stripped head bolts

[RichardL]

Let me add my voice to the long chorus of believers in Helicoils.  I've saved many a volkswagen case with them and those are under a lot more stress than your A7 will ever be.  In fact, I'd say that Helicoils will make your barrel better than new since the steel in the helicoil is much better suited to repeated unscrewing/screwing than the coarse-grained cast iron.  Also, since you get to use the original head bolts, you don't end up with some bogus pretty-looking, but soft stainless bolts that will fail.

(Alex, I think you clicked to the wrong topic when you replied regarding helicoils.)

I did some looking on the Emhart (Heli-Coil) and Alcoa (Recoil) websites to see if they showed the allowed maximum torque. It was a bit confusing, because the torque value offered was, I believe, that allowed or required for insertion of the coil. One inteesting thing from the Heli-Coil site was the warning to use anti-sieze when using hardened or stainless bolts in Heli-Coils. I have not previously known of or followed this advice. Of course, it is up to you (in the US south, where I am not, "y'all") to decide if you want to do this.

Richard L.

[beezalex]

Thanks richard...

[BSA_54A10]

Heli coils are made from stainless wire so you should always grease the bolts or they will glue themselves to the insert with very little provication. And this applies triple for plug inserts.
And just one little thing that everyone seems to forget.
A hole with a wire insert is not pressure ( read oil & combustion gasses) tight.
 

[MikeN]

Heres a thought. If you were to experience difficulty obtaining the correct imperial size thread inserts, You may wish to consider using metric.
  3/8" = 9.5mm so you would only have to go up .5mm (20 thou") to be on 10mm.
If you used a 10mm fine thread you could probably find some nice high-tensile cyl head bolts from a modern  J****ese  bike.
  If you used a        10 x 1.0 pitch it is approx the same thread pitch as 26 TPI.  ie. cycle thread

  If you were to use 10 x 1.25 it is approx  equivelent to  20 TPI .  ie BSF

  If you use             M 10 x 1.5  is approx 17TPI  which is close to Whitworth or UNC (actually 16TPI)

Mike
  

[MikeN]

Also, When i was a machine toolmaker apprentice I was taught to grease all nuts and bolts on assembly , regardless of what type of steel they were made from.
  And i also learnt that helical thread inserts are not suitable for spark plugs as they are not gas tight. I would fit a bronze threaded bush if the need arose.

[RichardL]

Helicoils are widely used for spark plug holes and, from my own experience and research, appear to work just fine. Starting a fire over this and sending everone to the closest machine shop to have their heads redone with bronze inserts does not feel like the best advice, and may not be your actual intent, but people could think it is.

Additionally, Whitworth and BSF wire coils made by Recoil (an Alcoa company) are readily available from various sources.

[beezalex]

Gotta agree with richard here mainly because 1.) no thread except a tapered thread is oil or gas tight, 2.)sparkplugs seal on the shoulder with a sealing washer.  There is, however, a common problem with helicoiled spark plug holes in that helicoils don't work that great in thru holes and tend to unthread themselves.  Locking solid inserts seem to work best here.

Just my 2c