Triumphrat

chefmv · 11-12-2009, 03:42 PM

My 73 Bonneville has 2 small, lawnmower engine sized spark plugs screwed into holes in the top end of the engine. The holes are evenly spaced which leads me to believe they are meant to be there.

My question is, why would the rebuild guy fit them with spark plugs and not bolts?
The plugs have vibrated loose on more than one occasion causing sudden and major compression loss.

Thanks!

Webby · 11-12-2009, 04:00 PM

Hi chefmv,

I don't understand exactly what your question is?
Do you have a head with a twin sparkplug conversion?
A few pics would help

Webby

chefmv · 11-12-2009, 05:18 PM

I'll try to post a photo later.
I suppose it could be a conversion? The holes are too small to hold a standard size plug though.

jimmy bush · 11-12-2009, 05:33 PM

Some people thought that putting the plug in the center of the combustion chamber would give a better fuel burn and drilled and tapped a hole between the valves. There isn't enough room for a regular size plug so they had to go with the smaller plug. The conversion proved to be a waste of time and money. The plug had to be recessed enough that the valves wouldn't hit it when they opened so the fuel burn had to start in a recess which didn't give the desired results. The stock plug placement actually provided better performance. There is also the added risk of the head cracking between the valve seat and the new plug hole.

Webby · 11-12-2009, 05:54 PM

Doe's the head look anything like this? (2nd photo down)
http://www.dresda.co.uk/development2.asp?CAR=4

Webby

I agree with Jimmy, it's a waste of time, mind you it might work with the 8 valve conversion

chefmv · 11-12-2009, 06:34 PM

Thanks, guys,
That's exactly what it is. Very informative responses!

I guess I will just leave the 2 small plugs where they are. Now that I've identified the problem and ratcheted them down tightly I haven't had a problem with them vibrating loose. Just don't like the look!

jimmy bush · 11-13-2009, 01:41 PM

Looking at the 8 valve head (4 valves per cylinder), it appears to have "only" the smaller plug in the center of the combustion chamber. The placement of the valves provide adequate clearance so that the center mounted plug can extend into the chamber (to properly expose the electrode) without fouling the valves. This is not the case with the standard 4 valve head.

RetroRod · 11-13-2009, 03:34 PM

8 valves? Double the problems!! RR

pookybear · 11-13-2009, 08:42 PM

Quote:

Originally Posted by RetroRod

8 valves? Double the problems!! RR

Well yes and no.

The increase in flow is well worth the extra valves. More flow
=more air and fuel=more power.

Pookybear