Cracked Slick 50 pistons

I don't think we have a thread on this, but it's a story that crops up once in a while.

The owner of this:



... recently had this happen:





DX-1800cc engined, it's a former Slick 50 car which won the 2001 championship and had a Jim Payne engine (aka RPM) with ignition-only ECU, so was one of the engines in the field with variable spark, which the latter front-running cars had.

The pistons in Slick 50 cars IIRC had to be cast or some rule made sure they were. The upshot is this cracking appears to happen, in this particular case after the head had been off, valves re-ground, and thus cylinder pressures were at a newly refreshed high.

I know we've had stories of cracked pistons before, so I thought this would be of interest.

This particular car now runs mainly in the Classic Touring Car pre'83 series. It is of course a Richard Lloyd replica car, and there is an owners write up on it in the current edition of VW Driver, which will probably be on the shelves still

 

Can't believe that if cast pistons only were permitted, that they were not properly heat treated!

Gravity die castings

Gravity die casting, like die casting, is suited to high-volume production. Typically larger than die castings, maximum weight of gravity die castings usually is about 10 kg, but much larger castings sometimes are made when costs of tooling and casting equipment are justified by the quality required for the casting.
Permanent mold castings are gravity-fed and pouring rate is relatively low, but the metal mould produces rapid solidification. Permanent mould castings exhibit excellent mechanical properties. Castings are generally sound, provided that the alloys used exhibit good fluidity and resistance to hot tearing.

Mechanical properties of permanent mould castings can be further improved by heat treatment. If maximum properties are required, the heat treatment consists of a solution treatment at high temperature followed by a quench and then natural or artificial aging. For small castings in which the cooling rate in the mold is very rapid or for less critical parts, the solution treatment and quench may be eliminated and the fast cooling in the mold relied on to retain in solution the compounds that will produce age hardening.

Some common aluminium permanent mould casting alloys are used (Alloy 366.0) for engine pistons, (Alloys 355.0, C355.0, A357.0) for gears, impellers, compressors, and aircraft and missile components requiring high strength.
Alloys 356.0, A356.0 are used for Machine tool parts, pump parts, hardware and valve bodies.

May be just 'Old Age'? Which some of us suffer from!

 

I think the pistons have been machined, though it is unclear, because rumours are that blocks were decked in this series - legally - and yet the combustion chamber had also been fettled on this one.

I will try to dig out the rules. I have a shot of the underside of the piston somewhere.

 

The piston in those pics is not like a std DX piston around the crown area. It may have been machined as there is no longer any sign of the asymmetric bowl which forms the lower half of the combustion chamber, and the raised area of the squish band at one side is also gone. Presumably block has been decked to increase compression and the piston modified to suit.
This being the case it could be that there is less metal between the piston crown and the inner top ring land which possibly decreases strength.

EDIT I need to type quicker!

 

Chris.

It would be interesting to see an image of the bottom of the Piston. Thanks.

But.

If the Piston is mass produced, it would probably not be manufactured as perfectly as possible.

If it's a Mahle Piston it would be as good as possible for a mass produced item.

But. There are many things which effect the durability of a cast Aluminum Piston.

I could write a fully detailed synopsis, complete with drawings, of how to make an Aluminum Gravity Die-Cast Piston as perfectly as possible.

Selection of Aluminum Alloy, Casting Process, Heat Treatment and Machining.

But. It's not worth the effort. As only 0.1% of CGTI really care what I say!

Dave.

 

Here's the piston, the number checks straight back to Kolbenschmidt (KS) listings I'm told:



The context of the Slick 50 series has to be seen as the basis of why this piston is used, rather than anyone pretending it was the ideal solution or a corner cut which shouldn't have been. In broadbrush terms, the engine exterior needed to look standard, but inside was regulated to a point that clearly prevented steel rods, forged pistons and so forth. Blocks were decked and heads were fairly free, "Pack D" being the notional tiered aspec which was commonplace. There were a number of builders, Jim Payne / RPM and JP Rose, who were delivering these engines and pushing the components to the point where they had compromised durabilty, for reasons which would need me to dig through the regulations to estabish.

Really I was keen on joining a few dots as every so often someone pops their head up with one of these cars, mentions a cracked piston, and it actually does seem commonplace, taking account of how few of these cars we see on here.

I'd agree with Danster that - taking his point he can clearly see the difference on the crown - that the strength is clearly compromised to the ring lands.

 

The slick 50 piston crowns I've seen normally have a smaller bowl in the centre after being decked... that one has had the bowl size brought back up...odd, as you need to reduce the bowl a fair bit.. well by half... to get the decent c.r needed, unless the head has been already skimmed to death.

The simple fact is, the cast piston just wont cope with the high loads both in terms of compression & sustained rpm.... forged piston are the only solution.

It's difficult to see exactly but it looks like dettonation damage which will quickly destroy the piston.

From memory they could run any internals as long as like you say the external appearance was unaltered... so forged pistons should have been a must on the sub 12:1 c.r's... some must have had this as some ran short stroke cranks with obviously bigger bores, so custom pistons in those cases for sure.

