Light Aircraft Association

The previous owner of my VW 1834 engine with Lucas SR4 mags ran NGK BP6HS plugs which are 12.7 mm thread, i.e. short reach, but with a projected nose. I recall previous mention on the Forum of people using B6ES which are 19 mm thread, i.e. longer reach, but without the projected nose. Out of curiosity (because my engine is currently in pieces), I have offered up some BP6ES, i.e. both the longer reach and the projected nose, to see if the piston will clout them, and it won't.

Apart from the fact that fitting BP6ES plugs will increase the compression ratio a little (and I can deal with that by fitting different thickness shims under the cylinder barrels to keep it at 8.0:1), I should have thought that long reach plugs projecting well into the combustion chamber with the additional benefit of projected noses would make for the best possible ignition of the mixture. Also, the longer reach plugs will leave fewer of the plug hole threads exposed to the heat of combustion.

But possibly I am wrong and maybe other factors come into play that I am not aware of, such as detonation; otherwise why would VW beetles not have been fitted from the outset with long reach or projected nose plugs (and according to my brief researches, they never were)?

I should be most grateful for others' views and first hand experiences of which plugs work best and, if possible, why they do.

John

Although I am no expert on this, I would have thought that having more exposed threads on the spark plug in the piston chamber could make them more difficult to remove, due to deposit build up? I use short reach Bosch W8AC plugs in my 1600 VW engine.

Regards

Gary

Gary,

Very many thanks indeed for responding.

You make a very sound point about build up on the plug threads which, I confess, hadn't occurred to me. It's very helpful also to know too that short reach plugs work perfectly well in your engine. My theorising is all well and good but you can't beat hearing from someone with real, first hand experience!

Kind regards,

John.

When I read this I thought you were having a wind up.
With Spark plugs it is important that the body threads NEVER STICK OUT.
They will get hot, cause preignition and the threads will burn, that alone will stop you ever getting them out again. The short reach ones have only a small amount of head thread left exposed and that will carbon up but not really a problem unlees you happen to fit a longer one in there, Ideally the plug thread should end just inside the head with no thread on either showing. This should be checked with heads off and stickout governed by washers under the body, they used to be available in the olden days but not today as manufacturer do somewhat better now making engines.
The spark plug would not change the compression to a noticeable amount as the volume of the end is small, but do not put a long one in.
Use the recommended one for the engine and if you worry about overheating go for a harder short nose, if they foul up then a softer one. as designated by the Spark plug makers chart,
PREIGNITION CAN DESTROY YOU ENGINE A FEW SECONDS AT FULL THROTTLE, on takeoff,about a hundred feet up over the woods if sod's law
applies, which is the usual way.

Many thanks, G. Dawes, for your very sound and helpful advice.

Having checked the heads, I find that a long reach plug in the original VW plug hole just reaches the end of the hole with no threads on either plug or hole exposed. This perhaps explains why I have seen mention on the Forum of people using long reach B6ES plugs.

However, in the second plug hole, which enters the combustion chamber at a very acute angle, a short reach plug is definitely necessary although some of the threads in the head will always remain exposed.

As for thinking this was a wind up, I am the first to admit that I am not very clued up about many things - hence my use of the Forum which has so far never failed to provide me with answers to my various questions.

Regards,

John.

Worth just adding again that NGK make plugs with machine-rolled threads whilst Champion use a die to cut the threads, thus making the champion threads a very effective tap to eat the threads in the alloy heads.
Bosch 'may' be rolled threads.

I have a set of ACRO heads with short plugs plus a set of 'unknown' heads with a mix of short and long, one of which had been recut at the seat and thus needed an extra copper washer to pack it out from the chamber.

Thanks Rob, I never knew that there were such important differences between the ways manufacturers produce their plug threads.

Out of interest, are any of the plugs in your various heads the 'projected nose' type?

I'll have a scout around the shed for the scrap and spare heads to see what is there. Basic NGK for this application is B6HS (from memory) which is the same as the sidevalve Morris Minor (1948-1953) so L82 or L10 Champion are the same 'rating'. BP6ES are the same as the later Morris Minor plugs such as N9Y. Finding the wrong plugs in a Morris Minor is not uncommon.
The projection of the tip is designed to keep the tip in the heart of the combustion. This works for three reasons. Firstly, the tip gets a clearer shot at lighting the fire. Secondly, the location of it means it get a sudden blast of hot gasses which help to burn off deposits that 'might' not be burnt off if the plug was recessed. The third reason is that it gets a better quick blast of cold air when the valves open so that it can be cooled down when operating hard and near the top of the temperature envelope. High rpm might give it a large heat input but that is countered by large throttle openings which flow more cooling air.
I can not see a god reason to prevent you using projected nose plugs so long as they don't slap the piston crown at TDC.
A thin wire tip such as a B6HS with a platinum or Indium tip will be better able to fire the ignition at low voltages (ie Mags rather than CDI/Leburg) since they eject sparks at low rpm better than fat centre electrodes.

Rob,

Whilst I would be most interested in whatever you discover, please don't put yourself to too much trouble rummaging around in the shed!

Thank you too for your additional comments on the projected nose issue and the benefits of Indium tips - all very useful grist to my mill.

John.

No problem as I only have 2 fights this week. Saturday to Egypt and Sunday nigh Egypt to East Mids andthen taxi home (at about 7am!!!)

I looked at the heads and the only head that might work with ES length plugs is a strange one where the lower plug has been cut at a different angle to the ACRO heads and has a long shot into the head. It is also cut for 10mm threads (beginning with C instead of B on the NGK chart) The top holes are 100% short reach plugs since the ES range have about 4 threads exposed inside the chamber

cardiffrob wrote:No problem as I only have 2 fights this week.

rough job eh!

Go to work at 2 pm. Fly for 6 hours to egypt (11pm their time). Clear customs/immigration (egyptian style = 1hr+). Suicidal taxi ride to dingy hotel. 24hr in hotel. Leave hotel just as the party-people are going to bed. Clear immigration again. Preflight and paperwork. Fly home for 6 hours. Postflight paperwork. Get in a taxi and go back to base (3 hrs). Drive home and arrive at 7am.....just as the kids get up and the neighbours start mowing lawns and doing DIY.

If it wasn't for the silly paypacket I'd have chucked it in a long time ago although it has improved a bit since I bought a £600 ANR headset.
In the height of silly season we can often pust 60+ hour in 7 days due to CAA flexibility on our flightime limitations. We used to do Cardiff-Egypt-Cardiff in one go, frequently exceeding 16 hours on the job!

Could be worse....I could be driving an Airbus!

I, on the other hand do fly an Airbus.

I go to work 4 times a month, maybe 5 at a push.

Mainly Florida, Vegas, Canada or Caribbean.

Good for parts, skydiving and seeing friends.

We report at 10, get back for 7 in the morning, refreshed after a light snooze on the ocean.

East - nah, only in the winter....

Oh, and I get to eat my dinner off a table, not a clipboard

Rob, it is your spelling of "Flights" Though after your description of the real job, I might prefer the fights