Rover 14 - 1935 - spark plugs - standard or extended reach?

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Rover 14 - 1935 - spark plugs - standard or extended reach?

Post by ChrisHallard » Sun Dec 24, 2023 3:47 pm

I’m reworking the head on my 1935 Rover 14.
It has been fitted with the recommended plugs (standard reach).
These present the electrodes well short of the combustion chamber.
I’ve tried an extended reach plug and - to my mind these present the electrodes in a much more sensible position.
Please see photos attached.
The standard plug is 15mm from sealing washer to ground electrode. The extended reach is 20mm.
There seems to be no danger that the extended reach plug will foul either a valve or the piston.
Any thoughts?

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Re: Rover 14 - 1935 - spark plugs - standard or extended reach?

Post by SHyslop » Sun Dec 24, 2023 5:36 pm

Dear Chris,

One of the supposed advantages of forums and websites is that you can find out things quickly. On that basis, I'm writing this and hope that others who know more than me can tell you more in detail but this is what I think I know. The plugs in your 14 would originally be 1/2" reach plugs with non-projecting electrodes. Plugs with projecting electrodes these weren't developed until the 50s, in conjunction with better design of "swirl" and gas flow, plus higher compression engines. What did come in between was that "standard" plugs tended to become 3/4" reach by the 50s/60s. In 14mm plugs, they had started off (1932/33) at 3/8" reach, Champion J8 for example, then your standard plug for the 14 and many other cars was an L10 ,1/2" reach 1934-38. By 1938/39, the standard Rover plug was an NA8/N5 - 3/4" reach. Now, that placed the point of spark further into the combustion chamber which, provided it doesn't put the plug in the way of valves or pistons, would seem to be "a good thing". Ideally, the explosion should take place in the centre of the combustion chamber , not too near the piston head so it doesn't damage one spot on the crown but not too far away either.

However, the design of this 3/4" plug was essentially exactly the same as the previous 1/2" plug in that the design of the heat dissipation path from the central electrode to the body of the plug was the same, in proportion to the overall length of the plug. That means that the plug is as "cold" in terms of its heat rating because the tip loses heat quickly through the plug body. In relative terms, it'll be a medium heat range plug but the added length of the thread won't make it any hotter or colder in relation to a 1/2" plug.

Quite separate, as a plug development, was the projected nose plug. This has an electrode that protrudes further from the body of the plug and therefore stays hotter longer. This gives the benefit of feeling as if the engine has 1 or 2 degrees of extra ignition timing advance and, again, provided it is not causing any damage to anything else, would seem to be A Good Thing if you feel your car is down on power. Before starting on any changes, it may be an idea to try and identify exactly what improvement it is that you would like to try and make and work out whether it is something a plug can help with. If it's reliability, is that because the standard plug is too hot or too cold, or would repositioning in the chamber help.

A projected nose plug should be more resistant to oiling up because it has a tip that is hotter, but a longer reach plug may be firing at a better position in the combustion chamber without needing to be a projected nose type.

My own experience with all of this is that I've run a Riley RMA with projected nose plugs in a rebuilt engine for several thousand miles and it has been excellent. The engine rebuilders fitted these plugs and had it not come to me with them, I'd have stuck with the L10S that's standard for these. I sent for a box of Accuspark plugs that were supposed to be their equivalent for L86C; AIRV86C, and when they arrived I got a bit of a shock because they are much longer than what they are supposed to be replacing. I wrote to Accuspark asking if they'd tested them as being genuine equivalents and a technical person sent a reply saying they'd found them to have no problems. However, I must confess I have yet to risk trying them in a 1930s head but I do have some spare heads and I shall try and measure the distances involved on these. If they work, they should be good, having multiple grounds and if you can find a plug with these that suits your needs, they may work with no attention for many many thousands of miles. Lodge developed these in various forms and were one of the reasons (so I was told many years ago) that the company failed because this type of plug lasted too long - Silver and Golden Lodge being their last iterations. Some twenty five years ago I fitted a set of Lodge CCL14 plugs from Tim Green to a six cylinder 1939 Rover and these never needed any attention. The principle is that you set each side electrode to the normal gap for the car. The spark (just one!) will then jump to the smallest gap first. As that wears, the spark will eventually jump to the next electrode with the smallest gap, repeating the operation four times and then the gaps will need resetting. It also means that in the event of one gap oiling up, another may remain fit for duty.

I was also thinking about the plug placement in side valve engines. Most SV Austins have the plug in a top centrish position and that seems to have served them all well. The overhead valve design causes more of a problem because roughly where you'd want to put a plug is already occupied with valves with single camshaft designs and of course the i.o.e. engine of the P3s tries to solve all these issues. This all tends to suggest that the most advantageous position would be central to the piston, near the top and so a plug which has its electrodes at that spot may be something good to aim for.

In conclusion, I'm not exactly sure whether your plugs are longer reach or projected nose (or maybe both?) because the photos haven't shown up but a projected nose plug may give you improved performance and the experience I've had with them does suggest they perform satisfactorily. Bear in mind the effect of the equivalent of the ignition advance; too advanced a setting on long stroke pre war engines can have a detrimental effect on the big ends so maybe an element of retardation would be worth trying first if using projected nose plugs. I say that with particular reference to the earlier 14 engine being a generation in design before the later 1901cc 14 engine. I've been running my own 34 14 on just the standard recommendation and I've been pleased with its go. Not as much as the later 14 engine but very respectable.

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Re: Rover 14 - 1935 - spark plugs - standard or extended reach?

Post by tony1935 » Fri Jan 12, 2024 2:50 pm

Hi Chris,
I have been using long reach plugs in our 14HP since the 1960's with only improved performance results, so go ahead & use long reach ones.

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Re: Rover 14 - 1935 - spark plugs - standard or extended reach?

Post by luli » Fri Jan 12, 2024 5:14 pm

Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD

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Re: Rover 14 - 1935 - spark plugs - standard or extended reach?

Post by David2021 » Sat Jan 13, 2024 5:38 pm

That is a very impressive result...I see that these plugs are specified for modern cars such as the Toyota designed for modern fuels.
Here in UK most "classics" I think will use 5% ethanol (98 or so Octane) whereas the Prius etc will be using 10% ethanol. I have no idea how that may affect the heat rating, but whatever it is those plugs look great. I am using NGK BP6ES at the moment. The original recommendation was Lodge CLNH - I don't know where that is on the Plug spectrum!
Perhaps they would be worth trying on my MG ZA Magnette which specifies N8B (3/4" reach)

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