Interesting development re: engine spec

[bertavfc]

Hi, having removed the head from my '69 +2 after a Cometic head gasket failed to seal (will go back to copper/ally, head/block both true), I had the opportunity to investigate my engine spec further after my suspicions were aroused by the fitment of sprint cams, details gleemed thus far are as follows:

* Head not stamped at boss,
* Head Numbers as follows: WM9403 - N2446 - 26E311,
* Twin grooved "Sprint" cams fitted,
* Weber 40 DCOE 31 carbs (I believe sprint/SE fitment),
* 180 PSI across all 4 pots on original (sound) head gasket,

Unfortunately I don't have a micrometer to give valve sizes, might try to beg/borrow one before reassembly. If anyone can help spread some light on the issue it would be very much appreciated, as far as I'm aware I have a standard block with the earlier style large cut-out pistons, many thanks in advance, Rob.

[trw99]

Hi Rob

Looking at the numbers you have found on the head only one bears a similarity to a Lotus engine number, N2446, albeit missing a fifth digit. Engine numbers in the N244## range were domestic Big Valves fitted mostly in the first few months of 1971 to Sprints. If you can find an additional digit lightly scratched in after the 2446, that would confirm your head as an early Big Valve. In addition, do let us know where that number is on the head, please.

40 DCOE 31's were indeed a Big Valve fitment, with 33mm chokes, a 120 main jet, a 155 air corrector jet and a 35mm accelerator pump jet as standard.

Overall it looks like your engine may have been given a Big Valve head at some stage.

Tim

[rgh0]

It really does not matter what the head was originally and many heads have had sprint cams fitted at some stage in their life. The basic head casting did not change with the introduction of the big valve engine

40 DCOE 31 Webers were used for a number of years on both big valve engines ( 33 mm chokes). and on standard ( 30 mm chokes) and S/E engines ( 32 mm Chokes ) and each choke size was jetted differently.

To convert a standard or S/E head to match the original big valve spec you need to do 3 or may be 4 things

1. Fit the bigger inlet valves and cuts the seats correctly for this slightly larger valve
2. Increase the compression ratio by shaving the head ( this has probably already been done during rebuilds over the last 40 years
3. Fit the Sprint cams
4. I you want it to be cosmetically as well as functionally a big valve head then you can also fit a big valve cam cover and stamp the boss with a suitable big valve letter and change the engine number on the head if it has one to one in the right range:D

A bunch of other detail differences with the rest of the engine some of which which are also functionally important such as tubular headers, different distributor advance curve, and some which are just cosmetic such as airbox style and different block casting number and engine number.

cheers
Rohan

[bertavfc]

Hi, many thanks for the replies, Tim, the head number you refer to is on the underside of the head on the exhaust side adjacent cylinders 2 & 3 (the middle number), I have had a good look and can't see any more digits unfortunately, I did however make a note of the number on the back of the head which is as follows:
L P (obviously)
A350 6783,
I should be able to get hold of a digital venier to measure valve sizes to comfirm wheather this is a modded standard head or an original big valve item, of course the PO could have had a standard head rebuilt to sprint spec in all regards, may never know, on an aside can anyone give me approximate compression expected on a standard block/head as appose a high compression big valve?, unfortunately I don't have the maths requisite to work out my compression ratio from bore/stroke/compression ect, again many thanks, Rob.

[rgh0]

Standard and S/E engines had a 9.5 compression ratio Big Valve engines had a 10.3 compression ratio. This was achieved by shaving the head from a standard thickness of 4.640 to 4.600 inches.

cheers
Rohan