Panda head

[michaelodonnell500]

I took a gamble on a Panda 30 head last year on a Buy it now or best offer and my offer was accepted!  Photos were a bit vague but it looked OK.  The main issue was that the former owner had started to hack away the leftover material to make it fit UK 126 cowling.




I took ownership then left it on the shelf until lat week.  Yesterday I took it to a welding firm to find out if they could sort out some of the pitting in the combustion face.



There were three engineers sucking in air through their teeth!
The left it for the cylinder head expert to look at. He phoned me this morning and explained that they have to fill the weld by forming a large puddle and allowing the corrosion to float to the top. The time and gas used to accomplish this would cost around £200 before I've even looked at machining the combustion faces back. I took the head home and am now considering a few other options.

I went home via tescos and picked up some groceries which improved my outlook a little!




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[drcdb15]

This is a normal alloy head, I guess, so not sure I see any problem with just using it as is. How deep is the pitting? I doubt it will significantly change the compression ratio, or impart any material imbalance between the two cylinders.

One option would be to machine out the pitting - don't fill the holes, just machine away until you're below the deepest pit. But that *would * noticeably lower the compression ratio. Having said that, the extra charge you get in through the twin ported head would be significant - say an increase in combustion charge of 100% in round terms. So that gets you about double the power all else equal (it isn't of course, but bear with me. In fact, iirc, a P30 head raises power from around 18BHP to around 30BHP, so a 68% increase).

Then the next question is, how much power would you lose by lowering the compression ratio? I don't know, but I doubt it would be as much as 50%. If it was only 10% you'd still be looking at about a 50% increase in power, to about 25-27BHP.

But to go back to my first point, why bother doing anything? The only issue I can think of is that the jagged edges of the pits might trigger pre-ignition or pinking, but this is only likely if the head has a particularly high CR anyway, and you might get around this by playing with the timing.

[Da Londo]

Why not machine the pitting out and flatten the head by 2mm or so.

[drcdb15]

Why not machine the pitting out and flatten the head by 2mm or so.

Because that would require prohibitively expensive machining around the contours of the combustion chamber, plus that would lower the compression ratio.

So you say lower the head by 2mm or so. Given the low slope of the squish area, I think you'd likely break into that area and risk the piston crown hitting the cylinder head. Plus on the outside, you'd be changing the effective length needed for the pushrod tubes (even assuming you had sufficient adjustment in the rockers) plus you would bring the lower most cooling fin 2mm closer to the top fins of the barrels. Given the space between fins is only a few mm anyway, this would have a significant effect on cooling air circulation around the very part of the head that needs it most.

IMHO this would be an extremely expensive engineering cost with very dubious benefit, and a real risk of making the head unusable. Better by far to use the head as it stands, for as long as it performs OK (and then scrap it), whilst saving up for a better one and keeping an eye open for the next one that comes on the market.

[Da Londo]

Maybe 126dave can shed some light on this topic, I believe he had the head skimmed 2mm, or 1.6.

[thepuddlejumper]

I have been told you can skim the head by 1.6-2.0mm to raise the compression as part of tuning the engine. Not done it though but will be on my 800cc build.

Ralph

[drcdb15]

OK, I wasn't aware of that established practice. Even so, it sounds like 2mm is about as far as you can go, and 1.6mm is the 'play safe' figure.

But in this particular case I think it's worth standing back and looking at the bigger picture. This head was a Friday afternoon punt off eBay, based on some somewhat less than high definition photos. It turned out the combustion chamber is quite extensively and deeply pitted. And the head has evidently already been hacked about (OP's phrase I think) to try to make it fit the 126 cooling tinware.

Now all of the measures under discussion are to compensate for loss of material removed in getting rid of the pitting.

It's by no means certain that this pitting *can* be "just" machined away, since it's so widespread over all the combustion chamber and no-one knows for sure how deep it goes. One firm has already considered the options and concluded it's not economically viable.

So what we're saying is that we have a head of unknown history,  of unknown quality, partly butchered, that will cost a small fortune to remove the pitting, and then to compensate for the loss of CR will need to have yet more engineering work to skim it.

If you were building a highly tuned engine and using known good components all of this might make some sense, but in the circumstances under discussion here it seems to me you'd just be throwing good money after bad. Far better to just use the head as-is as long as it works at all, and save your pennies and energies for a good quality head that will, sooner or later, come along.

[thepuddlejumper]

I agree, I would have the head skimmed to clean it up and just clean the combustion chambers of lumps and bumps and use. I know of a panda head that has the typical cracks between the valve seats and has been running for six to seven years without issue. Lucky or what?

Ralph

[Da Londo]

Yes, it's definitely worth running it like it is. It's too bad finding a good one is getting so hard.

[michaelodonnell500]

After I had the welding quote I had a look on eBay to find another one and as luck would have it there was another one with a Buy it now price that was about the same as the welding quote for the other one! 

I bought that and it is much cleaner. There are some cracks between the valves, but the engine rebuilder I took it too didn't seem too concerned. I'm having hardened valves fitted if it needs them. He thought it might be OK but said he'd find out as soon as he tried to remove them.










I also showed him the other head. He said it would probably be OK if it were Re faced and left as is. The only concern was a bit of pitting between the valves seats that could eventually work it's way under the valve seat, but might not.

That head has now been sold. I was probably being a bit fussy but I've always wanted one and I didn't want the risk of it getting worse. I plan on porting and skimming the other one so I'll update as and when that happens.

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[drcdb15]

After I had the welding quote I had a look on eBay to find another one and as luck would have it there was another one with a Buy it now price that was about the same as the welding quote for the other one! 

Glad it worked out like this, I told you there'd be another one along in a minute 

Panda heads are like buses....

[thepuddlejumper]

I thought the valve seats were hardened in the panda head, I have been running one in my Gamine for at least 8 years without modifying the valve seats.

Ralph

[michaelodonnell500]

This might not be the carbs I end up using. I bought them to have a look and a think. I find it easier to plan if I have metal bits to play with!





This will be the manifold.  The base will be machined in stainless.  If I can wait I should be able to get someone from work to machine it.
The pipes are 1.5" stainless pre- formed elbows. They will be shaped to fit the panda 30 manifold then welded together.
I think these pre- formed sections will push the carbs up too high. There are tighter radius versions which I might look into.

Small children and DIY keep stealing my car tinkering time so updates might be sparse!