27 replies to this topic

[The Matt]

hang on lads, I've got an idea!

If I were to take one of the 73.5mm pistons out of the spare 1380cc block and use that as a pattern they could copy the dimensions of that dish and machine it into the new pistons. The guys at work could probably do that for free! Then I guess they'd just need a static balance afterwards??

[Dan]

That plan will only work if the crown is thick enough to take the standard type dish. I think the dish in those is a hemi, which will cut down into the piston a lot further in the centre than a flat bottomed dish of the same volume as described by Mini Sprocket.

The engine builders you spoke to might have been interested in the shape of the dish purely to know if you wanted something exotic like swirl chambers in the top, I can't think they'd have a problem with making a normal dished piston.

Easier than machining down standard pushrods (getting the end profile right is almost impossible) is using MED's tubular pushrods. They are a lot lighter and stronger than regular pushrods but are a tube with removable end caps so you can machine to the length required and re-fit the ends.

[icklemini]

regarding the pushrods - this comes into the teritory of valve train geometry which if you want the best, you 'll have to work out anyway. Dont just lob off 2mm....

regarding how much you can skim off - you'll need to check how much meat there is on the block between the deck and the waterjacket...

On a large bore - dont deck the pistons to the top of the block as the gasket does overhang into the bore - if the pistons are flush you can knacker the firering... 10 thou down tops...

TTFN,
Dave

[The Matt]

Okey dokes people, thanks for the advice (again).

I am working on the basis that the pistons will take it (will verify when I get off me backside and get into the garage), I need a dish of 2mm depth and 64mm diameter, with a little 2mm fillet radius in the bottom (leaving the 5mm each side as suggested). This will give me the correct CC assuming I do get the deck volume down. I will get the engine guy to assess block/water jacket levels and suggest a slight skim. According to an email I recieved (from a well known engine guy who has built 1400s like this before) I want the pistons to be about 0.020" down the chamber. So the skim (dependent on block suitability) will be done on the basis of bringing the piston crown to that level.

[Sprocket]

Sounds like a plan

What CR are you running

[The Matt]

What CR are you running

Well, despite the fuel (octane) related problems, I am thinking of running about 10.5:1-11:1. I know its on the high side, but I think it'll smooth things out a little. Just need to make sure that centre main strap is on tight, lol. I am gonna be using 11 head studs too!

[The Matt]

Righty ho then, I have phoned a couple of places today about getting the dish and it aint a problem.

I can get it done by the CNC workshop that we use for work stuff if I give him the dimensions and do a quick drawing. I have worked out that to get the right CC I only need 1.7mm depth out of the piston, at the moment they are a shade over 6mm thick in the area concerned. The real bonus is that, with the deck skim and using a decent fast road head with 25cc chambers I can run the 1400 at 10.7:1CR and if I swap over to the 1380 block (just incase the 1400 doesn't deliver what I expected) I can run that with the same head at 10.4:1CR. So I am gonna give the CNC guy the go-ahead (it's gonna cost me bugger all).

Only thing I am wondering about is any kind of post CNCing surface treatment? Is anything necessary treatment wise??

[Dan]

I believe that pistons are just raw aluminium, not treated.

Personally with that CR and the amount of meat that's coming out of the block I would be inclined to go for a four bolt steel centre main cap instead of just a strap. And possibly even all steel main caps. The 1400's that MED used to build for Midgets had all steel main caps I seem to remember but then I only saw one once.

[The Matt]

I believe that pistons are just raw aluminium, not treated.

Personally with that CR and the amount of meat that's coming out of the block I would be inclined to go for a four bolt steel centre main cap instead of just a strap. And possibly even all steel main caps. The 1400's that MED used to build for Midgets had all steel main caps I seem to remember but then I only saw one once.

How much work is it to fit one? Getting the block drilled and tapped wouldn't be an issue, but do you have to do any refacing work to the cap to ensure it is perfectly level with the block? Also, where are they available from?

EDIT: Line bored, to get it spot on, then the face machined. Any ideas of cost for that work please anyone?

[Sprocket]

Fitting a four bolt main cap is IMHO a very good idea as it gives a bit more strength that the extereme over bore removes from the block

How ever, it will need line bore to make sure the bearing housing is perfectly round and central with the others. All three are usualy done at the same time to ensure all three bearings are true. This requires a quantity of metal removing from the mating face of the other two caps to achieve this.

Down side is the cost, at around 100 notes just for the machining, 80 notes for the cap and 35 notes for the drill and tap for the extra bolts (if you do have to pay for it)

It is the best insurance any high performance A series engine could do with, other than going steel crank.

[miniboo]

big difference in price between a twoer and a fourer then!!!!!

[icklemini]

...but then if your building a decent balls out engine you'd want to get it line bored anyway...

TTFN,
Dave

[The Matt]

Thanks again guys!

I will look into the four bolt centre main cap. I have decided that this engine is sort of dictating my project progress too much at the moment, it's really slowing me down and finding the time to run around getting bits machined here and bits balanced there is doing my nut in!

Building this engine is proving to be more of a challenge than I'd thought, both with technical knowledge and budget!