15 replies to this topic

[Mike D]

Hello folks, I’m rebuilding my old 1275 carb mini sprite at the mo, and it’s getting ready towards the time I whip the engine out to give it a coat of looking at. It’s a fairly low mileage unit with 58k on, and last time I was using it, it ran spot on, if a little lacking in pep. That was nigh on 13 years ago now, it still turns over fine, but I’m going through the whole car so it’s going to get a refresh as I assume the stem seals will have gone hard and it’ll smoke like a trooper! As it’s coming out, I’ll be whipping the head off to sort the stem seals, replacing the water pump and all the coolant hoses, and I understand the cam in these sprite motors is basically a bit of round bar with sod all lift, so I’d like to improve that, probably to an AC Dodd RS or Minispares Evo jobby. I’m not looking for all out power and revs, I’d just like a torquey little motor for daily driving. 70hp and some torque would do me fine. That brings me to the following – I’ve got a spare MPI head to get a bit of work done on, however this comes without the bypass hose connection. If I fit the MPI head, can I just change all the cooling hoses to SPI /MPI spec and fit a water pump without the bypass too? All the hoses need changing anyway so it makes no difference to the budget. Also, is it worth putting bigger inlet valves in the head while it’s out? Or will the 33’s do if I do a bit of port work? I’m aware of the 3 bearings cranks rev limitations so it wont be getting flung north of 5k all the time. Carb at present is a HIF38 but I guess that’ll need changing to a HIF44? Thanks for any advice, Mike :)

[Turbo Phil]

The MPI uses a front mounted radiator, so unless your planning on swopping to that the hoses will be no use. You can run without the bypass hose just drill a few 1/8” holes in the stat to allow circulation.
For your spec some simple valve and valve seat work will show some worthwhile gains & as this is where the real bottleneck lies you should prioritise this area first. If you do “port” the head, simply grind the guide boss away and give the rest of the port a good clean up, no real enlargement is needed.
I’ve started a series of videos on my YouTube channel covering some head modifications, you might find it useful.
https://youtu.be/n1brXc3qD5E

Phil.

Edited by Turbo Phil, 11 January 2021 - 10:32 AM.

[Mike D]

Thanks Phil, I'll give that video a coat of looking at when I get home :) MPI hoses are out then, I forgot about the front mounted rad on them. I guess the SPI ones should be ok? Or is the drilled stat the done way? Cheers :)

[Cooperman]

The Evo001 cam will suit you fine.
You need an HIF44 carb to optimise the head flow and the inlet valves should really be the 35.6 mm ones.
The flywheel assembly is a bit heavy and a bit if lightening is not expensive but it does improve the acceleration.
All that will give much better torque and you might expect 75+ bhp. If the exhaust system is also improved something around 80 bhp would be possible and with exvellent torque improvement.

[ACDodd]

Bolt on kit is possible to take that engine just over the 80hp mark on the stock cam.

Ac

[Mike D]

I'd seen a few of your posts regarding the sprite engines previously @ACDodd - the thinking behind the cam was purely a change as the engine was out - the price between that and the 1.5:1 rockers seems negligible, as it's going to be a daily driver I didn't want to run into any rocker alignment issues, or with such a mild cam are they not a problem? Would you recommend the 35.6 valves for the power i'm after or do you reckon a clean up and fettle of the stock ones would be fine? The exhaust at present is a sportex side exit single box (2" I think) I guess this should suit it fine still? Or is it a bit on the large side? Thanks for the advice, Mike :)

[Cooperman]

A 2" dia exhaust is a bit on the large size and results in a gas velocity which is too low for optimum gas extraction. The ideal size is 1.75" and either a Maniflow or RC40 twin-box system would be best.

The 35.6 mm inlet valves as fitted standard to the MG Metro, are a great improvement on the rather-too-small 33.5 mm ones.

Whilst the engine is out and improvements are being done it is easy to measure and calculate the compression ratio. Ideally for your car you can go up to 10:1 CR. Above that you would probably need to use 97 RON or better petrol, although that is worthwhile in engine performance terms.

At over 50,000 miles it is a good idea to fit a new cam and followers, plus a new oil pump. Check the crankshaft journals at the same time and fit new big-end and main bearing shells as well as new crank thrust bearings. Obviously fit a new timing chain and tensioner. Also check the bore wear and consider a light hone and a new set of piston rings. A re-bore should not be needed at 50k miles.

If fitting a new cam, make sure you check the installed timing and re-set it as necessary. It is not difficult and we are all here to help and advise.

However, to re-state, the gas-flowing of thehead is the key to making any A-Series engine go better. If you get David Vizards book 'How to Modify an A-Series Engine' it shows exactly what can be easily done to improve the head.

