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TTP 904s Sports Big Bore Kit
TTP 904s Sports Big Bore Kit

There are very few modifications you can perform on an engine that will improve power and torque right through the rev range, but TTP's 904s big bore kit is one of those modifications. Increasing cubic capacity and compression ratio give improved performance from idle to the rev limit, with no downside.

In use the extra top end horsepower is excellent for acceleration and top speed, but the revelation is the extra torque through out the rev range. It makes riding a Triumph twin far more exiting as when you open the throttle, the engine just pulls and pulls to the red line.

Designed by TTP, the 904s kit includes race quality pistons, a custom 904 MLS steel head gasket together with OEM cylinders which are bored, Nikasil plated and then powder coated externally to give a virtually new set of barrels.

Core must have no broken fins and no scratches on the bore or mating surfaces.
Shipping details will be on your emailed order receipt.

  • 12:1 compression ratio.
  • Larger valve pockets for +2mm valves.
  • Tight quench to improve tubulence & combustion burn.
  • Anti-friction coating on the piston skirts.
  • Pistons hand balanced to within 0.1gms of each other.
  • Nikasil plated barrels give tighter cylinder/piston clearance.
  • Hand gapped piston rings.
  • Custom 904 MLS steel head gasket.
  • Optional OEM gasket kit.

Available for the air cooled Bonneville, Thruxton, Scrambler, America
and Speedmaster from 2001-2016.

Click HERE to read the TTP X-Files
about performance bolt-on mods
for the Bonneville & Thruxton.

Dyno graph with 904s Big Bore Kit
904s 2-2 Exhast
Dyno graph.
Dyno graph with 904s Big Bore Kit
904s 2-1 Exhaust
Dyno graph.
1 Product Reviews - Average rating 5 / 5 (Show All)

Click to review this product

Great so far!

- 28th September 2018

Due to a piston seizure on my 2014 Speedmaster (don't ask), I needed to have the engine repaired in an economical way. The new 904s Sports Big Bore Kit seemed to be the best way available to get my bike back on the road and I was more than pleased to take one of the first kits to be delivered.

With 600kms of gentle riding to run the bike in completed, the engine is smooth, more economical and has more mid-range punch than it did before. Given the CC boost is only from 865 to 904 I wasn't expecting a step-change in performance but the bike does feel stronger overall and the economy boost is around an extra 30kms per tank so no complaints at all on that front!

Fitting was challenging in a few places but the gasket kits, clutch springs and start safe additions were all very well thought-through according to the mechanic who completed the work for me.

The exchanged cylinder barrels are matt black rather than having the original metal fins, but this does not detract from the look of the bike in my opinion and I believe the metal-finished versions are available now.

Overall I'm very pleased to have a working Speedmaster back to either enjoy or sell-on and the team at TTP were helpful throughout, offering advice and tips whenever we needed them.

The only downside for a non-engineer, layperson like me is the cost of the kit... I have no baseline of course but over £1200 is a lot of money for us and as a result, this will be our first and last Triumph. But without TTP's kit it would have been more than £2500 for a new engine!

Click to review this product

List Price: £890.00
Price: £890.00 (£1,068.00 Inc. VAT)
€1,009.87 EUR (+VAT)    $1,151.51 USD    $1,600.71 AUD

The later barrels have machined and polished edges which
after processing need to have the powder coat removed
and the fin edges polished back to a shine.

Like for like core exchange only.

Early Barrels  Late Barrels
Early Barrels                    Late Barrels
The performance camshafts we supply are the Triumph 790 units
which maintain the mid range torque and increase top end power.
They are the camshafts used in the 904s dyno charts above.
The Stage 2 Power Induction Kit comes complete with
fitting kit, DNA Performance pod filters, installation
instructions and the required jetting or EFI tune for
the 904s kit.

Carb Stage 2 Kit            EFI Stage 2 Kit
Carb Stage 2 Kit          EFI Stage 2 Kit
This gasket kit includes all of the OEM gaskets
required when installing the big bore kit.
(except head gasket which comes with 904s kit)

904s Gasket Set
904s Gasket Set

With a 904s Big Bore Kit installed,
the standard clutch springs are
pushed to their limit, so we offer
the Barnet Green clutch springs
along with a Triumph clutch case
gasket as an upgrade.

Barnet Green Clutch Springs
Barnet Green Clutch Springs

The Safe Start is a good preventative measure for
a standard engine, with the 904s Big Bore Kit
installed giving an increased compression ratio,
the Safe Start will give protection from the only
engine structural weakness of the air cooled
Triumph twins.

NB: If you're upgrading to the green clutch springs,
why not install the Safe Start while the clutch cover
is off, for complete peace of mind.

TTP Safe Start
TTP Safe Start

Installation of the 904s Big Bore Kit at our
workshop in Bournemouth, Dorset, England.


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