Tuned Pinto: Piston is touching spark-plug

[Comadis]

hi alltogether,

i recently changed my 1,6 to a 2l pinto. the engine has been built up by a guy in kent , who normally prepares engines for formula ford (doing this since around 30 years).

block has been decked to increase compression ratio.

when i started the engine first i used my "old" motorcraft sparkplugs (thread 18mm). after the engine was running i thought i will do something good to it and "invested" in a new set of Bosch D6BC (also M18).

started engine...running on 3 cylinders only.

intuitive i removed plug no.3 and here we go: the centre electrode was bent-->piston touched it.

when i compared the motorcraft and the bosch plug i recognized that the thread begins on the electrode side (motorcraft) and on the opposite side (bosch).

so this means: if the elekrode of the bosch plug is touching the piston, the elektrode on the motorcraft plug is facing upwards and therefore there is enough clearance.

a bit vague, everything. so any idea what to do?

taking thread inserts for using M14 plugs with a flat seat?

[Arthur Jackson]

I've seen this on a Pinto before. On that one the clearance was OK if the electrode was horizontal, but touched if the electode was vertical, up OR down. We made a thin copper washer and tightened the plug in until it was at the right position by means of a marker pen mark on the ceramic.

[oakdale]

This is a common problem, the standard answer was to have spherical recesses machined into the pistons, but slightly shorter tipped NGK plugs are available from Burton Power, I think it's the same part number minus the P.

[Comadis]

machining the pistons is too late now...engine already in car.... so the burton-plugs might be a possibility.

but whats about the idea using thread inserts:

i can use M14 plugs afterwards
i would have the choice of more plug-types
i would have a flat seat on these plugs where i could add copper washers
on the m14 plugs the electrode (however the screwed-in position is) would come 2mm higher

what do you think about that?

[Furyous]

Its either that or buy a bulk load of plugs, mark them with a marker where the electrode is and trial fit all of them to see which work.

Used to be known as indexing the plugs, to ensure the open face of the electrode was facing the inlet charge.











Or so I am lead to believe

[Comadis]

so it was by luck that on the other 3 cylinders the same didnt happen?

[Furyous]

Its more down to how or where the thread was actually started on the plug in relation to the electrode.

Any plug will only allow 4/5 turns before bottoming out.

[Comadis]

does anybody know where i can buy threaded inserts (m18 to M12) for using plugs with M12 thread?

reductions from M18 to M14 are more common and i already have a suplier for them.

checked today again my plugs:

there was "only" this 1 stupid plug from bosch where the thread started on opposite side. all others are similar to the motorcraft ones

[GreenV8S]

I assume it's got a conventional crush washer. With a new plug you could take liberties with this and over/under compress it by up to a quarter turn without any problems, this would enable you to make sure the electrode wasn't at the bottom. (You'd need to mark the body to show where the electrode is before fitting the plug).

[Freezing Ferg]

No, they don't have a washer, it's a kind of taper seal from memory.

[GreenV8S]

Freezing Ferg said:

No, they don't have a washer, it's a kind of taper seal from memory.

Are you certain? The crush washers are very common so worth a double check.

[Freezing Ferg]

[GreenV8S]

Looks like you're outa luck then.

How far did the electrode get bent? Any chance of 'straightening' it so it doesn't stick out sideways so far and then re-gap it? Or just try a few more at random until you get one with the thread in the right place? A friendly motor factor might let you look at a few plugs to find one with the thread you need.

[Comadis]

the pinto-plugs have a conic seat. flat seat would be easy..i could add copper washers.

the outer electrode was bent until it touched the centre electrode, also already flattened a bit.

sure, i could re-bend it and adjust the gap..but this will not solve the problem, also i would not recommend to use such a stressed plug again, the electrode could crack in the future and fall down.

so there is still one open question:

using M12 plugs instead M18 will result in a bigger distance to the piston too.

but where can i get such reduction inserts?

i´ve seen some from M18 to M14 in UK and over here. M18 to M12 only in US...4$ each plus 45$ shipping!!!

Edited by Comadis on Friday 2nd January 17:11

[Freezing Ferg]

Comadis said:

the pinto-plugs have a conic seat. flat seat would be easy..i could add copper washers.

We did.
Successfully on a rally car. No big deal.

[Comadis]

what you mean? you used flat-seat plugs on the standard conic-seat head?

i want to use the car on the road and the solution should be a "fit and forget" thing.

[GreenV8S]

I meant pre-shape the electrode on a new plug, not trying to use that original that's been bashed. But from the sound of it there may not be enough space even to do that.

If you can get/make copper washers the right size I expect they'd pull themselves up into a conical shape when you tightened the plug. The problem is that the thickness needed would depend on where the electrode was in relation to the thread start.

Can you find the plug manufacturer's site and get the specs on the plug? Perhaps there is an alternative with a shorter nose.

When you've done all this I suggest yo refit the plugs with some plasticine on the nose and see how much of a gap you have. The piston travel does change slightly with revs and temperature and it would be a shame after all this if it started hitting the plug again at high revs.

Have you looked at the valve/piston clearance, too? If you've moved the piston up substantially there's a risk of similar problems there.

[Freezing Ferg]

No, copper washer on the taper.

[Comadis]

valve-piston clearance is un-checked. engine hasnt done any miles yet.


the "only" thing i´ve done after fitting the engine to my car was starting it, incl. a short test drive around the block, using max 3000rpm.

how could i check the valve-piston clearance on an complete engine?



as the guy i bought the engine from is doing his job (preparing engines for formula ford) since 30 years i pressume that he knows what he does...i hope!!

Edited by Comadis on Friday 2nd January 23:05

[GreenV8S]

Comadis said:

valve-piston clearance is un-checked.

the "only" thing i´ve done after fitting the einge to my car was starting it.

an a short test drive around the block,

Since it's still running without nasty noises you have to suppose you got lucky, but you'd expect anyone building the engine to have checked the piston clearances (including the plugs) and advise you if special plugs were needed.