Have read conflicting things regarding the exact relationship between the 747cc Seven, 900cc Big Seven and 900cc Eight engines.
Some say the latter two are virtually clean sheet designs with little commonality between them or to the Seven engine, others say either the Big Seven or the Big Seven and Eight are both redesigns of the original Seven engine.
Also if the Eight engine does indeed share a relation stretching back to the Big Seven or Seven engines, was 900cc the maximum capacity the engine was capable of or was further stretch available (as seem to recall articles about Seven-derived engines being stretched to as much as 950cc if not more)?
And lastly is it known whether the Big Seven or Eight engines were recipients of an OHV conversion along similar lines to the LAP Head for the original Seven engine (possibly by Alta, etc)?
Apart from being sidevalve in-line fours, Seven and Big Seven engines have very little in common. Big Seven and Eight engines are quite closely related, but are not identical.
Nate - if it's any help to your question, you'll find claims that the Big Seven and Eight engines were different, but there really isn't a great variation. Certainly, as said, the Seven engines were totally different and I can't think of anything much that's interchangeable if that waa your thought. Good Luck, Cheers, Bill
12-12-2018, 01:16 AM (This post was last modified: 12-12-2018, 06:28 AM by Tony Press.)
(09-12-2018, 04:20 AM)Nate M Wrote: Hello
And lastly is it known whether the Big Seven or Eight engines were recipients of an OHV conversion along similar lines to the LAP Head for the original Seven engine (possibly by Alta, etc)?
I would think not- there have only been two types of Austin 7 OHV conversions I have heard of, both very heavy and I would say unlikely that the Big 7 and 8 were ever so fitted.
And lastly is it known whether the Big Seven or Eight engines were recipients of an OHV conversion along similar lines to the LAP Head for the original Seven engine (possibly by Alta, etc)?
I would think not- there have only been two types of Austin 7 OHV conversions I have heard of, both very heavy and I would say unlikely that the Big 7 and 8 were ever so fitted.
Tony.
For those pedantic - visually I don't know of any physical appearance differences in the Big 7 and Eight engines, apart perhaps from the paint colour. The only difference I'm aware of is in the timing and the carburetters. Big 7 = Ignition timing TDC. Valve timing TDC. Carb. Choke 23, Main 90, Compens. 50, Slow 60. Eight = Ign Tg TDC. Valve tmg 5 deg before TDC (7/16" on flywheel). Carb 19, 70, 60, 45.
Perhaps the above specs could be applied to improve your Big 7? In the postwar Eight, above details were different again. Maybe this is of help to somebody. Cheers, Bill in Oz
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And lastly is it known whether the Big Seven or Eight engines were recipients of an OHV conversion along similar lines to the LAP Head for the original Seven engine (possibly by Alta, etc)?
I would think not- there have only been two types of Austin 7 OHV conversions I have heard of, both very heavy and I would say unlikely that the Big 7 and 8 were ever so fitted.
Tony.
For those pedantic - visually I don't know of any physical appearance differences in the Big 7 and Eight engines, apart perhaps from the paint colour. The only difference I'm aware of is in the timing and the carburetters. Big 7 = Ignition timing TDC. Valve timing TDC. Carb. Choke 23, Main 90, Compens. 50, Slow 60. Eight = Ign Tg TDC. Valve tmg 5 deg before TDC (7/16" on flywheel). Carb 19, 70, 60, 45.
Perhaps the above specs could be applied to improve your Big 7? In the postwar Eight, above details were different again. Maybe this is of help to somebody. Cheers, Bill in Oz.
The momoblock of the eight is about three-quarters of an inch longer than the Big Seven, also I seem to remember the gearbox is also longer and possibly not interchangeable between the engines. This makes the rear mounting of the gearbox extend beyond the mounting point on the cross-member. It is definitely not a simple swap.
Ron Hayhurst
The 8 engine is quite easy to differentiate from the Big 7, the water outlet on the side is much shorter. On the Big 7 it is nearly the length of the engine, on the 8, perhaps less than half that.
The two types of engine are often muddled up on eBay....
Apparently, the 8 had a reputation for burning valves, probably a combination of the valve timing changes and the leaded pool petrol.
