TR4 & TR4A Forum

Leaded vs Unleaded... how to tell...

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IrishLA ryan O
Los Angeles, USA   USA
I purchased a TR4 a little over a year ago. Only driven it twice, for a total of probably 50 miles, maybe a bit more. It's been under restoration this whole time (new engine mounts, front/rear suspension, rebuilt carbs, new radiator, replaced slave cylinder and much more.

I will eventually have the engine gone through, but in the meantime, I was wondering if there was any way to tell if the car had been modified for unleaded gasoline without removing the engine head and valves.

From my knowledge, an leaded engine can use unleaded fuel in moderation with little damage to the valves. However, I heard somewhere that adding leaded fuel to an engine that is made for unleaded is much worse. Can anyone confirm this?

If I can't tell if the car has been modified or not, what should I do? My gut says to keep running unleaded and worst case, replace the valves in the future versus adding lead additive and hoping it's not been converted.

As mentioned, this car will be going through a complete restoration and hopefully the big finale will be this time next year. At that time, I will likely do many things, one of which is an engine go through. In the meantime, is there any way to tell short of removing the engine head and examining the valves?

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Desert TR Avatar
Desert TR Gold Member Jim P
Albuquerque, NM, USA   USA
The process for converting a TR engine to unleaded involves replacing the valve guides and seats with hardened versions. There is no way to determine if the seats where done without pulling the head.

The lead in fuel was meant to act as a lubricant to the valves. Running without it will cause additional heating due to friction and will reduce the life of the valves and seats. I've seen many discussions on how fast issues will arise, and the best I can glean is it will reduce the valve life to about 40K miles.

Running leaded fuel in an unleaded engine is a very big issue if you have a modern setup with a catalytic converter and O2 sensor. The lead will coat these parts and cause them to fail. Hardened guides and seats are not conducive to lead build up. In the worse case it could lead to loss of compression. I don't believe this is a major issue, but others on the site may have more insight.

Do you know when the engine was rebuilt? If it was done after ~1975, any responsible mechanic would have installed hardened seats as leaded fuel was being phased out.


TR3driver Randall Y
Confusion, Los Angeles, USA   USA
IMO, Go ahead and start running it. Odds are good that it will be fine. In many cases, previously running on leaded fuel leaves a lasting benefit.

If you do have a problem, the effects will quickly be apparent. The lash on the exhaust valves will close up noticeably over just a few thousand miles. If that happens, you can decide then whether to just live with it until you rebuild the engine anyway, or pull the head immediately for new seats (only takes a few hours to R&R the head), or start using lead substitute.

As Jim said, the lead substitute will shorten the life of a catalytic converter or O2 sensor. However, I don't believe it would harm anything if the engine has just been converted for unleaded use. Lots of engines back in the 70s were set up to burn either fuel, by just induction hardening the exhaust seats during manufacture. Worst case, you might get more combustion deposits and maybe have to change plugs a little quicker.

But, why go to that hassle and expense if you don't need to? It's a PITA having to carry a bottle around, and measure out a dose for every tankful of fuel.

FWIW, my previous TR3A did not have hardened seats, and got along just fine on unleaded. No sign of valve recession in the 50,000 miles or so that I drove that head. But my current "barn find" TR3 (stored 33 years) showed signs of recession almost immediately when I got it back on the road in 2008. Using Redline lead substitute at the recommended rate of 1 ounce per tankful stopped the recession.

56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Edited 1 time(s). Last edit at 2013-12-23 07:55 PM by TR3driver.

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twomanytriumphs Avatar
twomanytriumphs Gold Member Kyle Darby
Kelso, WA, USA   USA
1965 Triumph TR4 "My Baby"
1966 Triumph 2000 MkI "Bessie"
1970 Triumph GT6+ (MkII) "The Princess"
1977 MG MGB
Like Randall states, the valve recession will show up quickly. Engine in my tr4 was rebuilt in 1977 but just had the valves ground with new pistons and liners. At this point it was a $600 car with a bad motor and in primer. Crank and cam polished but left alone. It ran in this configuration until 2007 when we lost all the adjustment on #2 and #3 exhaust valves. From 2003 to 2007 was when the recession occurred. So I would just drive it for a year and see what happens. Worst case is you have to rebuild the motor, which you where planning to anyway. Kyle.

Desert TR Avatar
Desert TR Gold Member Jim P
Albuquerque, NM, USA   USA
Another option may be to just pull the head. A new top end gasket set (Head manifolds etc.) is ~$40 at TRF. It should be easy to see if hardened seats have been installed. If not, having the head machined and seats and guides installed is around $300. (Local Quote, including seats and guides) If you are planning on doing the engine at some time in the future, the head will be done. The extra cost will be buying another gasket set when you do the rest of the engine. The cost might be worth your peace of mind.


Vila Avatar
Vila Robert Beers
Mechanicsburg, PA, USA   USA
1933 Chevrolet Master "The 33"
1962 Triumph TR4 "The TR4"
1964 Vespa VNB 125 "The Old Vespa"
1984 BMW 633CSi "The 633"    & more
I have owned my 1933 Chevrolet since 1971 and had the cylinder head rebuilt in 1976, before the whole lead free cylinder head mod was ever thought of.

When I reinstalled the head, I torqued it down and adjusted the valves clearance period. Except for a head bolt torque check at 500 miles it has not been touched since , so it has been running for 38 years without any further adjustments.

I have read that if you are not driving your car hard, put rather drive it under normal conditions this should not be a problem. This is obviously the situation with my 33 Chevrolet.

Like others have said, if the valve clearance issue arises then have it modified and if not why spend the money.

1933 Chevrolet
1962 Triumph TR4
1984 BMW 633 CSi


Visit my personal website at:

Edited 1 time(s). Last edit at 2014-02-05 11:26 AM by Vila.

andymccl Avatar
andymccl Andy McClung
i have a 1966 just working on it to get it road worthy, Which additives do y'all use to replace the lead ??

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