Cylinder liners for the 180 in a 120 engine?

[Garry]

So my cylinder liner in the second cylinder is definitely ruined.
I’ve now come across a good cylinder block with pistons (75.5 mm) that would be suitable, but it’s from a 180.
Can I fit this into the RGS without any problems, given that the cylinder liner is longer than that of the 120?
Or would it be better to keep saving up and have another cylinder liner fitted, along with having it bored out and fitted with new pistons?

 

[sfcpiet]

The 180° cylinder block will slot straight in.

The lug for the front stud at the cam chain is narrower, the stud can be swapped for the shorter earlier version. For a right Royal Bodge, just add 4-5 extra washers...

You might contemplate adjusting the piston weights to the originals, it's only the skirts that were shortened a bit to suit the shortened liners, for whatever reason. Possibly to catch more splash oil for improved cylinder/piston lubrication.

piet

 

[Garry]

Thanks Piet

 

[Garry]

Piet weighed both pistons because I’ve got the 180 cylinder right here, but the one from the RGS is shorter yet heavier (371g. on the kitchen scales) and the one from the 180 weighs 363g.
So it won’t be possible to make them the same weight.
Both are Mondial pistons.
Is the top end thicker on a 120, then?

 

[Jota Rob]

Could be gudgeon pin weights. Given an 81mm Ross piston from 2009 weighs 284g naked you must be talking assembled weights?

Obviously gudgeon pin fit and length is critical but wall thickness/length and hence weight vary, so it "may" be the pin altering weights as well as piston design.

 

[sfcpiet]

To my knowledge the only difference between the 180 and 120 flat-tops is the skirt length, basically the same Mondial piston. As Rob has suggested, check the gudgeon pin. 75.5mm was not a factory option (75.4, 75.6mm). Is the stamping on the crown still visible?

piet

 

[Garry]

I’ve got the calliper and kitchen scales out again.
Piston pin 180: 63.41 mm long and 3.25 mm thick. 76 g.
RGS: 55.87 mm long and 3.73 mm thick. 73 g.
Pistons without pins: 180 – 287 g and RGS – 298 g.

And yes, the piston is 75.6 mm after all.

 

[sfcpiet]

Almost 8mm difference in pin length is huge, that would mean the circlip grooves would be deeply recessed within the piston. Does the distance between the circlip grooves coincide with the pin lengths? Shouldn't be much more than 1mm more than the pin length.

piet

 

[Garry]

The circlip on the RGS sits deeper in the piston than that on the 180, which is positioned close to the outside.
Another point is that, according to my latest measurements, the connecting rod is 22.7 mm wide. The clearance for the RGS in the piston is 26.6 mm, whilst that for the 180 is 28.2 mm. So this one fits more loosely. Could that cause any problems?

 

[sfcpiet]

The pistons cannot move sideways, no worries.

Forgot about the different inner width. Smaller gap = stronger piston = less bending forces on the pin. All pretty much academic in our case though.

All part of the many small improvements that went into the 120 engines.

The 120 rods had a lightened small end and did away with the small end bush altogether, the piston ended up slightly heavier.

piet

 

[Garry]

So can I just use it for the RGS then, Piet?

 

[sfcpiet]

Stick the pistons in as they are.

Both 180 and 120 OE "Jota" pistons have been obsolete for decades, literally thousands of 120s are running with 180° pistons, the aftermarket never considered the narrower 120 rods and generally supplied the wider spacing to accomodate all rod types.

piet

 

[Garry]

Thanks Piet
When Gijs gets back from holiday, I’ll take the engine over to him. He’s got a good RGS cylinder head that’s still to be reconditioned. He’ll put it all back together. Then I’ll be sure it’ll be sorted.