I am rebuilding a 1275 Cooper s engine and have yet to decide on the best course of action for the main centre bearing. Wondering what the general view is regarding steel straps as I am leaning this way, mainly to avoid line boring costs. It's a street car using GR's RE83 cam and minor head mods.
Welcome and appreciate any feedback.
Txs

I'm no expert but I believe you only need a centre main strap if your running high RPM?
GR suggested I didn't need one for my Supercharged engine as I was not planning for high RPM but use the high torque at low RPM. Having said that, when the engine was on the dyno and being " run in?" It would quite happily rev to 7000RPM

I have had the engine apart due to a gearbox issue and after about 2000km the bearings had no wear or marks.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

yeah, don't bother, RE83 runs out of puff by 6500rpm and a mildly tuned motor doesn't need it.... save your money

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did I tell you that I won a trophy?

Thanks for the feedback. I think I will spend the money on a good harmonic balancer and crank balancing then. Cheers for that!

Cooper S has a steel centre cap already, and it's design is stronger than the other 1275s.
No need for a strap on a road car really. Particularly the cheap crappy ones they sell from UK.
I like how they ask you to weaken the cap first by milling a big bunch off the cap to make it flat.
If you MUST have a strap, there are better ways to do it. Whilst keeping the cap almost stock.

Caps only tend to break when you buzz the engine to a zillion revs on overrun by downshifting at high speed, instead of using the brakes..

[edit] I made one for my 1310 > 1412 stroker, before I was told it didn't need it.
I used a chunk of EN25 steel from my local toolmaker, and grade 8 bolts.
Yes I suppose the strap could have been a poofteenth longer, but it's all the EN25 he had.
Cap was ground flat in the middle only, then the strap was stepped to have .002" crush on the middle when tightened.
Cap/block bore was checked after fitment, it was still round and on size.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yeah forget the centre main strap, spend your money on (the things you've mentioned) and a good, e.g. ARP, set of main studs. Most centre main straps use bolts that are are longer because of the extra height of the strap, which is a big backward step. I have seen people use grade 5 bolts, and they wonder why they have trouble . . .

Be aware that if you do want to fit a strap to an S block, most likely need to have one made as the (crappy) ones usually available are for non S 1275 blocks, which have different main bolt/stud spacings. As Kev says, there are better ways than machining the cap flat to start with (another backward step) if you must run a strap.

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Steve

The problem isn't so much with the cap, its the fact that we are running a 3 bearing crank. The crank will flex at high rpm (or any rpm for that matter) its the flex about the bottom half of the centre main that causes the problem. The crank flex pushes against the centre thrust bearings and the backwards and forwards pressure of this flex on the centre main cap eventually causes failure. (like taking a piece of wire and bending it back and forth a bunch of times until it snaps) You can machine a bit off your thrusts (the ones that sit against the cap not the block) to allow clearance for the flex. Basically you are only running thrust pressure against the top bearing but this isn't a problem. It costs you virtually nothing and will solve the problem. there are more detailed methods of doing this, PM me if you want to do down this route but for your application not necessary.
G.

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

Thanks so much for all your feedback. It is very much appreciated.

Here is a picture of the centre cap, it appears to be roughly ground flat already. Do you recognise this as a standard 1275 centre cap? If this is a stock cooper s centre cap I am inclined to fit it with ARP studs and balance the crank etc and leave it alone.

Leave it alone, it's an S cap.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Got it, thanks a lot Doc

Use standard cooper s studs and nuts.
They are well and truely up to the job.
Cooper s ARP stud kit does not fit.
Studs are too long, washers and nuts are too thick.
Foul on the gearbox case no iffs buts or maybes!!!

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The non S 1275 ARP studs are also too long for transverse engine applications as they too foul the gearbox case. It's not a big job tho to shorten the height of the studs and to relieve slightly the gearbox case so all clears.

I also agree that the original cooper s studs and nuts are up to the job, provided they are the original OEM studs and nuts and not recently manufactured replacement parts.

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Steve