Archived Message
Big Bore Thermostat
In a recent posting about an RV8 engine running hot, a few references were made about a "Big Bore Thermostat" to which Geoff King replied "Terry and Angus mention a big bore thermostat but tease us by not giving any details whatsoever". Whilst details of a stockist were later included, there has not been much further information, and not wishing to hi-jack that particular thread, I am attempting to start a thread about the advantage of the Big Bore Thermostat.

We are all aware of the overheating tendency of the original BGT V8 and I am assuming that there would be no reason why this particular thermostat could not be used in the BGT V8.

I would like to hear the impressions of those, who I think could possibly be Norwich Burblers, and hopefully some without an RV8, who now use this type of thermostat, as to its suitability for the HIF6 carb'd Buick, Rover V8, how it performs etc., perhaps including some comparison data with the standard set up, and if there are any downsides.

My vehicle Tundra(0805) which now resides in Texas, where we have already had our first week of temps over 102Deg this year, has an aluminium radiator, a 74 Deg thermostat and has Forlife coolant fluid. It still gets uncomfortably hot with the gauge heading for 4.00pm in slow moving traffic.

Thanks in anticipation,
Graham Cornford
04/07/2012 @  04:58
Under topic: V8
Archived Reply(ies) received for this Message
Big Bore Thermostat
Graham, although not strictly replying to your thread but nevertheless a suggestion would be to dispense with any type of thermostat altogether. In those kind of temps the engine already has a decent amount of 'pre-heat' from ambient temps (which you would see on your gauge before you start the engine). By 'decent' I mean you would not be starting a cold engine in the middle of winter in Canada and therefore, you don't need the benefit of a thrmostat to restrict the flow of coolant until the engine has warmed-up sufficiently etc.etc. Additionally, (and if not already done) I would wrap the manifolds in heat wrap. Also, you might want to consider 'richening' the mixture which will also make it run cooler. I did on my '33 L-type which used to suffer from overheating problems esp. in traffic! It didn't fix the problem entirely but it did make a noticeable difference in lowering the running temps. Maybe something else to consider in your application as with extreme temps there is not one magic cure but a number of small modifications, which together, will have the desired effect.
Pat Boghossian
04/07/2012 @  17:57
Big Bore Thermostat
The RV8 is a car that can gain from certain improvements, some great and some small with the small ones being perhaps either of the ‘clutching at straws’ or the ‘minor cosmetic’ variety.
My own car is an indulged and perhaps spoiled brat that is not always grateful for the attention lovingly given, at least in terms of any feedback. At least two of the ‘clutching at straws’ modifications that my brat has enjoyed are the fitting of Magnecor plug leads and the Big Bore Thermostat and I have to admit that I can see absolutely no difference in the car at all for either mod. But physiologically, they feel the right things to have done and are therefore money well spent within my unlimited hobby budget.
Regarding the BBT, that must be the most difficult item to quantify in terms of improvement. Just how would one go about that with the variations in trip distance and weather conditions that we endure? Such a measured result would have to be the result of an engine mounted upon a bench and operated within strict laboratory test conditions.
Why then, you may ask, did I go to the effort of fitting the BBT in the first place. I can only excuse myself by telling that I have a philosophy of ‘nothing is too good for my car’ and that sometimes, when making small adjustments to the original specification, you simply have to have faith. Besides, the feel good factor would simply not be there at the natter night when one is asked the question, “Have you fitted the....”
For a mere £23 I am content to believe that my car is, yet again, in the best possible state that I can achieve for it.
The only question from you that I can answer honestly is the ‘are there any downsides’ one, and the answer to that is, no.
Hopefully a BGT V8 owner, with some proper knowledge, will contribute more useful information.
As you are of course aware, the thermostat is only there to allow the engine to reach its proper operating temperature as quickly as possible by restricting the flow of the coolant after a cold start. Once the engine is running at its working temperature the thermostat is open to allow coolant flow, but is then simply in the way.
Were I in your position I would do as Pat suggests and take the thermostat that you have out of the car altogether. It is simply an unnecessary restriction to the flow of coolant and with the air temperatures within which you operate, you will not miss it.
Even with our pathetic summers here in Blighty I did that to good effect when I had MGs in my twenties. Perhaps the summers were better then though. I am sure that I remember them that way.
Angus Munro
04/07/2012 @  20:27
Big Bore Thermostat

The 74degC stat opens at a relatively low temperature but with the high ambient you have it is the ability of the radiator to reduce the coolant temperature that will determine the operating temperature of the engine.

