Cam Change - piston valve cut outs

[jono]

As part of an engine rebuild of a 1968 SE spec engine I am looking to change the cams to Sprint spec (carbs to be jetted accordingly).

My engine has been bored +20 (this was done around 35 years ago using the pistons avialable then) and the pistons have what appear to be rather small valve cut outs.

My head has been off to the machinist and had new valves, guides and seats - I have stuck with std valves. The block face was 'cleaned' by 6 thou and I will be using an Ajusa gasket.

On changing to sprint cams should I be concerned regarding the small valves cut outs - is there a risk of valve/piston contact?

If I hadn't already built the bottom end and fitted the sump I may have had the cut outs machined but I don't want to do that unless I have to.

Thanks for any pointers.

Cheers

Jon

PS: the cut outs are circa 32mm across the straight on the inlet and 23.5mm on the exhaust

[types26/36]

I don’t think there is a “one size fits all” rule with this, there are too many variables with a 40 + year old engine. I would do a “dummy build” with the plasticine check to determine what valve to piston you clearance you have. I personally don’t think you will have a problem but better safe than sorry……….just my opinion of course !

[promotor]

I don’t think there is a “one size fits all” rule with this, there are too many variables with a 40 + year old engine. I would do a “dummy build” with the plasticine check to determine what valve to piston you clearance you have. I personally don’t think you will have a problem but better safe than sorry……….just my opinion of course !

Exactly as Brian says - check it over with plasticene. New seats - if done correctly - are likely to bring the valve heads slightly closer to the pistons - presumably the old seats were recessed and valve shims were on the thin end of the spec, hence the reason for renewal?

[jono]

Thanks Al - yes, the valves were recessed and the head generally just needed 'doing' as part of recomissioning (Bill Bannister did the work)

I will do the clay test.

In terms of approximate initial timing - will the pulley marks be the same for SE and Sprint cams or do I need to start afresh?

Cheers

Jon

[promotor]

Thanks Al - yes, the valves were recessed and the head generally just needed 'doing' as part of recomissioning (Bill Bannister did the work)

I will do the clay test.

In terms of approximate initial timing - will the pulley marks be the same for SE and Sprint cams or do I need to start afresh?

Cheers

Jon

No problem Jon, just re-iterating Brian's wisdom really though!

Cam timing will possibly be slightly different between the two profiles - down to machining tolerances - but you won't have any chance to influence the timing if using standard dowels and sprockets providing you use the sprockets in the same positions as previously. It's always worth checking maximum opening point of the cams with a degree wheel and DTI though as this will show whether any mistakes have been made on assembly and also how accurate the machining of the cam dowel holes relative to the lobes is. If the cams are only a couple of degrees out from the recommended settings then you won't improve things so that you notice much of a benefit at all, but it's nice to follow the process if only just for peace of mind.

Original Lotus cams whether STD, S/E or Sprint spec were all designed to use a maximum opening point (MOP) of 110 degrees ATDC on the inlet valves and 110 BTDC on the exhaust valves.