Leaded Petrol For Bikes

[squirrel_hunter]

Originally Posted by -Ralph-

Leaded petrol is required for lots of classic bikes. What do you think all bikes used before the days of unleaded?

Thats sort of the reason for the question... As I haven't really heard of motorbikes being converted I was wondering if Lead/ Unleaded affected bikes. And if I were looking at getting something older then me its good to do your research, which might just mean asking a few questions.

Originally Posted by Sid Squid

And even before that it was - unleaded!

Now thats interesting. Which prompted me to do a little more research, it would appear if I go for something pre 1920's then its unleaded, but between the 1920's and 1970's it likely to be Leaded unless its Japanese. All good things to know when it gets late, I've had a couple of beverages, and I find myself on eBay.

And to end the speculation it was the TR6 episode that prompted me to ask, but yes they've done Unleaded conversions with others before.

[embee]

A few slight misunderstandings going on.

Lead (tetra-ethyl) was used to increase fuel octane number. The fact that lead compounds got deposited onto the hottest surfaces, in particular exhaust valve seat faces, was something of a side effect. It was realised that this provided some solid lubrication when engine speeds began to increase.

Valve seat wear (recession) rates are very highly speed dependant.

The lead content of fuel was progressively increased to gain octane rating allowing higher compression ratios and hence more power and economy, (thermal efficiency), and also supercharging especially in aero engines. Unfortunately these very high lead content fuels result in lead compound being deposited at alarming rates, clogging things up and fouling spark plugs.

In order to control the deposit rates, "scavengers" were also added (usually chlorine/bromine bearing compounds) which formed lead chloride/bromide which gets carried out in the exhaust gas. This however also made the exhaust gas and condensates (during short trips) very acidic so rotted out mild steel exhaust systems very quickly. It also ate spark plugs.

Some engines designed for high lead fuels (typically military use) even had valve rotators fitted below the springs to encourage rotation which scrubbed lead off the seats to keep them clean.

Getting rid of lead in fuel was good news all round.

Valve seat inserts come in numerous different materials, as usual in engineering the choice is a compromise of cost, machinability, wear rates etc.
Exhaust valves can have hard material faces (e.g. stellite) weld deposited in extreme cases in order to achieve acceptable wear rates. Most production engines don't use hard faced exhaust valves due to cost. Also modern valve gear is usually better controlled and seating velocities are kept to moderate levels (OHC designs vs. pushrod), minimising wear rates.

Typical 50/60's cars with cast iron heads had parent metal soft seat faces and generally need inserts to live for long with unleaded fuel. The modest engine performance means they are run at pretty high load most of the time (if you've ever driven an early Morris 1000 or A35 etc you'll know the throttle tends to be open or closed) so do suffer.

Slightly later cast iron heads had induction hardened seat faces, but still parent metal. These will often live fine with unleaded fuel. American engines were typically like this late 60's onwards.

As YC says all alloy heads have inserts, very often these will be adequate if rpm is kept modest. I've run my Triumph Tiger Cub, early Trident (T150V), and 1975 BMW on unleaded fuel ever since it was available with no significant valve seat wear. It's only if you're going to run them at high load/rpm a lot of the time that you'll get problems.

........and no, a few lumps of lead dropped into the fuel tank won't help.

[squirrel_hunter]

Originally Posted by embee

Very interesting stuff

And that is why I like the org.

Originally Posted by yorkie_chris

I can't help but think you're being overly pedantic here.

The reason I state the case above of the CG, and as Sid confirms above, they used to use leaded because that's what you got at the petrol station, NOT because they needed the lead content to run reliably.

I can only be pedantic if I'm disputing something, but I don't think there's any dispute.

Understand what your saying about the CG, but I don't think there's any dispute about that either.

All bikes used to run on leaded = agreed.

Not all bikes had to run on leaded, but lots of bikes do = agreed

Confused.Com

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