Melt-down on no 4 piston. Cause?

[dikko]

Cross-flow 1600

I presume the answer is 'a lot of things' but is there (according to knowledgeable ones) a familiar cause of no 4 piston eating a plug and starting to melt down?

It was a Thruxton a 20 second flat out run to the chicane and initially the engine couldn't pull above 6,000.

Timing, fuel...yes to all those and more?

PS If Roger comes on can he tell me if 112 degree valve timing is a good thing and does he want a cup of tea at my place?

[Paul Richards]

Weak mixture at high revs would be my guess. 

[OliverSedlacek]

Weak mixture or knocking, possibly caused by a weak mixture.

[Roger King]

Any number of things as you say, but fuel grade, fuel/air ratio, timing and spark strength are obvious starting points. If you do enough to damage to a plug by whatever means, it will then ignite the fuel poorly, which will lead to a slow burn and excessive temperatures in cylinder. What I'm saying is that it may be a plug problem that has then gone on to melt the piston - or it may not.

It may be significant that it is number 4. This is furthest from the water pump and therefore potentially the hottest area of the entire cooling system; I once worked on a single seater with a tuned Crossflow that kept doing what yours is doing. The final solution was to add an extra water outlet in the rear cylinder head core plug to make sure that water would flow well in this area rather than sitting stagnent. The outlet was fed back into the water rail leading to the radiator. This proved to be a 100% fix and we put it down to the much longer water runs used in a single seater than a road car and the pump being asked to do far more than it was designed to.

But do the obvious first and check mixture, timing, etc.

As to cam timing - I assume this is FF1600 spec and it's not something I ever did a huge number of, so I'm rather vague on the subject.

[7 wonders of the world]

Going back a few years when we saw many tweaked crossflows on the rollers we found a couple of things over and above all the noraml checks:

On from Rogers point re No 4 overheating we had some headgaskets modified so the main flow holes were larger on No 4 it works on a similar principle to balacing the rads in a house allowing more flow to the furthest.

But by far the most common problem was caused by usin 2 seperate air filters, at high RPM the rear carb vibrates at a different frequency to the front, casung fuel frothing and hence weakening off the fueling - this can easily be seen on the rollers, accompanied by a drop in power.

So either fit a filter with a one piece backplate or brace the two carbs together using a piece of ally plate on the underside of the carbs ISTR there are 4 suitable screws on each carb - also makes a neat plate to mount a throttle cable to!

 

[dikko]

Interesting guys. There's only one carb by the way 7 wonders, so maybe not that.

But the head gasket flow might be worth a look at.

On the heating/cooling front I'm using Evans waterless and did wonder if flow may be slower and cause what Roger was saying - a heating at the far end from pump. Evans has the consistency of brake fluid and wonder if flow is effected.

Could be just a spark plug and my imagination is running away with me. There is an issue with pinto engines and Evans not being happy partners - I've forgotten what the full problems were -  and maybe head/gasket design doesn't suit it on crossflows too...

Piston melt was down to the top ring. I stopped none too soon. 1min. 31.3 sec lap - 7 secs off...

In the meantime a glazebust and a rebuild is the order of the day. Oh for a bit more room though.

[OliverSedlacek]

Can someone (Roger?) point me at how to tap water off the core plug at the cylinder head?

[Roger King]

You simply have to get a new core plug and weld an outlet onto it. It makes fitting it a little tricky, but it is doable.

At the same time, because having a hose hanging off it adds extra stress to the core plug, it is worth drilling and tapping three holes into the rear of the cylinder head around the core plug and screwing in some little homemade tabs that overlap the edge of the core plug. These will simply prevent the plug from pulling out. You must put sealant on the screws that you use.