Our first 1959 Bugeye Sprite (Wally) came with numerous spare parts, including a spare 1098 cylinder head. Wally's previous owners had upgraded much of his engine and drivetrain with later-model MG Midget components like disk brakes, 1098 cc engine, ribbed case transmission and a 3.9 final drive. Barring any disasters I didn't anticipate needing to ever use the spare head.
Fast forward to 2023! This Summer we purchased a second 1959 Austin-Healey Mk1 Sprite. The big difference is that Ms. Scarlet has, for the most part, remained intact with its 948 engine, smooth case transmission and drum brakes all-round. So why the interest in that spare cylinder head now?
The cylinder head in question is a 12G295 MOWOG head purchased on ebay in 2015. It was described as a MG Midget 1100 Rebuilt Cylinder Head and sold for $355.00. The seller listed the following specifics: Magnafluxed, new guides, oil seals, resurfaced, valves ground and the spring pressures set. Casting fitted for larger intake valves. Should I consider using it on Ms. Scarlet?
What are the differences between a 948 cylinder head and a 1098 cylinder? Can a 1098 head be fitted to a 948, and if so, is there any benefit? It seems that the 1098 head can be used on the 948 block and has better (larger) inlet ports and valves.
What about the head gasket? Conventional wisdom is to use a head gasket that matches the larger 1098 head. The 1098 and 948 gaskets may even be the same part/size.
What about skimming the head? The 1098 head has larger combustion chambers and must be skimmed ~ 60 thou if used on a 948 engine in order to bring the compression ratio up. It's recommended to check if it's been skimmed already. Measure the thickness of the head - they ALL start life at 2.750" . Any less then that is what's already been skimmed off.
In doing my due diligence on this topic, I came across a fantastic article in the Mini Spares website "Articles" section - Cylinder Head 2022. This very detailed article lists casting specs, history, cylinder head identification, and descriptions and photos of each head.
Specific to this post, here's a sample excerpt from the 12G295 description:
"Keith Calver says- The 12G295 invariably has bigger intake ports and very shallow short side radius. For most road applications the ports are too big as standard – It does not need port work doing, just sorting out the valve seat shape and valve throat area. The inlet valve throats often have very, very thin seats so the valves seem to be barely hanging on. Also generally the throat has what appears to be an insert fitted because the underside is square stepped in shape like a shallow insert, they are cast like this. The 295 was produced for Group 1 race/rally homologation in mind at a time where big was beautiful. Keith Calver is still amazed at some of the valve sizes they crammed into them. The 206 is a much better head for road use – including those with a sporty camshaft. Far less prone to cracking as well.
Often described with the wrong description and wrong inlet valve sizes as an alternative to 12G295. Even our catalogue had inlet valve size wrong with a typing error from the late 90’s on hard copy which many have copied without research but I can confirm the following. The Inlet valves were original 1098cc size at 1.156?. Cast with MOWOG above 12G206 between 2&3 rocker posts, it has a recessed back and Wellingborough symbol between 3&4 rocker posts. Has square inlet ports and no locating ring recess and same 12G202 inlet valve size but often increased by .060” to the 1.218 inlet for either valve seat repairs or performance. The better flowing combustion chamber shape is 28.4cc but smaller and deeper at around 0.450” whereas the 12G295 is larger and shallower at around 0.430”deep, usually 20-25 thou difference. Although listed as part number 28G191 by BMC in AKD3509 parts list for very early 998 Cooper with low 8.3 CR, and a few 9-1CR- (I can only ever remember seeing the raised D shape pistons for 9-1 CR with 12G295 castings back in the 60’s}. Mainly fitted to inline 1098cc 10CG engine Sprite/Midget 1962-4, pre 12G295 fitment, plus a few found on A40, Morris Minor with 1098 engines but probably had Compression Ratio modified aftermarket.
Note 11. 12G295 1.218? inlet valve 1963on"
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