Crossflow block

[shaun]

Hi
Just finished rebuilding my twin- cam head. I will be putting this on a 711m block.Please could anyone advise on all the extra parts I will need, spacers etc. Also could anyone advise me how far the pistons should be down the bore at TDC after decking, I have been advised 6-10 thou?Also has any body any information on fitting 1.75 SU carbs?

Cheers

Shaun

[tdafforn]

You need a spacer for the timing chain chest, longer chain, some way of adjusting the timing sprockets (vernier or offset dowels), spacer for the head to block breather. Ideally a lengthened chain tensioner sprocket carrier would be good, but they come from the elan factory and are currently out of date..
Most of the stuff is available from QED who do a 711M block conversion..
Cheers
Tim

[rgh0]

I normally aim for piston deck to be 20 thou / 0.5 mm below the top of the block. I believe this was the Lotus orginal setup also. This plus a nominal 1mm compressed gasket thickiness gives 60 thou / 1.5 mm clearance between the piston top and the head

My understanding is that the piston / rod assembly stretch plus bearing clearances can lead to the piston being up to around 40 thou / 1 mm higher at top dead centre at high revs especially after a bit of bearing wear than you measure when static. Thus you need to aim for around 60 thou / 1.5mm static clearance between piston and head and check the valves around TDC for a similar 60 thou / 1.5 mm clearance


You can go less and a lot of racing engines are built with the pistons flush with the block or slightly above to increase compression but you need to be careful with valve to piston and piston to head clearances if you do this. i have seen engines and heads with slight marks from where the piston was gently kissing the head at high revs because the clearances were to tight.

cheers
Rohan

[steveww]

The SU carbs will bolt straight on but there are space issues around the bulkhead and bonnet (hood). There is really nothing in the performance (airflow) between the two. Some good ram pipes and a high flow air filter will give more improvement.

[tdafforn]

I used Rohans' recipe re: piston clearance and the engine runs well..
Tim

[kmoist]

Rohan, when you say piston deck to be 20 thou / 0.5 mm below the top of the block, do you mean that as a range? 0.5 mm is about .012 vs the .020 quoted. Can you clarify what the tolerances you show mean? thanks, Ken

[CBUEB1771]

0.5 mm is about .012 vs the .020 quoted.

Actually, 0.5 mm X 0.0394 in/mm = 0.0197 in. It is typical for the piston to be about 0.02 inches below the deck at TDC, measured at the outer diameter of the piston. There is more to this discussion of course. The piston crown has to be shaped and positioned to result in the desired compression ratio as well as to avoid piston-to-cylinder head or piston-to-valve contact.

[kmoist]

You are right! I had the wrong conversion factor, thanks

[rgh0]

Sorry for the confusion I normally quote dimensions in both metric and imperial given the wide readership of the forum. I also use in my head 40 thou of inch equals 1 mm to do the conversion as I do all my own work in metric

Tolerance on the piston top to block top number when aiming for 20 thou / 0.5 mm would normally be in the range of around +/- 2 thou (0.05mm) on the assembly of crank, bearings, block, rods and pistons if all the machining of individual components has been done well

cheers
Rohan