The ‘Big Bore ‘(or any other) thermostat, won’t know whether the engine is 3.5 or 3.9 litre with Efi or carbs. The difference between a ‘standard’ and the ‘Big Bore’ is simply that the flow through the latter will be more than the former. However, if the flow is increased too much the engine temperature could actually increase as there may be insufficient retention time in the radiator. I suggest you are cautious if you do remove the thermostat as it might not give the result you would expect.

Your ambient temperature of 102 Fahrenheit or 39 Centigrade is not much more than we have here in France on a good (or bad, depending on how hot you like it) day. My 3.9 has a normal 88 degC Wax Stat, normal twin yellow fans and an uprated radiator from Clive Wheatley (probably less efficient than your aluminium radiator). I also have one of these to speed the water pump up slightly and the engine seldom exceeds 95C, even in stationary traffic.

I’m not concerned with the engine temp but I’m curious to know what effect an 82degC stat will have so I’ll order a Wahler or a ‘Big Bore’ if the Wahler is smaller.
Geoff King
05/07/2012 @  03:11
Big Bore Thermostat
Before you go and buy a "Big Bore" thermostat, try running without one and see what happens?
Alan Spence
05/07/2012 @  20:58
Big Bore Thermostat
Pat, Angus, Geoff and Alan,

Thank you for the informative replies.
I have the original cast iron exhaust manifolds and crab claw filters fitted which I want to retain. Instead of exhaust wrap, I am considering ceramic coating both inside and out of the exhaust manifolds which, I am told moves the radiating heat further back down the exhaust system. I have already covered the firewall, transmission tunnel and floor with fiberglass/foil insulation which helps cool the passenger compartment.
I thought about removing the thermostat all together, but recall information against that as per Geoff's comment- too fast circulation for the radiator to cool. There is a thermostat blanking sleeve used in racing MGB 1800cc's offering less restriction than a thermostat which was recommended. I tried that but the inlet manifold internal casting would not allow it to seat properly. I considered altering the sleeve to make it fit, but abandoned that idea on the basis that that may cause some turbulence and an unwanted hot spot.
I suppose what I am trying to achieve, as we do get icy cold weather here as well, is a year round, one size fits all approach that will not require altering season to season, but it may not be possible owing to our extreme temperature variation, however, I am still searching for "Nirvana" !!
I think my next move will be the BBT which can be supplied with different temp ratings, and Geoff's different, presumably smaller circumference, water pump pulley.
Angus, like you, I cosset my car, my first MG, to my mind, the best, so why change it. We have had this relationship since 1976. 36 years during which time she has not always performed impeccably, but been forgiven. Even my wife of a mere 18 years accepts her and does not resent the time I spend in the garage with my mistress!! Indeed it was Anita's insistence that when we retired to the US that the MG came too.
Graham Cornford
07/07/2012 @  01:22
Big Bore Thermostat
Going back to basics the engineers at Abingdon were faced with a Mission Impossible – to shoe-horn a V8 engine with more than twice the volume and heat-generating capacity into an engine bay previously used for a small four-cylinder engine. In addition, these engineers had to produce this new model at maximum speed and with minimum development time and cost, so our over-heating problem was designed in from the start. For 10-15 years I religously changed from Winter to Summer thermostats each Autumn/Spring etc, then one year left the thermostat out entirely for the Summer - this was an improvement but still not a full answer. That is when I went for the four-core radiator (see W/N 399) which gives faster water-flow due to greater-diameter vertical tubes, has more fins for heat dissipation, and increases the total cooling liquid capacity by around 30%. Now I no longer have overheating problems. I suppose the engineers at Abingdon would have got round to this solution if they had been given a longer design period instead of being presured by the Board following the success of the Costello V8s.
Gordon Hesketh-Jones
13/07/2012 @  00:08
Big Bore Thermostat
My Big Bore Thermostat arrived today, 16th, ordered on 10th. Not bad, from Hampshire to NW of Houston Tx., 6 days with a week-end in between. Congratulations, you must have your Boy Sprout badge in thermostat identification, as it is indeed, a Wahler.
Now I just need the Water Pump Pulley to get here without the 8 week languish in customs, as some heater hoses ordered did!!
Graham Cornford
17/07/2012 @  07:06
Big Bore Thermostat
For my money it must be an advantage to have higher flow through the radiator. True, this may mean the water temperature as it re-enters the block may be higher than with a lower flow, but total heat extraction must be higher if the mean radiator temperature is higher, all other things being equal. So a higher water pump speed should give a lower mean block temperature. The only proviso is that if taken too far there can be cavitation effects at the pump impeller. It's necessary to ensure the water temp at the pump is well below it's boiling point (at 15psi assuming pressure cap is operating correctly). Elevating the boiling point reduces the chance of cavitation. The 50% anti-freeze standard coolant is better than pure water in this respect, although it reduces specific heat and so heat transfer rate, but some other coolants may be better, eg "Forlife".
Bob Owen
19/07/2012 @  00:16
Big Bore Thermostat
As stated before, I am loath to remove the thermostat all together for fear of too fast a flow through the system. Geoff's ears may be burning now, but from his previous postings, he appears to know what he is talking about. I believe that he has built his own car, and he does sometimes get the high temps that I do as he now lives somewhere down in France. Anyway I have confidence in his ideas and am willing to try them.

