Skimming a Nikasil lined barrel...

Do you mean use MAP pistons in the Gilardoni or MAP rings in you 8.6:1 pistons?

I was gonna ask them about using the pistons. But they’re gonna need dead accurate bore measurements and supply dead accurate pistons for it to work.

I've got one of these barrels. Reading this, I'm afraid tv he pistons I've got are std. What's the size of the correct pistons?
If I remember correctly, mine has the fine UNF? threads, did I read somewhere that this can be machined to take the stronger UNC?

I assume you've already skimmed the head?
Have you considered special made pistons? I remember Comnoz writing about some pistons he had made specifically for his Commando, I asked him about the price, I don't remember the price but was surprised it wasn't higher. It was a minimum of 4 pistons though.

Dan

dantriumph said:

I've got one of these barrels. Reading this, I'm afraid tv he pistons I've got are std. What's the size of the correct pistons?
If I remember correctly, mine has the fine UNF? threads, did I read somewhere that this can be machined to take the stronger UNC?

I assume you've already skimmed the head?
Have you considered special made pistons? I remember Comnoz writing about some pistons he had made specifically for his Commando, I asked him about the price, I don't remember the price but was surprised it wasn't higher. It was a minimum of 4 pistons though.

Dan

Click to expand...

Skimming the head doesn’t have the desired impact on the squish gap.

Re the threads, the earlier barrels had UNF threads which (apparently) can strip easily. Later barrels had UNC threads and are stronger. To convert a UNF barrel to UNC you’d need helicoils or inserts.

Re the piston size, I only have vernier calipers at hand so am not able to be accurate, but the 8.6:1 pistons (for iron barrels) are coming up around 2 thou bigger than the 7.9:1 pistons (for alloy barrels). Moreover, the 8.6 pistons are tangibly tight in the bore of the Gilardoni barrels.
Given how tight the clearances are in the alloy barrels anyway, I’d be unwilling to run the ‘tight’ pistons me thinks.

Specially made pistons isn’t something I wanted to do. However, someone like MAP may be an option as they also do Nikasil barrel kits themselves.

I was also wondering how you were going to get the correct (around 0.5 thou ish?) tighter bore clearance using non-nikasil specific pistons? The Asso pistons were graded A and B I seem to recall. I have pondered MAP as they do a number of options, not sure if Gilardoni compatible is one of them?
What has always put me off personally, is spending a bunch of dosh and losing what is currently such a sweet, powerful motor, even if it is only running 7.something:1? The higher comp pistons will possibly be heavier(see below though) and the flame front compromised. With the cool running thanks to the alloy heat dispersion AND the tight bore clearance, I have never experienced detonation (and, yes, the low comp helps in that respect), even with the undoubted lack of squish.

I conversed with Jaye Strait of Britech in Maryland about 5 years ago and he has run a Gilardoni kit in one of his street bikes for 50,000 miles. Here was his response back then on my request for Gilardoni type pistons:-

"I'm familiar with the Nickasil kit I've got 49000 miles on one I fitted from (Triumph LF Harris) in 1988 to my T140ES. It works great. I can supply a set of Coated Forged pistons with Nickasil compatible rings to fit that cylinder @ 2.991" or whatever size you need".

in a further mail

"Pistons forged 9-1 or 10-1 std $429.90

Coating both $99

Gapless $79

Pistons are lighter.

520 sprockets $188.00


We have a Triumph 750 long rod w/ 6 full seasons of racing undefeated in Open GP


BSA A 50 with 61,000 miles.


T140D with 43,000"


Jaye has a good, if somewhat divisive reputation, no idea if he still does this or anything else? I also seem to have E&V engineering down as a possible source but never conversed.

Also, DanTriumph, J H motorcycles in Watford used to have new UNC studs for these kits. (my UNF threads are fine also though)