Big Bore Kits

Also see Cylinder Barrels and Cylinder Liners

Replacement Routt Big Bore Kits

Part #DescriptionBore 

Routt Big Bore Kit
650cc 9 stud kit
C/W 8.9:1 Pistons & Head Gasket

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76mm (745cc)Routt Big Bore Kit 9 Stud

650cc 10 Stud Kit
To Suit Pre-Unit or Unit When Fitting a TR7/T140 10 Stud Head
C/W 8.9:1 Pistons & Head Gasket

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76mm (745cc)Routt 10 Stud Big Bore Kit - Triumph TR6/T120
PS230500cc 8 Stud Kit
To Suit T100 Daytona, Trophy Trail (TR5T)
C/W 8.9:1 Pistons & Head Gasket
71mm (530cc)

Comprises of Cylinder Barrel, Pistons and Head Gasket

Routt Bore Kits were originally designed by Hubert “Sonny” Routt for the T120 RT’s1969.

In May 1970 the top 200 dealers in USA were sent a letter from the Baltimore and Duarte Offices stating that “Triumph were conducting a nationwide market test of a limited number of 750cc Bonneville models” Each participating dealer was sent one of the new machines which were built to homologate Triumph’s B-Range twins for the AMA’s new 750cc racing formula.

Dealer Price was $1,199 and suggested retail was $1,599-about $150 more than the regular Bonneville. All of them bore the regular model code T120RT stamped on the crankcase serial number pads. The “T” being added in either Baltimore or Duarte, NOT in Meridan.

Triumph wanted to compete in the new 750cc Twin Dirt track events, but AMA required 200 “production” examples of any race engine to be built for sale to the public.

Therefore Hubert “Sonny” Routt was contacted. Routt ran a successful Maryland business making accessory big bore kits for Triumphs when he wasn’t building record-setting twin engined drag bikes.

“Rod contacted me in 1969 for an order of 240 Big Bore kits, which included 40 spares,” Recalled Sonny Routt. “The job costed out at $88 per kit, including a pair of 10.5:1 ForgedTrue pistons, rings and wrist pins. Triumph got a great deal, because my stock kits sold for about $200 retail!”

According to Routt, the special barrels carried embossed part numbers on opposite sides of their base flanges, front and rear. Interestingly the Motor Castings logo (Routt’s regular cylinder barrel supplier), which identified every regular Routt kit-an “MC” inside a tiny upside down triangle- remained in the casting mould for the T120RT job. On the 200 RT barrels, the logo appeared where it usually was on the base flange, next to the tappet holes.

Taken from “Triumph Motorcycles in America” 

Aftermarket Big Bore Kits

All Made in England


BSA B44 Big Bore Kit (See Instructions Below)
Square Barrel Type, Nicasil Plated Bore
C/W 10.5:1 Piston, Head Gasket & Push Rods

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84mm (500cc)

BSA B44 Big Bore Kit (See Instructions Below)
Round Barrel Type, Nicasil Plated Bore
C/W 10.5:1 Piston, Head Gasket & Push Rods

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84mm (500cc)
71-1219A**BSA B50 Big Bore Kit
Square Barrel Type, Nicasil Plated Bore
C/W 12:1 Piston to suit Methanol, (20mm Dia Gudgeon Pin),
Copper Head Gasket
92mm (600cc)
71-1675RBSA B50 Race Crankshaft and Conrod assembly 

*Fitting Instructions for BSA B44 84mm Barrel & Piston Kit
Machine out Crankcase mouth to 3.490″/3.495″. The sphere of the cylinder head must be increased in diameter to match the 84mm bore size, this can be done by either placing the head on a lathe faceplate or carefully with hand tools.
**Fitting Instructions for BSA B50
The Crankcase mouth requires boring out to suit. No work is needed to the cylinder head.


Morgo Big Bore Kits

Part #Description 
Morgo650c.c Triumph Unit or Pre Unit Big Bore Kit 9 Stud Head
C/W 9.5:1 Pistons & Head Gasket
See Below For Details
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Manufacturer’s Description

Outstanding in Design and Performance the Morgo conversion is made to fit any 650c.c Triumph Unit or Pre Unit motor (9 stud head to be used on the latter) without any alterations to any of the standard parts. No reduction in bolt size or separate liners are employed in this conversion. The cylinder barrel is made from high quality cast iron, cast exclusively for us. The casting is one piece and only weighs a few ounces more than the standard barrel.

Supplied with cycle threads as standard, UNF threads to order only. Fin area increased by 20% to give greater cooling efficiency. We have introduced a fin tower at each side of the barrel to eliminate fin ringing at high speeds and to give more rigidity. Also the fins are much larger to give added cooling and blend in with the shape of the cylinder head. Most important of all the strengthening pillars go up through the fins to give maximum strength to the base of the barrel.

Thread depth lowered by one full fin section to make available the possibility of removing top fin etc., for special purpose building without loss of thread depth, i.e., short stroke motors.

The pistons are manufactured in England by one of the world’s largest piston producers. All pistons have cutaways for larger valves.

Standard compression ratio is 9.5 to 1 with a standard gasket. Alternative gaskets are available to give compression ratios of 8.5 to 1 and 10.5 to 1.

The standard range of pistons give the following capacities:-

STD 2.975in. = 740c.c. 45 c/ins.
PLUS .020in. = 750c.c 45 c/ins.
PLUS .040in. = 760c.c. 46.4 c/ins.

Care should be taken when using Thruxton Cams on valve to piston clearances.

Now after thirty one years production and continuous research this product is used by thousands of discerning enthusiasts throughout the world.

All kits are now supplied with the Morgo Super Ring Pack which consists of:-

Top Chrome Ring
2nd ring Morgo M10TP
3rd ring Morgo twin rail oil ring,
All Morgo components are made from the best materials to give long and trouble free service.


MORE POWER 750cc. No loss in R.P.M.
MORE STRENGTH All one piece
MORE ATTRACTIVE Design blended to head
MORE SILENT Made from cast iron
MORE RELIABILITY Good engineering techniques used
MORE ECONOMY Improved power to weight ratio
MORE EASY TO START Moderate compression ratio

MORGO the 207.9 m.p.h. name.

Our own test machine, which is a 1963 unit 650 Bonneville, reached a speed of 127 m.p.h. at just over 7,000 r.p.m.

The Conversion comprises of:-

1 Morgo cylinder barrel
2 Standard pistons complete with rings, pins & c/clips.
1 Standard cylinder head gasket. (9.5 to 1)

Optional extras at time of ordering:

Cylinders with Thruxton tappet block holes.
U.N.F. Threads.

The above options are at extra cost.


The success of this conversion depends to a large extent on your assessment of the condition of the rest of the engine, obviously worn parts deter performance and may collapse under added power. The variation in the balance factor on fitting this conversion is negligible.