The BBT if my maths are correct has a surface area approx 1.8 Sq. Cm. larger than a normal stat., so that should allow quite a lot more fluid through it. Once I get the smaller pulley fitted as well, I shall watch it carefully and hopefully the flow will not be too great and cooling will be satisfactory. I shall report back, but I may be some time!!....To quote Capt Oates (1912)
Graham Cornford
19/07/2012 @  04:51
Big Bore Thermostat

I did indeed build my car. From a 1.8 GT wreck to a V8 roadster, initially with a 3.5 and now a 3.9 engine with a few modifications.

I’ve driven the car in -15C to + 38C temperatures, although not on the same day. The high efficiency radiator from Clive Wheatley works well and the engine has never overheated (the temperature gauge is calibrated). I fitted a smaller water pump pulley when I installed the 3.9 engine, not to increase the water flow but to give clearance for the Range Rover bottom pulley with a trigger wheel for EDIS ignition. And the reason I’m considering fitting an 82C thermostat (currently 88C) is because the non-adjustable fan switch is only just above the temperature that the thermostat opens and the fans are running when they don’t need to be.

By the way I now live just to the west of Lyon in France after 30 years close to Aberdeen in Scotland.
Geoff King
20/07/2012 @  06:36
Big Bore Thermostat
I wasn't actually advocating removing the stat, just confirming that in my opinion higher flow must be better, ie bigger opening on stat and smaller pump pulley would be advantageous - with the proviso that pump cavitation is to be avoided. (I've heard of cavitation damage on V8 pumps if the engine is habitually run without a pressure cap. This is because the water boils locally on the low pressure side of the impellers). There was no intention to cast doubt on any of Geoff's work and if that was how it was interpreted then I apologise unreservedly.
Bob Owen
22/07/2012 @  04:36
Big Bore Thermostat
Just to clarify, I didn't interpret your postings as recommending removal of the thermostat, nor I felt were they casting aspersions on anybody's views or suggestions, etc., so personally, I don't feel an apology is necessary.

We all, depending on our engineering prowess, have idea's and preferences in all matters and we can always learn from each others experiences. That is to me, why this Bulletin Board is so successful. Someone raises a problem and there are many volunteers willing to assist with said problem. In fact, I would suggest that there has been no bigger subject discussed than V8's overheating, which has been amply and well documented by the V8 Workshop Notes series and this BB., and many of us are still searching for the answer. I have no doubt the overheating cure/debate will continue on.

My attempt at another remedy will be the Aluminium Rad and Forlife fluid which I have used for at least 15 years and am still only on my second fluid change!!. These items will stay and then the Big Bore, Wahler thermostat, and the small water pump pulley will be fitted. If this combination causes the water to circulate too fast then I will think again and adopt a mix and match approach, but I am hopeful that it will be a success this time,
Graham Cornford
22/07/2012 @  12:08
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