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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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 Posted: Thu May 08, 2025 4:47 pm Post subject: Some like it hot. |
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...and some like it cold. I am of course talking about adjusting the tappets. I have never been sure if the valves should be adjusted with the engine hot or cold.
The reason for my confusion is that I have always assumed the reason for clearances in the first place is to allow for expansion of the push rods in particular.
If this is the case then cold adjustment should be sufficient and hot adjustment would only increase valve gear clatter; especially when cold and operational inefficiency.
Given that there is no drop off in compression when a cold adjustment is made I would imagine all is well ...but what do I know? |
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MikeEdwards
Joined: 25 May 2011
Posts: 2701
Location: South Cheshire
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| Posted: Thu May 08, 2025 6:17 pm Post subject: |
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I always do my Vauxhalls with the engine cold. The Haynes manual gives an adjustment figure of 0.003" for a cold engine, I forget whether I add that to or subtract that from the recommended gap, but I obsessively check the book when I'm doing it anyway.
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1976 Vauxhall HP Firenza, 1976 Vauxhall Sportshatch (x2), 1986 Audi coupe quattro, 2000 Audi TT |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4231
Location: South Cheshire
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| Posted: Thu May 08, 2025 6:42 pm Post subject: |
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I have always thought that a hot engine temperature is going to be more consistent for tappet adjustment rather than than cold, so logically a hot adjustment should be more accurate?
Cold could be anything from near freezing to warm. All that said I doubt it will make a huge difference! |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Thu May 08, 2025 9:00 pm Post subject: |
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So, if a hot engine gives a more consistent tappet adjustment, should that be the recommended figure? As I say, surely the clearance figure is to allow for heat expansion.?
I suppose one should make a record of hot AND cold measurements. That might help to identify the source of unwelcome clatter.? |
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Rusty
Joined: 10 Feb 2009
Posts: 272
Location: Bunbury, Western Australia
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| Posted: Fri May 09, 2025 12:57 am Post subject: |
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| Some makers actually have two specifications, one for "cold" and one for "hot" with the hot one being slightly less, allowing for expansion. |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Fri May 09, 2025 9:44 am Post subject: |
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| Rusty wrote: | | Some makers actually have two specifications, one for "cold" and one for "hot" with the hot one being slightly less, allowing for expansion. |
I am grateful to you for that. I didn't know.
I should, of course, refer to the workshop manual - if there is one - but I haven't looked at one of those in years. 
Some makes, like my MG TC, have the recommended clearance visible on the rocker cover but that it would seem only relates to the standard camshaft. Without a dependable history it could have been changed; possibly several times. 
My MG TC for example recommends 0.019" for exhaust AND inlet ...which I think would be fine for competition work but seems too much on a hot engine in normal use. As I say, that only corresponds to the standard camshaft...but I cannot see why? It is surely only the valve stems and push rods that extend considerably when hot; anything else - like the camshaft - must be minimal, I would have thought...but as I say, what do I know? |
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Miken
Joined: 24 Dec 2012
Posts: 577
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| Posted: Fri May 09, 2025 10:29 pm Post subject: |
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I'm pretty sure that unless stated otherise , All adjustments should be made with a cold engine.
But anyway, cast iron and steel have a similar coefficient of linear expansion. So I don't see why anything much would change with an engine with an iron block and head.. The valve stems will be transferring heat via the guides into the head. The push rods are wanging about in the valve chest at a similar temperature to everything else due to the mass of the iron block and all the oil splashing about.
I've had old air cooled motorbikes (Triumph, BSA) with iron cylinder barrells and alloy push rods. I can imagine valve clearances changing a bit with them when hot. Although, admittedly I have never checked. |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Sat May 10, 2025 10:15 am Post subject: |
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One thing I do know is you want to be careful not to have too tight a clearance on Austin Seven exhaust valves (I've had mine for over 40 years) as they will easily stick open slightly and burn. Having said that, it has never happened to me but I believe it is quite common.
Better to rattle than regret... 
(Incidentally, Trundles runs like a well oiled sowing machine...lovely!) |
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alastairq
Joined: 14 Oct 2016
Posts: 2117
Location: East Yorkshire
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| Posted: Sat May 10, 2025 10:53 am Post subject: |
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I always pondered why those French cars, Simca/Talbot/etc, used to have such 'rattly' engines?
Was it 'valve' clearances being ignored?
Or was it something more intrinsic to their design?
Yesterday, I found I still have an ages-old SPQR valve gap adjuster in my 'shed!'
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Sat May 10, 2025 11:04 am Post subject: |
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| alastairq wrote: | I always pondered why those French cars, Simca/Talbot/etc, used to have such 'rattly' engines?
Was it 'valve' clearances being ignored?
Or was it something more intrinsic to their design?
... |
My guess is if it had been owned by a Frenchman it would have been ignored.
(Probably with a shrug of the shoulders.  ) |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4231
Location: South Cheshire
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| Posted: Sat May 10, 2025 3:31 pm Post subject: |
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| Ray White wrote: | | alastairq wrote: | I always pondered why those French cars, Simca/Talbot/etc, used to have such 'rattly' engines?
Was it 'valve' clearances being ignored?
Or was it something more intrinsic to their design?
... |
My guess is if it had been owned by a Frenchman it would have been ignored.
(Probably with a shrug of the shoulders. ) |
My first car in 1979 was a 1971 Simca 1301, I was still at school and it was owned by our librarian, who had know idea about cars and had just been charged £140 for the fitting of a new battery, the car wouldn't start sold he sold it to me for £70, I turned up with a new set of plugs, (60p at the time) fitted them, it started and I illegally drove it away as I hadn't passed my test.
Most folk won't know what a Simca 1301 is , in retrospect it resembles BMW's of the same era, at the time my dad was initially cross that I had purchased a car, mega annoyed that I drove it, but pleasantly surprised at the spec and quality.
I vaguely remember the valve clearance spec was a hot figure, whatever I cleaned the car up sorted a couple of bodywork issues and sold it for £500  |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Sat May 10, 2025 4:21 pm Post subject: |
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| ukdave2002 wrote: |
Most folk won't know what a Simca 1301 is ,
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I am sure you are right. Even when new there were eyebrows raised in curiosity. I remember an old chap who had just bought a new estate version (it actually might have been 1501,) and he went about telling everyone about what a brilliant car it was.!
The build quality certainly put the BL cars of the time to shame...but then so did almost everything else! |
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bjacko
Joined: 28 Oct 2013
Posts: 516
Location: Melbourne Australia
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| Posted: Sun May 11, 2025 10:24 am Post subject: Tappet Clearance |
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Most Morris Manuals for the older models quote a figure hot, but who wants to work on a hot side valve engine with very limited access. Most owners just add 0.001" to the quoted figure..
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1938 Morris 8 Ser II Coupe Utility (Pickup)
1985 Rover SD1 VDP |
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Ray White
Joined: 02 Dec 2014
Posts: 7075
Location: Derby
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| Posted: Sun May 11, 2025 11:32 am Post subject: Re: Tappet Clearance |
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| bjacko wrote: | | Most Morris Manuals for the older models quote a figure hot, but who wants to work on a hot side valve engine with very limited access. Most owners just add 0.001" to the quoted figure.. |
You make a good point about working on a hot side valve engine; especially as many of them not only had limited access but also had the exhaust on the same side. |
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alastairq
Joined: 14 Oct 2016
Posts: 2117
Location: East Yorkshire
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| Posted: Sun May 11, 2025 2:08 pm Post subject: |
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I believe there were some engines where one did the tappets whilst the engine was running! So, never mind scorching the fingers........
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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