Question: I have been told that the MG XPAG engine in the “T” series cars is a long stroke design. This observation is often followed by some kind of a remark indicating that this is a very bad thing. My question is simple what are these people talking about?
Answer: The XPAG engine is considered an under square design, this means the stroke length (90 mm) is larger than the bore diameter (66.5 mm). This was a very common design before WW2, as a matter of fact the British taxing system encouraged long stroke designs. The tax paid for any given car was based on the bore of the engine, the smaller the bore the lower the annual road tax.
A long stroke engine tends to provide more torque for it’s size than a short stroke engine making them easier to drive. Have you ever noticed how easily our MGs pull away from a stop, even in second gear, that’s the result of the relatively long stroke. The downside often noted is that these engine are not supposed to rev to very high RPM levels and that tends to be true. However our XPAG engines rev very nicely to 5,000 RPM or more depending on their state of tune. The long stroke design has been abandoned in recent years for several reason the two most important of which are: 1. piston speed and therefore friction is higher at any given RPM in a long stroke design and 2. With a larger bore there is more room for bigger valves so the engine can breathe easier and produce more horsepower for its size.
So how bad is the XPAG? Here are some simple comparisons that may shed some light on the matter. A 1948 Packard 288 ci engine has a 3.75” stroke and the 356 ci Packard engine from the same year has a whopping 4.63” stroke! The Famous Chevy 350 engine has a 3.48” stroke and these engines rev safely to over 7000 RPM. Our humble XPAG has a 3.55” stroke, doesn’t look so bad now does it? The XPAG is an old design but a very well engineered design. The engine will self limit its maximum RPM because of the small valves and the bearings are huge for the size of the engine.
Bottom line – Don’t worry about it, the XPAG engine is a fun engine to drive and very robust! So have fun and drive the heck out of it.