Triumph Big Bore Kits

There are many “big bore” kits available today to increase the displacement of your Triumph. This is an overview of the ones available today.

JRC Engineering

The JRC Engineering 650 to 750 big bore kit is made of cast iron by Aerco in England and retains the stock appearance of a 650 cylinder. This kit has been in production for many years and is both economical and successful. This kit includes aerco pistons, rings, piston pins, circlips, gaskets, and instructions. These kits retail for $699.00 are for use only with Triumph 9 bolt heads, and are usually available.


Now in the 31st year of production, The Morgo 650 to 750 kit 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 Morgo. The casting is one piece and only weighs a few ounces more than the standard barrel.
Supplied with cycle threads as standard. Fin area increased by 20% to give greater cooling efficiency. A fin tower at each side of the barrel eliminates fin ringing at high speeds and provides 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.
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.
These kits retail for $645.00 and have been used by thousands of enthusiasts worldwide. Please call or email to check availability.

MAP cycle

M.a.p. cycle produces a 650 to 750 billet aluminum big bore kit that has wide cooling fins shaped to blend with the head. It is similar in appearance to the Morgo shape, and is for use with 9 bolt 650 heads. Also available is a T140 750 kit. As prices for this kit are not published, please contact us for pricing and availabilty.

Triples Rule

Triples Rule 750
This 650 to 750 big bore kit is similar in appearance to a stock cylinder, however it is sand cast aluminum. Features are:
Sandcast Alloy 356 Aluminum Block
Centrifugally Cast Iron Sleeves by Northwest Sleeve
Better Heat Transfer From Piston To Block = Detonation Resistant Engine
Tighter Clearances = Better Sealing = More Power
7 lbs. Lighter Than Stock Iron Block
Bolt-on: No Modification to Cases Necessary
Kit Includes Cylinder Block With:
Strong, Wear-Resistant Alloy 7075 Tappet Guide Blocks Installed
Cast Pistons, Rings, Wrist Pins, and Clips
Recoil® 3/8-26 BSC Inserts Installed In Head Stud Holes
Exhaust Tappet Oil Feed Retained
This cylinder is made in USA, and can be ordered with nikasil cylinders for the ultimate in light weight and durability. This kit is priced at $1185, please call or email to confirm availability.

Please be aware of the following:

Increasing the bore size on your Triumph may change the balance of the engine, as most larger pistons are heavier than stock
A 750 kit may not make your bike “faster”. Cam selection, carburation, timing, and gearing are equally important, if not more, than displacement