 

I WOULD LAY MY MONEY ON DETONATION, if you look closly at the top piston slightly behond last major missing piece you can see pitting marks

 

I would also say detonation without a shadow of a doubt......I had a similar thing happen on a Peugeot GTI engine I built once running a high CR and fueling went to pot

 

Thats what I was looking at... was'nt sure if was general dirt or not in the pic.

For the compression it is likely to be running, the squish band is'nt wide enough, so that wont have helped... plus would be interesting to have seen the deck height installed in the block. Then there's the ignition timing to take into account. There is'nt an awfull lot of colouring on the piston either... maybe on the lean side. But either way, just the wrong part in the wrong application.

 

Bear in mind there's some debris which has gone round, so it could also be that.

The squish is modified:

 

If there's still a piston in the block measuring it would be handy... it needs to come out the block @ TDC by at least .020 thou if the common multi layer steel gasket is being used.

 

All of my high performance two stroke engines, one of which produced 148 bhp/litre, used 'Dykes' top rings.

 

That's spooky, I was going to mention Dykes rings when I commented on the pistons in the Nissan touring car engine in the BTCC engine thread just now but foolishly thought no one would have heard of them . Yamaha flirted with them in road going DT's and RD's didn't they.

What two strokes did you run Daved? I have a soft spot for two strokes, still have a VJ22 RGV 250 v twin in the shed, ran it around the yard at the weekend and toyed with the idea of putting it on the road for 6 months - if I wasn't racing I would but funds don't really allow at the moment [:^(]

In a rash moment standing in the paddock between runs a few weeks ago I contemplated putting two of them in a single seater (I have a few spare ones here), chain drive to a common axle. Easy enough to get 70+ bhp per engine in a fairly reliable package, easy to link the 2 gear changes, 140+ bhp from a 500

Anyway off at a tangent but I just though it was spooky as I was thinking about Dykes rings not less than half an hour ago.

 

Tony.

I saw your contribution in the TCC thread! Thought you might be on the same wave-length as me.

Tva Tackts.

1969 - 1964 Greeves 24 TFS Trials Bike - This had a de-tuned Griffon Engine so was easy to re-tune for Enduros. Approx 20 bhp from 250cc.

1970 - 1965 Saab Sport - With the data, and drawings, supplied free gratis, those were the days, I tuned it to about 90 bhp from 850cc.

1974 - 1974 ISDT MZ Works Replic. 37 bhp from 250cc.

Dave.

ps

Pround to say I never owned Jap Crap!

My son had a Honda NSR 125 R though. 28 bhp from 125cc. I bet that had Dykes rings?

 

Being in the 0.01% of folk that care or notice what daved says. I had a Dykes top ring in my Fantic GranTurismo 50cc Minarelli engined motorbike.

How about gas drilling the piston crown to top ring land?

 

Now your talking Moto minarelli engines had 6 speed boxes back in the day and that's what gave them the edge over 'jap crap' 50's.......better leave that there now I think

Out of interest, going back to Daved post, have you heard the Saab 95 out in the British Historic rally championship, that sounds really crisp! On a different tack I once went out in an ex works Saab 96 with a v4 on cross over 45's, that was a quick motor car - for what it was... My father was a big Saab fan, had one of the very first Saab turbo 99's with the factory water injection kit, over 200php back in 1979 I think it was. Turbo the size of a truck - in fact I think it came off the Scania truck libe, turbo lag like you wouldn't believe. Was mental in the wet

On the other side of the valley (I live in Welsh Wales), nearly in throwing distance, John Jones (audigolfhits) was starting building some of the early VW MK1 Golf turbos at around the same time.

 

I care Daved

 

Pfft, thread hijackers!

Reminder:

1) pistons
2) series rules

!!

 

I find rules normally stifle creativity but that said creativity has it's place and it's probably not in this thread so to make amends.....

I think the guys are right, the pistons have clearly been machined down and I guess the block has been decked to suit. I did the same on my engine although after chatting to Mr H I decided to go for Forged pistons. More on that in a minute. The problem would have been to get the right volumes and clearances etc. using cast pistons. Lots of compromises not doubt.

I don't think detonation is the issue though, I think it's simply too much machined from the piston and in wrong place. If you look at where it's gone and compare to a std piston there isn't much strength left in the area above the top ring. As the piston reaches top and bottom of the stroke a lot of force is thrown on the top and bottom of the diagonally opposed corners of the piston. In addition I reckon, I could be wrong on this, but the outer 'skin' (for want of a better word) of any cast item is the toughest part and as you get further in to the casting it gets less dense it gets so machining the toughest part off it hasn't helped. All of this adds up to what is shown in the picture.

As Mr H said the squish seems far too thin, if this was left thicker then it would have worked better and probably left another 1mm or so more material and that might have been enough to avoid the problem.

I've got a set of pistons here out of a 2E turbo engine that have been machined down the same but the dish has been left std width leaving far more material at the edges, they seem to have survived ok.

As I said I had a similar problem but I run in Mod Prod so anything goes so I used an off the shelf set of Wiseco Forged ABF Turbo 16v pistons, machined them down to get the right bowl volume, then decked the block to get the squish as tight as I could. I could have had custom pistons made but I needed them fairly quickly to get the engine assembled and they did the job, the squish band was nice and wide so it all worked fine, pic below.

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