Pistons and Rods

[Pete H]

    Hi All

I am new to the forum and the joys of Austin 7 ownership.

I have recently become the proud owner of an RTC Trials special. The engine was a little smoky but also had clutch issues. So I have stripped it. Compression before was 107-110 psi. Which I thought was probably OK? The clutch issue, violent vibration, was I believe caused by a loose lever adjustment grub screw which meant one lever was considerably lower than the other 2.

The engine has confused me!!

Its a 2 bearing Ruby I believe with a 1 1/2 inch pressure fed crank. From the picture attached can anyone recognise the piston and rod. What I find  surprising is that the bores show little wear, no ridge and within 0.001 throughout but the piston fit to me is very loose 0.007 and ring gap 0.025. Big end shells are perfect and unmarked with part number 2390RA std. However the pistons have a little play on the pin,very slight, which appears to be an interference fit in the rod.  Is there a reason for this fit or is it just poor build quality. I wonder if second hand pistons have been used with new shells and rebore.

It also has larger inlet valves double springs and modified ports. 

Thanks

Pete

[Stuart Giles]

Rod is Renault R4  The piston looks to be Renault also -what diameter are your pistons? most (the ones from the 850 motor) require a +0.84" overbore IIRC.

[Pete H]

Piston is 2.281, Bore 2.288 

Thanks

[Zetomagneto]

Piston is 2.281, Bore 2.288 

Thanks

Yes, Renault 4 rods and pistons,you are probably right about old pistons into new bores.

[Bob Culver]

.025 ring gas are not ideal but many old books give that sort of figure as a refit limit. I suspect the coarse finished rings soon develop such gaps anyway.
It seems a waste to not utilise the good block. In the 1960s and 70s there were many knurling, peining etc process to expand pistons. Often a disaster if cracks started, esp on split skirts  but many successful. Do any vintge specialiats still do?
The black varnish is characteristic of 1930s style non detergent oils.

[Roland Alcock]

Koetherising used to be one system to enlarge worn pistons. 

http://www.oldclassiccar.co.uk/piston-refurb.htm

[Douglas Alderson]

Any chance of a photo of the RTC, to compare with mine?

[Stuart Giles]

Piston is a bit on the slack side, but the ring gap of .025" really is a bit generous. Here's a scan of Hepolite's piston and ring fitting datasheet that I keep on my garage laptop. Ring gap  should be .008-.009"  using their figures.

[Pete H]

Thanks all for the help. Looks like I need to try and find some new R4 pistons, any ideas ?

Here is a picture of the car in action.

[Zetomagneto]

Thanks all for the help. Looks like I need to try and find some new R4 pistons, any ideas ?

Here is a picture of the car in action.

I don't know of a supplier of these piston without the liners, others may do.It will be interesting to see what is available, the Renault rods are nice, but wrong length to use with A7 pistons.
MG MMM pistons are a similar diameter, but I don't know the compression height.
Time for Phoenix/ Hadley/Saenz rods and slipper pistons ?