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MG MGB Technical - '67 BGT running temps ok?

Hey all,

I've been using my '67 BGT just about every nice day I can so far this season. I just had her to Cleveland this past weekend (ran perfectly the entire time) but was wondering about one thing..

The car ususally runs in the 170-180 degree range but since it was near 100 degrees outside today in Cincinnati, the old girl was running around 200 degrees on the freeway, this is at 65mph (no overdrive). Is this an acceptable temperature? I watched the temp gauge like a hawk and it didn't get any higher than 200 and went down to 190 when I got off the interstate.

One other question comes to mind while I type...My car has been converted to negative earth at some point in the past. The electric tach was disconnected when I bought the car about three years ago. I've left it be because the car also has a pertronix ignition installed (which is a Godsend). I was going to pull the tach and send it to Palo Alto Speedometer (they did a beautiful job of rebuilding my speedo) for them to "convert" the tach to negative earth and to make any necessary modifications so I can use it with the Pertronix system. Is all of this necessary or can I accomplish this myself? I understand something has to be changed with the board in the tach so it will recognize the pickup signal from the electronic ignition. Truth or not?

Also at what RPM should my car turn at 70mph on the freeway (as stated previously I don't have O/D).

'67 BGT

Hi Aaron,
200 is fine, as long as you've got a cooling system in good repair. I'd say that 170-180 is too cold. I run a 195F thermostat year-round, and the temps here sound like what you've got. As long as the engine is not boiling over and/or running poorly from the heat, it's fine. A hotter engine will run more efficiently (more power/better economy), last longer, and is generally happier.

Rob Edwards

formula is

MPH = driven wheel circumferance x 60 x rpm
5280 x gear ratio x final drive ratio

For exampled depending upon ratio's
MPH = 6.339ft x 60 x 1000 = 23mph
5280 x 0.802 x 3.909

High oil temps are not good for the oil especially mineral, but I doubt that this is an issue.


Lower temps aren't good for the oil either! ;-) The water from condensation and other contaminants (like fuel washing past the rings at cold start) don't boil off as fast at lower temps, and multi-weight oils don't come up to their proper viscosity as fast (remember that a multi-weight oil is a thin stock with additives that make it act like a thicker oil at higher temps). That's why a hotter engine will last longer....

Oils used to be a lot less temperature tolerant and would coke at relatively low temperatures -- that's why older cars were generally fitted with 170 or even 160 thermostats. Modern oils are much MUCH better in this regard. With the common availability of supercharged and especially turbocharged cars, they have to be! In comparison, a B engine barely stresses the oil at all....
Rob Edwards

Aaron, Pauls method is accurate but this is also close. Non overdrive MGB with standard diameter tires, mph is about 18 mph per 1000 rpm. 4000 rpm would be 72 mph.

Clifton Gordon

With regard to oil temps aftermarket thermostats open @ 180F

Porsche 32vDOHC V8 (Coolant cooled)

Oil intercooler thermostat opens at 87C and is believed fully open at 93C

Detroit Diesel(DD) 24v SOHC 12.7 litre 500hp

Engine ECM programmed to issue an oil temp warning @115C and to power down (progressive shut off) @121C using a 15w-40 mineral HDEO.

The DD oil shut down temp corresponds with Porsche's two staged ECM warning programme @ 118C and 120C

A track car will have oil prewarmed to 90c to 115c expected race temp.

I like synthetic oils to operate in the 90-110C range for maximum durability and performance.

With regard to Aaron temps - oil tends to run at 1.2-1.5 water temp and rise 1/3rd deg per deg rise in ambient.

Expected norm oil temp 96c 120c with 200f coolant
111c - 140c. 140c is pushing it an a mineral but in Aaron's case this is short term and should not be n issue and I'm speculating at oil temps. I believe BMW suggest not using mineral for extended use with temps above 130c.

For a Turbo car I would use a synthetic ester oil.

Core engine warm up speed is important and Porsche use flaps to control airflow during warm up to speed up the process.

There is very little difference in warm up time between say a 0W30 or 15W40, and I doubt any difference between a multi grade mineral using viscosity impovers and a multigrade ester oil which uses no viscosity improvers. The main difference is that a synthetic ester oil will thin less at higher temperatures and provide better cooling and less friction.

The above is based on sump temps whearas oil at the bearing will be in 150c range.


Aaron - I'm with Rob Edwards. I too run a 195F thermostat year around and our MGB runs consistantly at 195 - 200 regardles of ambient temperature. As for converting your tach to negative ground, there are instructions in the Moss MGB catalog for making the two internal wiring changes in the tach to convert it to negative ground. Additionally, you have to reverse the direction of the white wire through the pickup loop on the back of the tach. To do this, place two pieces of tape about an inch apart on the wier on one side of the pick up loop. Cut the wire between the two pieces of tape and then cut the wire on the other side of the pick up loop about the same distance from the loop as ont he other side. Now splice the wires together, each wire with tape to the side with no tape. YOu are now ready to go. Finally, get one of the lables from Moss that say "IMPORTANT This Vehicle Wired Negative Ground" and put it on the panel next to the hood latch. This is important so that the next person who winds up with your car wil know that it has been converted to negative ground. Good luck - Dave
David DuBois

Tach conversion:

Not sure about the changes needed for a pertronix.
Mike Polan

Take Dave's advice. My 67 BGT was converted to Neg ground (the alternator should have been a give away)but did not have it marked. My brother, Remembering that Pre 68 MG's are positive ground, hooked the batery up and blew out the electronic ignition and the alternator. $160 later, I have purchased the sticker along with painting on the edge above the battery cover NEG Ground.
Bruce Cunha

Re: Engine temps:
I too do as both Rob and David do and get the same results ~ 68 GT, 18 GF engine.
Bob Muenchausen

This thread was discussed on 26/07/2005

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