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This month has seen various areas of the ’46 progress well, with the rear subframe undergoing a complete overhaul, new brake lines being fitted, the cylinder head rebuild starting, and the crankshaft receiving a full re-grind and balance. Starting with the subframe, I was initially thinking of powder coating the whole lot, but the more I thought about it the more I wanted to find something with a little bit more longevity to it. Although powder coat is reasonable hardy, I find it hard to believe it would really stand the test of time without flaking and/or peeling off. So instead I purchased some two-part epoxy mastic from Bilt Hamber – essentially it’s super glue that’s coloured black. With Zinc-phosphate embedded within the resin, once set it provides a supertough, corrosion-resistant surface that should help preserve and protect the subframe.
The subframe was taken back to bare metal and a liberal amount of epoxy applied. Whilst the underside of the car was all apart, I discovered the hard lines for the brakes were close to corroding through, so replacements here were also produced. Once again, I wanted to make sure that whatever went back in the car would last, so Cunifer pipework was used. A copper-nickel alloy, Cunifer is significantly more resilient than steel or copper alone with the trade-off being a relative lack of flexibility – although this didn’t prove to be a problem in the slightest.
By comparison, rebuilding the cylinder head has so far been a breeze. Valves, springs and retainers were the first items to be re-installed. With all the valves numbered on removal it was a case of fitting them in the correct port and giving them a quick lap. Lapping is done by applying a small amount of paste to the seat of the valve, then rubbing it back and forth on the valve seat for a few minutes. This process grinds a small layer of material off the mating faces, helping them to create a better seal with each other. Once the lapping process has been completed, it’s important to check both faces seat around the full perimeter. This was done using engineer’s blue, which was applied to the valve, then the valve was seated (with a fair bit of force) into the head. Once seated, remove the valve and check for an even coverage of blue on the cylinder head valve seat. All valves passed so it was onto the next step. This involved fitting new rockers and rocker shafts, with the former of the DLC (Diamond-like coated) variety. With the engine receiving a set of race-y cams (full specification to be revealed later), it seemed sensible to upgrade to the more slippery, harder-wearing DLC rockers.
And so onto the crank, where a fair amount of head scratching and research has been involved this month. First and foremost, I need to thank Daren of Crosthwaite & Gardiner for doing a superb job of regrinding my crank. It turns out the crank was in a bit more of a state than it first appeared, yet after a few tense hours of careful measuring and grinding, it’s ready to spin in anger. Regrinding the crank was incredibly eyeopening for me. There’s a lot of discussion online about how marginal the big ends are on S54s, with numerous people (including myself) removing shells after ~100k miles and finding they’re in a terrible state. I knew I was in for a regrind on the big ends, but what became apparent was that the mains weren’t too great either.
Despite the main journals looking fine, and measuring up OK at first glance, the crank was bowing considerably (0.125mm eccentricity was measured between the mains). After a bit of straightening and a light polish, the mains were running true again, although it became evident they weren’t so healthy, with noticeable scoring to the journal surfaces showing. How deep the damage went was very surprising – it was only when 0.2mm had been removed from the journals (of the 0.25 total) that all the scoring was removed. It was a similar story too with the big ends, with all of them requiring considerable material removal before cleaning up. On the big ends it was always the top and bottom side of the journal that showed the most damage, evidently from the combustion stroke and rod/piston inertia during the exhaust stroke.
Considering the state of the crank, I decided it was worth doing a bit of research into the shell/big end issues on the S54 to see if a solution could be found. Reading back through information online, it was news to me that these engines originally shipped on a 5W30 oil, before numerous big end failures forced BMW to switch to a 10W60 Castrol (as used now). Unfortunately this appeared to make matters worse, and it was soon after that BMW increased the clearances in the big end shells (forcing a recall of many M3s). Looking at how tight the shell/crank tolerance is for the S54, to me there’s no doubt these engines were designed around the 5W30, with the change to 10W60 a knee-jerk reaction to try and solve the big end issues.
It was from here that I started speaking to Driven, manufacturers of Joe Gibbs Racing Oils. Its range of oils are developed from research undertaken when trying to push the limits of highly-strung Nascar engines. These struggle to cope with incredibly high stresses on the camtrain, leading to premature wear and power loss. The Driven oils, therefore, focus heavily on reducing engine wear, as well as friction reduction for a competitive advantage. Upon discussing the S54 issues with the development team in the US, the first thing that became abundantly clear is that oil changes every ~15,000 miles in a performance engine is simply not OK. Like all products, oil has a lifespan, and when it’s being pushed hard in what’s essentially a road-going race engine, expecting it to last so long is very wishful thinking. To create oils that operate over a wide viscosity range, modifiers have to be added to the formula, and over time these break down, reducing the oils’ performance. To try and make an oil last 15,000 miles, its chemistry must also be heavily biased with detergents, leaving less room for anti-wear components.
On top of this, levels of #ZDDP ( #Zinc-dialkyldithiophosphate ) in oil have been reduced considerably. This is an important anti-wear compound, but due to its slightly negative effects on catalytic converter performance, governments are restricting its use in commercial/OEM engine oils. Driven has worked incredibly hard to reduce wear, with all its oils having considerably higher levels of ZDDP. Driven even has extensive dyno results to back up its claims. I’ll be looking to run the DT40 5W40 from the Driven range; with regular oil changes (every 6000 miles or so) this should keep the motor running sweetly and ensure the crank doesn’t lose any more precious material from its journals. Regular oil changes can’t be recommended highly enough for M car owners…