Esprit2
Member
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Matthew,
Add ten-pinning to your list of machining jobs. The narrow 907 block & main bearing panel were stiff enough for the short-stroke crank and modest power output (okay for the street, marginal for racing). The long-stroke 2.2 crank brings more vibration along with it, and most people who invest in the conversion also go for more power. In that case, the stock 907 block & MBP are not stiff enough. For the 910 & 912, Lotus went with a wider and significantly stiffer MBP.
The slender 907 has one locating ring dowel at the front, and one at the rear. The wider 910/ 912 added a third one at the middle. You can significantly increase your 907's rigidity by putting a ring dowel around the base of every main stud. There are ten main studs, so it's called ten-pinning.
A 'good' machine shop will place the ring dowels as accurately as possible, and mirror the pattern accrately on the MBP. But perfection is absolute, and invariably it all ends up 'off' by a tiny amount that can't be ignored. So, after ten-pinning, the block & MBP must be align-bored together.
However, boring oversize is a problem, since over-size OD bearing shells are becoming unobtainium. It would be unwise to align-bore oversize unless you already have a set of over-size bearings in hand.
As an alternative, fly-cut a little off the mating faces of both the block & MBP... like 0.005-0.010" per side. As a result, the bore becomes an undersized football shape. Now align-bore it back to the standard round diameter on the original centerline. Choose a skilled shop to do that work.
That process shortens the distance between the cam and crank centerlines. That introduces more slack into the timing belt loop by twice the depth of the cut taken on the bottom of the block. As the tensioner pulls that extra slack out of the belt loop, the cam sprockets rotate a few degrees in the retard direction. Not enough to be a concern for a street engine, but a racer might want to invest in adjustable cam pulleys in order to get the cam timing spot-on.
On the other hand, Lotus' current composite head gasket's compressed thickness is about 0.5mm / 0.020" thicker than the original steel-asbestos-steel head gasket, and that increases the distance between the cam & crank centerlines. That gives the opposite affect of advancing the cam timing a few degrees.
The net effect is like robbing Peter to pay Paul. The amount of retard due to milling the bottom of the block is typically less than the amount of advance due to the gasket's greater crushed thickness. And as I understand it, the various aftermarket clones of the composite gasket have different crushed thickness... complicating the calculations... ie, what values do you use?
In the end, for a street engine, I recommend that you don't sweat it, let the milling retard deal with the gasket advance, then wait to see what the net affect is in the end. For a street engine, don't sweat it unless the pedantic you simply can't sleep at night as a result.
Back in the mid-1990s, I built a 2.2 converted 907 hotrod that put out about 230 very streetable Hp (used non-adjustable Lotus cam pulleys), rev'd to 8000 rpm, and I flogged the crap out of it autocrossing. There were never any bearing issues due to lack of stiffness in the ten-pinned 907 block & MBP.
Regards,
Tim EngelLast edited on 04-10-2020 06:38 pm by Esprit2
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