Forum for competition engine enthusiasts. RIP Guy Croft 09/11/2020 - This forum will continue both as a source of information and as a memorial to Guy.
My current project is the engine for my Skoda Felicia rally car. It's an ex-works engine that I'm building to the same spec as bought (the crank had snapped), and I have sourced the original works-spec pistons which I have had machined with the correct valve cutouts.
They are cast pistons, and have a variance in weight - the lightest one is 269g, the others are 270g, 272g and 272g.
I need to remove 3g from the heaviest piston, but my question is where, and how do I remove the weight? They all look to be identical in dimensions. I've attached pictures of one piston so you can see what they look like.
you asked a perfectly reasonable question but also said 'any comments or suggestions welcome'.
So here it is chapter and verse:
I know those pistons like an unwanted relative. They are about as weak a piston as I have ever used. Don't make any mods at all to balance or anything else. I wouldn't even mount them in lathe. If you do fit them pray they don't break up.
Thanks for the honest (if not exactly best news I've heard) advice. I wasn't aware that forged pistons were available for the engine (it took a fair bit of looking to find these), so I take it that they would have to be custom-made one-offs and as a result very expensive with a long lead time?
The problem I have (aside from a limited budget - this is the first engine that hasn't been mostly standard) is that the cylinder head from the engine (which has some very high-quality looking porting work, unlike the engine currently in the car) has been modified to suit the domed pistons; the standard 136 pistons are flat, so I can't take advantage of the cylinder head as the compression ratio would be completely wrong. This was my original reason for sourcing the pistons as seen above; I didn't realise the works team used such low quality parts in their builds, and this news has devastated me.
The engine currently in the car has the redline set to 7200rpm, and it was only recently that I found out that the supposedly higher-spec pistons are in fact standard; the engine was fitted in haste after delays in my build last year. Due to having to keep standard bottom end components (for Group A rules) and the lack of availability of higher-spec conrod bolts (ARP don't list one, and while they said they would make a batch to the right specification it would take quite a few owners and still be expensive compared to their normal products), the RPM limit on the engine will be no higher than this anyway. As it sounds as if you've had a fair bit of experience of these pistons, are they weaker than the standard Skoda items?
Anyway, I am now unsure how to proceed; this is the last year I'll be able to do Rally GB (as HANS is becoming compulsory next year, meaning an extra £2500 bill to compete in the event, not something I can fund) and having seen the quality of the work on the cylinder head on the car and the totally standard nature of the rest of it I really wanted to build a good, solid engine for this year, using Group-A legal MPi inlet manifold, etc. But I don't want to build a hand-grenade and spend £4000 entering the event only to have it blow up straight away.
I don't talk about price here so I won't steer you to my pistons. I don't charge design time and the prices can be gleaned more-or-less from the pricelist in GC V/W. My list prices are based around a 2 week lead time from order to shipping to me which ain't bad for a tidy GC custom designed piston. The rings can be OE Skoda.
The problem is that the designers of this piston rather naively tried to make a cast piston in the shape of a lightweight forged one. As a consequence they produced an item in material with half the tensile strength with no bulk reinforcement where it's most needed. Given that aluminum fatigues and that their design is extremely weak under the pin boss and where the pin boss region joins to the crown region (which are the places where cast pistons always fracture if they're going to), I would not run them at all at any rpm in any engine.
