000’s in the same period. Superseded by 219015 with smaller exhaust valve for 74-75 then 219016 with lower CR for 75-77” https://www.tr-register.co.uk/forums/index.php?/topic/37668-cylinder-head-identication-218225/
https://www.triumphclub.co.nz/?page_id=653 also provides some interesting additional info:
Compression Ratio ~ 9.5:1 and “HE”/High Compression. The original distributor was a Lucas 25D6 with max advance 11 degrees at 3000rpm. It also suggests the 2.5 PI Mk2 produced 132BHP at 5500rpm, but that’s obviously including the PI – I’m currently thinking carbs, at least to start, and have a couple of HS6’s ready and waiting.
For the Cylinder Head it also shows height should be 3.4″, should use flat pistons, has 32mm exhaust valve (diameter?) and 8 1/8″ push rods – might be needed later.
Camshaft
The engine came with a spare camshaft, labelled 307621, which is for 2500TC & S MM20001 on, 2000 TC ML1 on and Mk3 GT6 and the one for my engine should be 311399 (Mk2 PI from
MG75001, TR6 CR****, 2500 TC 1974 MM1 – MM20000) according to https://www.chriswitor.com/datasheets/camshaft_applications.pdf. The two cams seem very different – the 307621 has lower lift, shorter duration and “earlier” timing.
What I know about camshaft design:
lift is an easy way to get air fuel mixture in faster, but only within reason (too much lift can cause airflow issues and strain on the valve train). The two lobes for a cylinder tend to be one just before the 12 o clock position 9 (intake) and one after (exhaust). The closer they, the shorter the “overlap” between the intake opening and the exhaust closing. Overlap is good and optimises airflow, but the larger the overlap, the more “aggressive” the engine.
Obviously in the above you need to remember that a complete cycle (induction, compression, power, exhaust) is 2 rotations of the crankshaft but one rotation of the camshaft, so it’s actually intake opens (then exhaust from the previous cycle closes), intake closes at the end of induction, then no valve action during the compression and power phases, then exhaust opens before the intake starts to open again.
The 307621 spare I have is listed as 10/50 (i.e. its symmetrical/both lobes have this profile, which is pretty normal). This will mean the intake valve opens at 10 degrees before TDC and closes at 55 degrees before TDC, and duration is therefore 360 – 40 = 320 degrees, but it’s listed as 240 degrees! A definition of lift includes a measure of lift for the duration – i.e. its not normally 0″. I.e. the valve will lift a little before it is considered “open” for the purposes of measuring lift. So maybe the valve is a “tiny bit open” for 40 degrees on each side before it is considered open – doesn’t sound quite right to me. What am I doing wrong here?
Duration (aka how long the valve is open in total) – the longer the better to get the gas in but obviously not so that the overlap means you are opening too early or closing too late. I have seen designs where the intake port opens before the exhaust phase has completed which allegedly helps the airflow (the airflow out the exhaust port actually helps draw the AFM in the intake port!).
Theoretically, a really “steep” lobe would be good as you keep the valve as open as possible as long as possible, but to do this you need stronger springs to keep the follower/pushrod against the cam. The springs work against the camshaft and therefore against the engine, so stronger springs introduce more drag on the rotational movement of the engine. I.e. its a balance (quelle surprise).
Intake Manifold
Purchased from eBay and claims to be from a 2500S. However, it has an unusual, thin tube running through it rather than the bulkier housing I see in the Repair Operation Manual. Having a look online, I think I might have what may or may not be part no. fzx1263fr which someone on eBay claims is the HS6 conversion manifold. I may have landed on my feet here.
It’s stamped with a British Leyland badge and Stanpart, but it seems a bit roughly engineered to me. Lots of rough seams on the inside and the divider between e.g. #1 and #2 seems a bit flawed. I might give it a bit of porting just to tidy that up if I can – from what I’ve read, attempting to change the shape of the thing is not a good idea for an amateur. There is tonnes about how these things are a bit of a weakness of these engines and you can see the path for the air in #2 and #5 involves a bit of a bend in the road which I’m sure the air won’t like much. Still, only alternative would be Webers or PI/EFI. I might work up to EFI later, but not yet…
Differential
The Diff is stamped with BTB 866 and the Crown Wheel/Worm Gear are marked with 4167 (with 009 as a suffix on the crown wheel). The 009 appears to be the backlash figure (0.009″) which is used to calculate the required shims for positioning the crown wheel properly against the worm gear and ensuring the right pre-load. Each side of the diff case has an additional measurement relating to this – for me that’s A+3 and B+2. I think this is the amount of material taken off each side of the casting in the factory to ensure the bearing are fitted correctly (but don’t quote me!).
allanwatkins 