Stroked 2 1/2 engine

From what I understand it is possible to install a 3 1/2ltr crankshaft into a 2 1/2ltr engine on a MkIV or V…
Has anyone done this and if so what other changes were required eg carbs or carb jets.

I don’t think there is any doubt that it fits but you would need to check that it doesn’t come too close to any webs in the block and possibly grind a little off the block to ensure clearance.


Hi Graham,
I think Ed Nantes has done this before. It ends up with around 3.0 litre capacity.
Cheers, Jon.


Sounds interesting and I am sure it can be done, but I am wondering how this should get built when the factory manual states that in the 1 1/2 Litre and 2 1/2 Litre engine the pistons with con rods must be installed from beneath (as the con rods would not fit through the bore) and on the 3 1/2 Litre they instruct to install the pistons together with rods from top as they state the rods would not fit though the bottom due to the crank and block clearance being too small.

Different rods? Install pistons with rods into the block before installing the crank?


Yes, there is no difference. You can’t fit the 2 1/2 rods through the bores and you can’t fit the 3 1/2 rods through the bores.

I’ve always fitted the pistons to the rods and installed them in the bores before mounting the crankshaft. My engine is bored out to 60 thou so my pistons are bigger than normal but I believe with standard pistons it is just possible to get the pistons and rods past the crankshaft by rotating the shaft along with a piston located at a slimmer part of the shaft. I can do this with some cylinders but not all. This is probably impossible with the 3 1/2 crank.

You do need special pistons with the 3 1/2 crank because the stroke is longer so the pistons need a lower crown height such that they don’t pop out the top of the bores.

It is tricky compressing the rings when entering the pistons from below but I’ve done it on quite a number of engines. The skirt rings are rather fragile and need special care.


On older engines, where that was standard, was how I did it. My 'rent’s Auburn was like that.
Always appreciated the “crooked” bottom ends of rods, on A Series Austins and early Triumphs, addressed that problem!

The crooked big ends are fine until you come to a totally seized engine. I have in mind a Wolseley 6/80. Pistons seized in the bores and valves seized in their guides so you can’t rotate the crankshaft and you can’t access some of the big end bolts that are well up inside the crankcase.

There’s no way to remove the camshaft if you can’t open all the seized valves simultaneously and you can’t take the head off with the camshaft in place. I did manage it in the end but it was a real struggle.

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Yep! I can see how that’d be… a bit difficult.


Greetings All,

I know that the Standard Flathead can benefit from this increase.

Not sure that it’s worth doing to an OHV engine? Far easier to get a 3 1/2 liter example. Not an option for the Flathead.

Now finding an OHV with the elusive SS logo is a whole other story.

The extra engine size isn’t worth it as the standard car is quite fast enough for the brakes and handling but the trade here is to get a set of steel rods. The power increase is just a small bonus.

So the costs are: Buying a 3.5 crank, a set of XK rods and special pistons.

The alternative is: Buy a set of modern bespoke rods. Some years ago this cost about £1600.