Billmack wrote:6800 rpm . That's what formula ford used.Best FF engines do 7200 and crankshafts break. Forget 85 mm bore. 95 percent chance it won't end well. Stop at STD bore for an elan maybe .020 past that.
We seem to be reviving an old post.
Yes I agree with a standard crank I would not use over 6800 rpm. Seen lots beak when used at 7000 rpm for extended periods on the track.
In terms of what bores are possible it depends on the original block bore casting diameter and how centred the original bore is in the casting and then how to offset bore to maintain maximum wall thickness and how thin a wall thickness you are prepared to go with. I prefer a minimum wall thickness of 3mm / 120 thou for race engines but could be talked into 2.5 mm / 100 thou for a road engine depending on the location and extent of the thinnest spots.
Lots of devil in the detail e.g. if the original bore is well off centre versus the casting you can struggle to get it even all around as you end up boring into air on one side !, the amount of offset possible is limited which limits the amount of bore increase in a block with non centred bores which is normally the case as it is rare for the bore to be central in the bore casting due to tolerances on the core locations
In general with careful measurement and offset boring 83.5 mm is safely obtainable in around 70 to 80% of blocks. 85mm is obtainable safely in maybe 5% to10% of blocks. No difference between the various Block versions and or between L and non L blocks of specific casting number versions in terms of these outcomes
The South African AX blocks are supposed to have a thicker bore wall casting but I have never measured any to see what they have and what variation exists. However the gap between the cylinders pretty much precludes walls beyond the upper end of the tolerance range across the standard blocks due to sand casting requirements with non siamesed cylinders so they cant be much better than a standard block at the top end of the tolerance range.
The modern Ford motorsport blocks appear to have tighter tolerances on the bore casting OD and location and you will struggle to go beyond 83.5 mm in any of them from what I have measured but they will all also go to 83.5 mm
cheers
Rohan