Boring out a 1725cc Alpine engine

This article originally appeared in The Alpine Horn, the magazine of the Sunbeam Alpine Owners Club. It was originally written by Pete French. Submitted by Karl Gilchrist.

BIG ALPlNES

The Americans say that there is no substitute for cubic inches and in many ways that is true. Here are a number of ways to get a few more "cubes" for your Alpine.

First get a 1725 engine if yours is a 1592. The recommended maximum you should rebore the block is 40 thou which will give you 1775cc but Hepolite pistons are also available in plus 60 thou and even plus 90 thou(!) which will give you 1789cc and 1823cc respectively.

This is a very worthwhile increase and involves no extra cost over that of a standard rebore. The Hepolite piston part number is 18861/K 0.060 or 0.090.

That was the easy one. For those of you wishing to go further it is possible to go to 1922cc by using 1500cc Avenger pistons of diameter 86.1mm as against the piston diameter of 81.5mm.

The following method of installing these pistons in an Alpine engine was developed several years ago by Swaymar Race Engines of Leatherhead (Tel: 037 23 79495). They tuned a 1922cc engine to give about 200 bhp and installed it into a Rally Escort. They said at the time that it was cheaper for them to get 200 Bhp from the Rootes/Chrysler engine than to get the same from a Ford engine. I shall describe the full works conversion.

The Whole Hog

Method. Take your 1725cc block and have it overbored to take 1500cc Avenger cylinder liners. That is the expensive part. Avenger pistons are 115 thou shorter than Alpine pistons and also have a large bowl in the top giving a very low CR if used directly. The way round this is to obtain con rods from a 1500cc iron head Hunter engine which are 220 thou longer than those fitted to 1725cc engines. You then need a set of low Compression 1500cc Avenger pistons. Part number 18854 (Hepolite). These have a bowl in the top about 88 thou deep and almost as large as the piston itself. If the rim of the bowl is machined off, you have a flat topped piston 68 thou shorter (203 thou shorter than the standard Alpine piston).

This short piston combined with the long con rods gives you a 1922cc engine with flat top pistons just about flush with the top of the block. (In the standard engine the pistons do not come fully up to the top of the block.)

If a standard head is fitted the CR will be totally unusable (over 12:1), even the H120 Holbay head will give 11:1 CR which is far too high for road use (good for racing though). The Holbay head has a chamber volume of 43cc and the standard head a chamber volume of 38.6cc Either of these can be used but they must be ground out to a volume of approximately 50cc to stand a chance of running on four-star petrol.

Obviously only fairly new heads which have not been skimmed can be altered to this degree. It is also possible to fit even bigger pistons using this method but any thing bigger would require special head gaskets and some other modifications.