1380 build

[Kztom]

Hi guys,

I am doing a budget 1380 build, I acquired a 1275 engine for £15. Completely seized. Iv stripped the engine down and was going to overbore it to +020 or something, just to get past the bore pitting. (Its been left outside with the head off). Doing a bit of shopping online for new pistons and came across a set of 73.5mm second hand powermax, Af20659 STD 9cc dish, so I bought them. I know about the offset bore etc etc and im no stranger to engine builds.

My question is, is the 9cc dish too much for a regular engine, ie.. none turbo? I've worked the compression out (roughly, as the engine is not even bored out yet, so I cant mock build to measure deck height etc) everything is standard apart from the bore size and 9cc dish pistons, using a bk450 headgasket. I keep working it out at around 8.4:1 , which i would like it to be more.

I've seen so many conflicting reports on these 9cc pistons , mainly from the mini guys, some say say fine, some say best for lower comp turbo builds. I dont mind skimming the head and block to achive admit decent comp if anyone has any rough figures its a 12g940 head.

[IslipMinor]

We have a 1380 in our Minor, and it has Omega 11cc dish pistons in. With a cylinder head volume of 21.5cc, this gives a CR of around 10.3:1. The head is a 12G940, converted to 11-stud and worked on by Peter Burgess with Cooper 'S' MkIII size valves (1.4" inlet and 1 5/32" exhaust).

The block was very lightly skimmed only to clean it up, and the piston tops are 0.008" below the tops of the bores. The head was skimmed to achieve the chamber volume of 21.5cc, but I do not know the actual thickness exactly.

The whole engine runs very well indeed - if you are anywhere near Oxford, please call in and sample it for yourself.

[Monty-4]

May I add - consider the camshaft your'll be using for the build. Different static compression ratios will suit some better than others - dynamic compression is a consideration. AC Dodd recommends no higher than a 9.3:1 static CR for his tame but torquey road cams, for example.

[IslipMinor]

Yes, very much so.

I am using a Piper BP270 (276° timing), and when first fitted I timed it 4° retarded, which gave a very flexible engine response, with no really noticeable 'on the cam' feeling. At a rebuild nearly 3 years ago, I took the advice from Piper to set it 'straight up', and now there is a very noticeable reluctance to get 'on the cam' until nearly 3,000 rpm. It still runs very smoothly below 3,000 rpm, just not a lot of real grunt!

Not had the chance to get it on a rolling road with the new cam setting, but with the previous one it had 112 bhp @ ~6,000 rpm and >110 lbf ft torque at ~4,000 rpm.

[Kztom]

Yes, although I've not even thought about camshafts at this stage, I have had expirience in the past with rotax flat track engines I used to build. They were strictly race only as the tick over had to be stupidly high just to idle.

[Kztom]

We have a 1380 in our Minor, and it has Omega 11cc dish pistons in. With a cylinder head volume of 21.5cc, this gives a CR of around 10.3:1. The head is a 12G940, converted to 11-stud and worked on by Peter Burgess with Cooper 'S' MkIII size valves (1.4" inlet and 1 5/32" exhaust).

The block was very lightly skimmed only to clean it up, and the piston tops are 0.008" below the tops of the bores. The head was skimmed to achieve the chamber volume of 21.5cc, but I do not know the actual thickness exactly.

The whole engine runs very well indeed - if you are anywhere near Oxford, please call in and sample it for yourself.

I should imagine my 9cc dish would be okay then! I will also have to skim the block as its rusty. I should imagine a couple of thou will clear that off though.

[IslipMinor]

With a 1380, there is not much material left between the bores for the head gasket to bed on, so both block and head faces must be perfectly clean and free from even minute blemishes.

[Kztom]

With a 1380, there is not much material left between the bores for the head gasket to bed on, so both block and head faces must be perfectly clean and free from even minute blemishes.

Thanks for the tip, its going to be skimmed until it's right. I dont have vizards book and so many conflicting reports online, is it 10 or 15 thou offset. Cheers

[philthehill]

If you are building a 1380cc engine you do need the Vizard book.

The correct cylinder off-set is:-

Cylinder No: 1 & No: 2...…………...0.015" forward and 0.010" towards the camshaft side of the engine.

Cylinders No: 3 & No: 4...………….0.015" rearwards and 0.010" towards the camshaft side of the engine.

Be aware that the transverse oilway at the rear of the block may be broken into when off-sett boring. This can be repaired but needs specialist repair.

The skimming the top of the block (especially the depth of skim) needs to be taken into consideration when determining the dish of the piston as the amount of skim does have an effect on the compression ratio.