LCGB Forums

I am playing with a friend’s 200 at the moment.

It is re-made head, but the spark plug tip hides up the hole. This must not be ideal for igniting the fuel.

The thread goes all the way to the bottom of the plug hole, so it must be the shoulder of the plug coming up against the top of the plug hole too soon.

Has anyone come across a way to re-cut the top face of the spark plug hole so the tip will protrude more into the combustion chamber.

I have had to modify the counterbore depths in cylinder heads but always borrowed the bits from a place I used to work for.

Both the spark plug hole & the cylinder stud nut holes have been modified in various heads over the years.



If I were to need to do so in the future, I’d look on eBay for a bit that serves the purpose, rather than try the alternative which is a longer plug with a packer. However, that is yet another solution to consider.....

Thanks WT1,

Even though I have access to a milling machine, that looks an expensive tool to buy and a head in an awkward thing to mount for machining.

Does anyone know a business that can perform the machining for me?

Can't help thinking it would be cheaper to source another head Martin, how much needs come off btw

If your using an NGK plug,I'd think about changing to a Champion,the tip is further past the threads than an NGK.

Knowledge wrote:Thanks WT1,

Even though I have access to a milling machine, that looks an expensive tool to buy and a head in an awkward thing to mount for machining.

Does anyone know a business that can perform the machining for me?

The counterbore depicted is Ø12.4 mm pilot x Ø18.8 mm cutting size so would just about work (though the sealing washers do vary from about Ø18 to 19 mm TBH)

(The spark plug thread tapping drill is Ø12.7 mm)

The UK sourced eBay price of that bit is £6.34 ("that looks an expensive tool to buy") Direct from China is cheaper if you can afford the wait.....

I can only dream of having "access to a milling machine" but if the shank bit is not too large for the standard home pillar drill, the head is clamped to the table which is usually able to be swivelled to bring the plug centre in line. It takes a while to get the set-up accurate, but patience is a virtue, they say.... Why else do we modify Lambrettas?

As I’d mentioned counter boring the cylinder head mounting holes deeper in my earlier response, again, the pillar drill is very useful for that to allow bigger diameters for the heavier (Ø21 mm x 4 mm thick) washers &/or better thread engagement in the case of packed-up cylinders or any nuts that are not proper, high tensile steel

Depending how for the plug tip is inside the thread take the sealing washer off

dscscotty wrote:Depending how for the plug tip is inside the thread take the sealing washer off

With respect, I would strongly advise never to do so. The only time that I have ever found leaks on doing a pressure test is with brand new plugs (& washer) so I'm very wary of that fact. Surely, it's the last thing most of us would expect to leak, so that made me look a little further into the issue:

The standard washers on NGK plugs seem to be more prone than others in my experience & although I have literally dozens in stock, it is one reason why I have reverted to Champion, particularly with engines using points ignition, where their fine wire plugs are excellent along with proper, non suppressed plug caps.

Without wishing to take this subject further off track, differing thickness washers are available, including solid copper, but their use is not so convenient as they are not captive on the plug like the folded steel type.

My apologies to the OP if any are needed...

With respect, I would strongly advise never to do so

I realise this is not ideal, however I can assure you I have not had any issue when doing so, several road engines I've built have run like this in order to get the tip of the plug flush with the combustion chamber, one is currently around 3rd in the LCGB BSM and another 3 have just finished very high in the BSSO race championship, all pumped up when leak tested with no loss of pressure and none have had had any issues on road or race track. Like I said, its not ideal but it doea work with a little thought!

dscscotty wrote:

With respect, I would strongly advise never to do so

I realise this is not ideal, however I can assure you I have not had any issue when doing so, several road engines I've built have run like this in order to get the tip of the plug flush with the combustion chamber, one is currently around 3rd in the LCGB BSM and another 3 have just finished very high in the BSSO race championship, all pumped up when leak tested with no loss of pressure and none have had had any issues on road or race track. Like I said, its not ideal but it doea work with a little thought!

Thank you for your perseverance & not taking umbrage!

On your saying so, if I were in a similar scenario, I’d give it a try. Presumably it works from the outset with a ‘dry’ plug in a ‘dry’ head as I’m sure you’d have mentioned it if even oil/grease was likely to have been present in effect toward ensuring the seal.

I guess that what I can only verify is that the sealing washers themselves can give problems, although they should be a component that ought to perform their function reliably as intended. It's worth mentioning that cylinder heads that have had inserts to restore the thread can leak down the outside thread which is when the larger, 'flatter' sealing washers must be used.

Once again, apologies to the OP if any are required, but even information gained in such a 'tangential' manner must be worth sharing

Warkton Tornado No.1 wrote:.......................On your saying so, if I were in a similar scenario, I’d give it a try. Presumably it works from the outset with a ‘dry’ plug in a ‘dry’ head as I’m sure you’d have mentioned it if even oil/grease was likely to have been present in effect toward ensuring the seal.

I guess that what I can only verify is that the sealing washers themselves can give problems, although they should be a component that ought to perform their function reliably as intended. It's worth mentioning that cylinder heads that have had inserts to restore the thread can leak down the outside thread which is when the larger, 'flatter' sealing washers must be used.

Once again, apologies to the OP if any are required, but even information gained in such a 'tangential' manner must be worth sharing

Apologies for continuing it but I recently built a GT 240 engine and the leak down test revealed a significant leak from the plug. It was a brand new Denso so I tried a new NGK and a used NGK and it still leaked. I concluded that the oil and carbon in a used plug hole would help for the seal. I ended up putting a smear of copperslip on the thread which did the trick nicely

I'm glad you mentioned this, because I am not sure I have ever done a leak-down check around the spark plug, assuming it was sealed.

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