It is always hard to know how far to go with engine building. You can spend lots more and gain little extra. For example, some may advise buying all new ARP studs and bolts/nuts for main and big ends. However, unless the engine will be seriously revved and driven extremely hard, they are not necessary. In fact, the originals should be fine to re-use. It is different for a race engine, but you are seeking better torque and thus better acceleration. 

[ACDodd]

If you are changing the cam fine no problem. Yes 35.7mm inlets are the way to go. The head need a full porting for road use to make the best of the engine. Select a road cam and set the compression at no higher than 9.75:1.

Ac

[Spider]

I'm pretty sure you'll find the cylinder head on your 1275 also has no By-pass Adaptor.

 

I run the MPI heads on many of my builds. There's nothing better or special about them, it's just that I was luck enough to land on a few new ones and these already have seats fitted for unleaded fuels.

 

They don't have the hole drilled for the Heater Take Off, but that's straight forward to drill out and the 2 x 1/4" UNF Holes are already drilled & tapped. They also don't have a pocket for the Temp Sender. There's a good many ways of getting around this, but I just drill and tap these. They are not quite straight forward as the hole is stepped, but that's just a matter of drilling right through on the lower of the threaded holes that's in the casting with a (from memory) 10 mm drill, then drilling about 12 mm deep with a 14.5 mm drill, then tapping 5/8" UNF.

[Mike D]

Thanks all, Vizard's big yellow book arrived last night so i'll get reading that - quick question about the bigger inlet valves as the mpi head seems to have unleaded seats on all 8 valves, do I need to remove the inlet seats and buy larger ones? or machine what's there? Spider, my sprite head definitely has the bypass hose fitted. ACDodd, would your RS cam be suitable for what I've got planned? Thanks, Mike

[Turbo Phil]

Are you sure it has inserts on the inlets? I’ve certainly never seen this on a standard factory head. Any pictures?

Phil.

[Mike D]

Turbo Phil, I think that was a mistake on my part, I've not got the valves removed yet and I think I've mistaken the recess in the chamber around the valves. Nice work on the videos too, very informative, I'm looking forward to part 3 of the cylinder head work :)

Re the spec of the rebuild, as with everything on this 'quick weekend recommission' it's all getting out of hand. The shopping list I've got is below:

check wear on block, bore to +20' if necessary - swiftune die cast pistons

bit of work to head, bullet nosed valve guides, 35.7 inlet race spec valves, standard race spec exhausts

HIF44 carb, minispares manifold

1 3/4" exhaust and LCB minispares

double valve springs

minispares hi grade cam followers

either AC Dodd RS cam or minispares evo001

New big end and main / thrust / cam bearings / oil pump

recurved distributor to suit cam

stock 1.3:1 rockers

centre oil pickup

duplex Vernier timing gear

 

I'd like to get it to about 9.75:1 compression, or whatever it'll happily run on normal unleaded

 

I'm not going to be wheelspinning around like a hooligan, is it worth getting a cross pin diff or should the stocker be ok? Is a lightened flywheel necessary / worth it?

 

Cheers, Mike

 

*edited as it sent the formatting all over first time!

Edited by Mike D, 21 January 2021 - 10:16 AM.

[Turbo Phil]

Thanks for the positive feedback on my videos, and glad you’re enjoying them.
For your intended spec and use the road spec valves will be fine & with a back cut applied flow within a hairs width of the race valves.

Phil.

[Mike D]

I've got on a bit more with this recently

 

Engine is stripped. Bores look OK but i don't really want to end up pulling the engine in another 30k so i'm thinking of going +20 with some swiftune die cast pistons. That should up the compression a smidge too.

 

Crank and rods are out, please see pic below for crank measurements:

 

 1275 crank sizes.JPG   50.19K   6 downloads

 

Crank is stamped YGY

 

Bearings and lands all look good apart from there seems to be a bit of a score on the centre main.

 

Bearings fitted are:

mains - bottom shell = TAM2144

           - top shell = TAM2145

 

big ends - 12G1301 STD

 

thrust - bottom = TAM2131

           - top = TAM2130

 

While it's out - is it worth having it reground to suit some +10 bearings? I'm buying new anyway so it makes not much difference. The sizes there look OK to me but i've read elsewhere about the new bearing shells being a bit generous on clearance?

 

While it's all out it's getting a lightened flywheel -does this need balancing with the clutch and crank?

 

Cheers for any advice,

Mike :)

[ACDodd]

Those journals need a regrind aim for 1.741” on the beige ends and 1.991” on the mains. Check the clearances with your new bearings.

Ac