Spark plugs are hitting the pistons. Help! - Page 2

Bill D · #21 (**permalink**) 09-14-2018, 01:46 PM

Here is a comparison of the length

Bill D · #22 (**permalink**) 09-14-2018, 01:54 PM

I measure both plugs

Old .940
New .787
Shorter by .153

1795 · #23 (**permalink**) 09-14-2018, 02:25 PM

Hope that it works Bill!

dallas_ · #24 (**permalink**) 09-14-2018, 02:50 PM

possibly get a bit thicker washer too?

spdbrake · #25 (**permalink**) 09-14-2018, 03:08 PM

Were all the plugs mashed the same? If so its detonation.

If some were sheared to the side or twisted 20-30 deg left of or right then it could be a piston interference problem.
Since each plug is "Clocked" differently in each cylinder. (Unless your using Moroso dead soft copper indexing washers). If its interference they'd all have somewhat different damage.
To be honest unless your running a super high compression domed pistons the contact issue is pretty slim.
Last thing on interference. It should have reared its head in 10 seconds or less during your 1st fire up I'm guessing.

The FR5 plugs you removed are a good sub for the Autolite 3924.

Bill D · #26 (**permalink**) 09-14-2018, 03:57 PM

Spdbrake, they still smooshed when I disconnected the coil and cranked the engine

Gaz64 · #27 (**permalink**) 09-14-2018, 04:53 PM

It doesn't take much of a kiss to bend plug ground straps.

The current set have been incorrect for the engine since they were fitted.

The piston tops will have some marks but should be ok.

Note, your new set have cutback ground straps, end of strap inline with centre of centre electrode.

ACademic · #28 (**permalink**) 09-15-2018, 04:40 AM

Another solution is to shorten the electrodes with a Dremel and side gap the plugs. This old racer trick also places more of the spark/flame front directly toward the piston.

undy · #29 (**permalink**) 09-15-2018, 05:04 AM

With the plug electrodes getting hit I definitely wouldn't be comfortable with the chance that it's not only the electrodes getting hit but possibly piston-to-valve clearance issues too. When the engine is run hard the PTV clearance seems to diminish somewhat. I'd pull a head and add a bit of clay on top of a piston to check the actual clearance.

Goopie 18 · #30 (**permalink**) 09-15-2018, 07:11 AM

Bill, I overtightened the plugs on my Roush 427R and flattened one plug. I'm guessing that happened to you. I like the looks of your new plugs. What is the part #?

olddog · #31 (**permalink**) 09-15-2018, 09:31 AM

If all it did to the plug was push the gap closed a bit, I doubt that hurt the piston. The shorter plug should fix this issue.

What would concern me more is the valve to piston clearance. Did you take the time to put clay on the piston, mount head and rotate, then check the clearances, buy the thickness of the clay? If not, I would be really scared right now.

Had you done this with the plugs installed you would have found the problem then. I have to admit, I have no clue if the plugs were installed, when I did this. Note to self, always have the plugs installed that you plan to use, when putting clay on the piston.

Bill D · #32 (**permalink**) 09-15-2018, 05:53 PM

Quote:

Originally Posted by Goopie 18

Bill, I overtightened the plugs on my Roush 427R and flattened one plug. I'm guessing that happened to you. I like the looks of your new plugs. What is the part #?

NGK 4091 gapped to .032

Danr55 · #33 (**permalink**) 09-15-2018, 06:12 PM

Bill put the new plugs in and if they seem to fit okay run a compression check if the compression check is okay go with it.

jon@harrison.ne · #34 (**permalink**) 09-16-2018, 08:48 AM

I had this problem on a 327 fuel injected Chevy 2. One or two plugs had the gap closed a little and I installed indexing washers in all plugs to increase electrode clearance. That did it, but your interference might be too great. It's an easy, cheap thing to try that might solve the problem.

Bill D · #35 (**permalink**) 09-16-2018, 06:01 PM

Well, the engine quit running today and I can’t get it to start. Covered the car and pushed it aside. Time to move on.

spdbrake · #36 (**permalink**) 09-16-2018, 06:42 PM

Bill, I think taking a break sometimes is the best approach. When I'm faced with a round peg VS square hole issue (aka conundrum), I find it helpful to take a walk and mull it over. Issues like these can be a real *buster especially when another item breaks and forces the troubleshooting to skeew sideways.

I'd also PM Herr Blykins while your on break as he is a true engine guru and gives sage advice.

Don't give up, you've got common sense on your side so you will win.

Phx Mike · #37 (**permalink**) 09-16-2018, 06:47 PM

Bourbon also helps.

Gaz64 · #38 (**permalink**) 09-16-2018, 10:23 PM

Quote:

Originally Posted by Bill D

Well, the engine quit running today and I can’t get it to start. Covered the car and pushed it aside. Time to move on.

Time to contact the engine builder now?

Danr55 · #39 (**permalink**) 09-17-2018, 09:26 AM

Bill, think about it. There are only three things that will keep an engine from running. Fuel spark and Air. If the car ran fine before you change the plugs you've obviously taking something loose somewhere. Go back and check all the connections at the coil and make sure that everything is hooked up correctly then try to start it. If it cranks you can still check the spark plugs to see if the electrodes are compressed.

MOTORHEAD · #40 (**permalink**) 09-18-2018, 04:00 PM

Bill; you left this thread after "pushing it aside" because it wouldn't start, then you start a another thread with the engine running, but you can't get the carbs synced ,but then you did and it's running great! There's a huge gap in this story,; What was the plug mashing cure ? Why wouldn't it start ?
Inquiring minds want to know !

PS if the fixes were too embarrassing to post, just say "no comment", and we'll understand!

Ted