Commando Head corrosion

Hi all. Just after some feedback on a combat head I'm looking at to see if it would be OK to run with combustion chamber like this or need a big tidy up:



Thoughts?

It doesn't look very good if you are using a helite flame ring gasket.

Mensch said:

Hi all. Just after some feedback on a combat head I'm looking at to see if it would be OK to run with combustion chamber like this or need a big tidy up:



Thoughts?

Click to expand...

Sucks, but it will run, especially on the street.

You might write Jim Comstock and send a picture and see if it can be made better but it seems like a daunting task to cut all that out, maybe have to install new seats, maybe have to shorten the valves, and then to have lower compression.

If it were a bike I was building to sell, I would find another head. For a personal rider, I would use it.

I bet before you cleaned it, it was smooth(er) - carbon tends to fill those pockets.

marshg246 said:

Sucks, but it will run, especially on the street.

You might write Jim Comstock and send a picture and see if it can be made better but it seems like a daunting task to cut all that out, maybe have to install new seats, maybe have to shorten the valves, and then to have lower compression.

If it were a bike I was building to sell, I would find another head. For a personal rider, I would use it.

I bet before you cleaned it, it was smooth(er) - carbon tends to fill those pockets.

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Thanks Greg
It's actually one I'm looking to possibly buy as believe it or not the one I got with a broken combat engine was even worse. It would only be as a personal rider.

Ps I've actually been reading through all your combat work over the last few days. Excellent stuff

Mensch said:

Thanks Greg
It's actually one I'm looking to possibly buy as believe it or not the one I got with a broken combat engine was even worse. It would only be as a personal rider.

Ps I've actually been reading through all your combat work over the last few days. Excellent stuff

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In that case it’s a no brainer IMO… don’t buy it !

There is no shortage of good used 750 heads. With a little patience and diligence, you’ll find a good one.

Considering the hack job the factory did on the Combat porting job it would be better starting with a Std 750 head and getting it ported properly.

Mensch said:

Thanks Greg
It's actually one I'm looking to possibly buy as believe it or not the one I got with a broken combat engine was even worse. It would only be as a personal rider.

Ps I've actually been reading through all your combat work over the last few days. Excellent stuff

Click to expand...

I would keep looking in that case. However, if it has the rockers and is cheap, it might be worth getting for them. Often, people selling heads strip them so having a set of rockers is good. Rockers are over $500 delivered new. Valves and springs are not very expensive - about $150 delivered.

Caveat, often one or more rocker spindles are bad and sometimes one or more rockers is bad so if buying for rockers, it needs to be CHEAP!

Great advice all. Appreciate it. Think I will pass on this one and wait for something a bit straighter to show up. No rush I guess as I'm still rebuilding the bottom end

Mensch said:

Great advice all. Appreciate it. Think I will pass on this one and wait for something a bit straighter to show up. No rush I guess as I'm still rebuilding the bottom end

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Good call.

I don't think this is corrosion, more likely erosion due to engine knocking (detonation). Someone may have run the donor bike on low octane petrol for an extended time.

- Knut

Mensch said:

Great advice all. Appreciate it. Think I will pass on this one and wait for something a bit straighter to show up. No rush I guess as I'm still rebuilding the bottom end

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I have a good head for sale on gumtree in Australia if you're interested have a look.

That head is all but junk.
Find a good 750 RH1 which can be lightly ported and decked 0.20" as an option.

In saying that, finding any head now that has good threads, seats etc and not busted up is getting harder to find and any head that has those problems cost decent money to rectify these days.

Time Warp said:

That head is all but junk.
Find a good 750 RH1 which can be lightly ported and decked 0.20" as an option.

In saying that, finding any head now that has good threads, seats etc and not busted up is getting harder to find and any head that has those problems cost decent money to rectify these days.

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It is expensive but on the flip side there are more options to repair cylinder heads these days
If you can find someone competent

Why not just fit the head? it might run reasonably well and if it doesn't what have you lost, a bit of time and a few gaskets. It would give you time to find a good replacement.
Just a thought.

cash said:

Why not just fit the head? it might run reasonably well and if it doesn't what have you lost, a bit of time and a few gaskets. It would give you time to find a good replacement.
Just a thought.

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Well he hasn't actually bought the head as yet
There's still better ones out there I reckon

baz said:

It is expensive but on the flip side there are more options to repair cylinder heads these days
If you can find someone competent

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In my case I bought a good RH1 off US eBay and had it posted to Jim Comstock. He did the exhaust and 20TPI threads in Al/Bronze along with new KW guides and cut seats.
It came as no surprise under magnification his seats were better than those on the brand new FA head.

I do have two spare RH1's but they are not for sale, one being the original off my Fastback and the other a like new casting wise from the UK.

The UK head, they are out there with a bit of searching and that head is most likely what the Combat got before the factory elected to overdo it as far as porting and decking.
It would be easy with a good (stock) RH1 to make sensible Combat spec head but with less decking and modern velocity porting to suit 32 mm carburettors.

Anyone know what the RH stands for ?

Fast Eddie said:

Anyone know what the RH stands for ?

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Yes
It's from when John Pertwee worked at NVT
It stands for "rong head"

marshg246 said:

Sucks, but it will run, especially on the street.

You might write Jim Comstock and send a picture and see if it can be made better but it seems like a daunting task to cut all that out, maybe have to install new seats, maybe have to shorten the valves, and then to have lower compression.

If it were a bike I was building to sell, I would find another head. For a personal rider, I would use it.

I bet before you cleaned it, it was smooth(er) - carbon tends to fill those pockets.

Click to expand...

I agree that the damage might be pre-ignition ("knocking") but it is most likely the result of the corrosion of the magnesium in the head alloy. The alloy in the Commando heads from the factory (and, so far as I know, all the Dommie twins with alloy heads) was a high-magnesium aluminium alloy, developed by High Duty Alloys (see https://en.wikipedia.org/wiki/Hiduminium#High_Duty_Alloys_Ltd ) for Rolls Royce aero engines in the 1920s, with later use by other areo companies, including use in Whittle's first centrifugal flow jet engines. A special characteristic of RR53B (the alloy used in Commando was called RR53B and it combined significant levels of magnesium with nickel. Unfortunately, magnesium -- even in alloying molecular quantities -- is an active metal and it tends to oxidize or corrode leaving the surfaces in an old Commando head pitted and rough. This corrosion is more evident on cast surfaces and less so on machined surfaces (I can only guess that machining "wipes" some base aluminum over the magnesium particles but that's only a guess on my part).
Since magnesium is very active, it tends to form a metal compound with carbon, especially under pressure. Where do you find carbon, magnesium, heat, and pressure in a Commando engine -- inside the combustion chamber of a cylinder head, of course. You can't do much about the heat and pressure, but is you remove the carbon regularly, you cut down on the corrosion. That dark deposit that looks like plain carbon that Greg refers to is likely to be this carbon-magnesium compound. (But the corrosion is really a factor that's built into the alloy itself.)

(The latest latest "FullAuto" heads are made of a modern alloy that's not a lot different from RR53B but with modifications that make it stronger, more resistant to what called "creep" under pressure and heat, and more resistant to direct heat damage as well as component changes in the allpy that make the magnesium component less likely to form the micrp-corrosion seen in earlier alloys.)