Cheers Dave
I had to redraw the back plate mainly because I made the outerpiston from thicker 5/16 plate this would impact by reducing the clearance at the end of the stroke to 1mm so decided to increase the width of the plate to accomodate this change, also I realised that by not having adjustable jib strips I was changed the over all shape off the engine, so decided to increase the height so that it would look as though ut had jib strips fitted.
A made the back plate from some scrap yard 16mm thick ally jig plate.
First op square the plate up to size.

With the dro I find it easyer if everything is dimentioned from a common centre point also with my digital height gauge this also makes marking out easy.
Just zero the height gauge on the centre point, then you can make adjustments up and down to bring it on the size you want.

Just as a sanity check I centre poped the positions as an insurance I'm drilling in the right spot.
Set the plate up in the mill on some parallels so that I wouldn't drill into the table, and with a stop set up square to but the plate up against.
Find the edges of the plate and Zero the DRO on the centre point, first drilling op centre drill the zero position so that I have a datum point off set and centre drill for the bearing housing position, then using the PCD feature drill and tap 6 stud holes M4, and on the bearing centre line drill the edge of the plate to take 2*3mm dowels to aid alignment with the covers ets.

Then it was just a matter of walking round the edge to each poistion to drill M4 clearance and counterbore with a 5/16 slot drill to take the stud pads that will be fitted later.

The plate was too big to do all around it at one setting, so unclamp reposition and using a centre wound into the datum point and with the plate up against the stop, reclamp zero the dro on this new position and finish off the rest of the holes.


So far so good.

Now to maching the hole to take the bearing housing.
With my big four jaw independant chuck in the lathe with a wobble bar set up in the centre clock it up true to get position.

Just a matter now to put a bigger centre in the job drill the meat out then finish off to size with a boring bar.

Job done.

Still needs the bolting pads fitting and the edges rounding off so that it looks more like a casting, but I'll do that at final assy.
Stew