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Warning – Modifying 12/4 Forged Pistons

Observations on the modification of RRSL forged pistons for 12/4 engines



The initial batch of 12/4 forged piston have been produced and when inspected were somewhat thinner in the crown area than the existing die cast pistons. These new forged pistons are designed by Omega and all the dimensions meet the Omega design criteria for a durable piston to replace the previous die cast items. Incidentally, in other areas they are of greater thickness, so they met our criteria of having the same weight and centre-of-gravity position as the previous die cast pistons. The alloy used for the forged pistons has the same expansion coefficient as that used for the cast pistons so there is no requirement to amend the boring specification over what was recommended previously.

So, it is seen as necessary to caution those members who have chosen to modify the RRSL product to meet the needs of a competition engine or to provide enhanced road performance.

Subsequent, to the initial inspection we have supplied additional manufacturing requirements to Omega to enable the next batch to address our concerns.


  1. The RRSL terms and conditions state that we supply parts for normal road use. And that RRSL are not liable for wear or failure of a part used for competition.
  2. The terms and conditions also state that our parts should be fitted without modification, so RRSL are not liable for excess wear or failure if parts have been modified.
  3. We are of course aware that members at their own risk do on occasions modify parts.
  4. We are aware that members reduce the diameter of the top of top land of the piston to allow longer rods with the resulting increase in compression ratio. Historically, this has been a reduction in diameter of say 5mm for a length of 3 to 4mm dependant on how much the block and head has been skimmed. If you chose to perform such a modification you are reducing the integrity of the piston and potentially subjecting the piston rings to higher temperature that may have a detrimental effect on sealing. Such modification will take the thickness in the top land area close to being detrimental for durability.
  5. We are aware that members also pocket the crown area of the piston to enable large valves, increased durations & high lift camshafts. This is an area where RRSL has significant concern. Any modification to the crown needs careful consideration in the light of modern fuel that will run with large amounts of ignition advance and a higher compression ratio without the tell- tale evidence of pinking. This results in significantly higher combustion temperatures with the potential to damage your engine. Please search the internet for Manchester XPAG and read.

If you do decide to pocket the crowns of your RRSL supplied piston you need to consider the fact that any pocket deeper than 1.4mm at the maximum material removal point will take the crown thickness to the minimum recommended by the manufacturer for durability.

This batch of parts are marked Omega 731 inside the crown.