Engine – Main cap issues
I have been talking to more and more people recently about main caps, main straps and steel main caps and 4-bolt steel main caps. Most are somewhat surprised that I do not advocate using anything other than the standard caps. And that steel caps should really only be used where the standard one has broken or been lost (not a problem I have encountered!) - in other words as a rescue fitment rather than a planned replacement. Various reasons for this, but mostly because, contrary to popular opinion, main caps do not break – they are broken. Broken by errant drivers who continually carry out savage down-shifts – use the brakes to slow the car they are FAR more efficient at it. Or who continually over-rev the motor through missed gears. And broken through badly balanced rotating assemblies. Not a problem at road speeds, but in race engines they can be very destructive.
To be properly effective, a centre main strap needs to be ground flush as does the main cap face once machined off with great accuracy. For those that have cared to look at a main cap/strap when disassembled after some period of running – you will see where the main cap has rubbed an elongated, shaded in figure of 8 between the main cap bolts. Obviously the strap is not holding the main cap down any tighter/straighter. And really the main bearing housing should be line-honed after the strap is fitted as it slightly distorts the cap in a different way than when using no strap. Not the cheap fix/protection many believe. And just because some say 'I've always fitted main straps just as they are and not had any problems'. The question then is – have you tried running without them?
And steel main caps – whether 2 or 4 bolt types, need to be of a decently quality steel. I have had extensive discussions with friends and colleagues engaged in the performance automotive world as engineers and engine designers. They all have the same thing to say about the 4-bolt main cap. Without spending a fortune on proof testing the installation, so from a purely engineering point of view and considering the block to which it is being fitted, it is likely to weaken the block rather than increase the strength/support of the centre main. Yes, I know some of the A-series renowned specialists almost insist on their fitment – that does not make it correct. I suspect they are fitting them more to try and fend off the results of idiot drivers doing what I described earlier. Those that have taken the time to look will also notice that the bolt heads settle in a lot more in to a steel cap with no washer than they do in to the standard cast caps. This is because the standard material is much tougher/harder. A sobering thought eh?
Whatever you decide to go with, one thing I will say is a must – the use of proper, hardened steel washers under the main cap retaining bolt heads. I believe that most of the main cap movement in the performance A-series comes from the main cap bolt head settling in to the cap material in use. This allows the cap to move around, stress the cap material and cause the cap to shuffle and settle in to the block face, effectively releasing/easing the torque setting of the bolt - or stud if using them. The result will be a misshapen bearing housing causing oil pressure loss and ultimately cap failure. The centre main suffering most as it tries to contain the whipping mass of two piston and rod assemblies. ARP do some standard engineering type washers that I am using – part number 200-8530. It is likely you will need to grind a small flat on the edges of a pair of the washers – usually for the front main cap – to clear the crank web, check this with the crank and thrust washers in place. Where used in a Mini transverse (i.e. Mini) application you may well need to relieve the gearbox casing under the rear main cap bolt head positions, or grind the main cap bolt heads down to give clearance.
Caption – pic 1
This is a full race, 140bhp/106lb/ft torque Midget engine run to 7,800 to 8,200 rpm that has now completed two full and successful seasons without missing a beat or requiring anything other than a strip, check, clean and re-assemble using all the original parts including rings and bearings. Note the centre main is neither 4-bolt or strapped, and that ARP washers are used on all bolts.
Caption – pic 2
Here is a prime example of how the main cap bolt eats in to a steel main cap where no decent washers are used. Obviously if the bolt head is eating in to the main cap like this, it is releasing the pressure on the main cap – causing it to 'shuffle' on the block, and allow the crank to jump up and down in the bearing – causing bearing failure, oil pressure loss, oil feed loss to the big end bearings and the piston to possibly hit the gasket/head. All of which I have seen over the years.