Why do you want to build a stroker crankshaft engine (long)?

We start this page with a reminder; a stock-stoke offset crankshaft, due to increased torque and less interial weight feels like adding 100cc to a stock 650cc BSA or Triumph engine or 125cc to a stock Norton 750cc engine. Building a stroker engine may add more 'real' cc but the desired increase in performance may still be available at less cost using a stock-stroke crankshaft.

That said, Ed G Cranks will build any custom stroker crankshaft that a Customer wants. The question that still comes up, before you spend the money, will still be "do you need a stroker crankshaft? After you read the complications inherent in building an engine with one of these custom crankshafts you'll know."

A stroker crankshaft engine is not a simple project. We can supply crankshafts that do not require any modification before installation for your engine but that does not mean the building of a stroker engine is simple. Piston selection is a problem when building most Triumph strokers. It is less of a problem with most BSA engines. Stock high compression pistons cannot be used when increasing the stroke by 5, 7.5 or 10mm. This means that you must find a low compression piston with the correct flat-top profile, deck height (distance from top of wrist pin to top of cylinder) and wrist pin diameter. Puma, Tony Hayward and other suppliers make Nikasil 750 and 830cc kits but they do not supply low compression pistons. If you plan to build the ultimate Triumph stroker and big bore engine you may be forced to purchase another set of pistons such as 7.4:1 T140 pistons. You may also have to delve into the world of automotive pistons built for 4 cylinder Ford, GM, Honda, Mazda, Subaru or Nissan engines.

BSA engines using a modified 84m stroke A10 crankshaft, or a modified Norton 89mm to crankshaft are easiest to build. BSA made a variety of pistons for their 441 and 350cc singles that can be used with many BSA big bore kits sold by SRM and others. These pistons use the same wrist pin size (.7500") as the twins but have a different deck height so test assembly is often required to determine the deck height of the cylinder barrels. You may have to use a stroker plate, when assembling these engines even when the best calculations and measurements are done. It is also possible to use Triumph T140 pistons which are available from some suppliers at compression ratios as low as 7.4:1. These pistons have the same wrist pin diameter as stock BSA pistons and are available in .020", .040" and .060" oversizes. If you are considering using automotive pistons most have 18mm wrist pins; not .750"

750cc Triumph big bore kits for T120 engines have been around for more then 40 years. When these barrels are sold they usually come with 9.5:1 or 10:1 pistons. Low compression pistons were never made for these kits. Why not use a low compression T140 piston? T120 pistons have a .6875" wrist pin diameter while T140 pistons have a .7500" wrist pin. The deck height of these pistons is different too. A large bore stroker engine using a modified Norton crankshaft with 89, 91 or 93mm stroke is possible but piston selection becomes difficult and a custom-length connecting rod is mandatory with the correct wrist pin size. As mentioned in the previous paragraph it is not uncommon to have to shave 1 to 1.5mm, or add the same amount using a stoker plate, when assembling these engines even when the best calculations and measurements are done. Although stroker Triumph engines, using stock 360 degree-type crankshafts, is common modification information on building these engines is scarce. Most builders that have successfully built one of these engines are; a) not willing to share their secrets or b) will charge you an arm and two legs to build an engine because something always unforeseen pops up or c) are too embarrassed to admit to the number of costly mistakes they made building their stroker engine.

Unforeseen problems? Like what? Crankcase clearance is one problem. Crankshaft cheeks may not clear the cases and connecting rod clearance may also be a problem. Camshaft clearance with flywheel and counterweight is not a problem with an Ed G cranks crankshaft as flywheels are smaller then stock and counterweights easily clear the highest-lift camshaft. BSA A65 crankcases can fit any crankshaft up to 91mm stroke but good luck finding a dished piston for the longest stroke if a big bore kit is being used. Some car pistons can be used but as mentioned before they have a 18mm wrist pin which requires a modified connecting rod small end. Pre-unit Triumph engines cannot fit a modified Norton 89 to 93mm crankshaft unless the oil sump scavenge pipe is moved. This requires welding and modification to the timing-side crankcase half to make a BSA-like scavenge system. Unit-construction Triumph engines have all the clearance required for up to a 93mm stroke crankshaft. R&R connecting rods fit; we don't know about other rods but are willing to test the fit if you send us the connecting rods at your expense.

Connecting rod clearance must also be checked so they don't hit the cylinder barrel skirt. An engine should be assembled with plasticene or other clay mounted in the side of the connecting rod to verify that there is at least 1/4" of clearance between the side of the connecting rod and the skirt. Some high-lift cam profiles have clearance problems with the flyweel of stock crankshafts. ED G Cranks are designed so that lifts at high as .600" can be used without touching the flywheel.

Drive and timing side bearings may become a problem. A stroker BSA engine, with larger then stock barrels, must have the timing-side roller bearing modification done. Stock-size BSA barrels, using lower compression pistons, will easily handle an A10 84mm crankshaft with lower compression pistons and the stock BSA timing side bushing if a high pressure oil pump is used. A modified Norton crankshaft, in a BSA engine using stock barrels, should have the timing-side roller bearing conversion done. Did you know that Triumph T140 drive and timing side bearings, to fit 30mm crankshafts, will also fit a T120 or pre-unit Triumph cases? This larger bearing is recommended when building a stroker engine. The BSA A10 drive side bearing, 30mm ID, can also be used to replace the A65 drive side bearing that is 1.125" ID as the OD is the same. What about using Super-blend bearings? They may not be required; see the next paragraph.

Output drive shafts may not be strong enough to handle the extra power. When the Norton 'Combat' Commando first came out they had problems with the long drive shaft breaking at the point that it joins the drive-side crank cheek. They solved this problem by using Super-blend bearings that allow the crankshaft to flex. Ed G Cranks solves this problem another way. The stock drive shaft meets the Norton crank cheek with a tiny .030" (.75mm) radius. We create a larger .080" (2mm) radius at this joint and nitride the crankshaft to reduce the flex as most bearings are ground with a .1" (2.5mm) radius. On a Norton engine outrigger bearings may be used to reduce the drive shaft flex too. Most modified Norton stroker cranks, used in BSA and Triumph engines, are shorter then stock Norton drive shafts so they do not flex as much and they are ground with the large 2mm radius between the crank cheek and the drive shaft.

The last and final point to consider when building a stroker engine. Do you have the strength and the leg to kick-start a large displacement engine? Yah, sure, in the day I could kick start a stroked Panhead but that cast-iron head engine had 8:1 pistons. And how old were you when you did this? I have a 600cc short-stroke A7 BSA with a stock, pre-unit frame, that eats any stock 750 Triumph twin that has not been modified and it handles like a featherbed with improved suspension and forks. Think about cams, piston weight, connecting rod weight, valve train weight and the properties I mentioned that are inherent in a stock-stroke offset crankshaft engine. When you have decided to make the expensive and troublesome leap into the world of big-bore stroker engines call us and we will happily build you that stroker crank. We will also help you with advice along the way before you make expensive mistakes.

Information outlined on this page is based on the experience gained from building offset and custom crankshafts for British twins for more then 15 years. Over that time period we have learned from many others along the way including Customers that made the leap to build large displacement strokers using my crankshafts long before we made a similar leap with our own engines. Advice and your experiences are always welcome. Feel free to tell us we are wrong too; let us know so we can share your experiences with others. If anyone is willing to pay our way, to a rally in North America, to make a speech about 15 years of building offset crankshafts send, us a message. We'll rent a van and bring some bikes along so you can sample what we're writing about.

Drive-shaft radius modification

Stroker crankshaft clearance

Drive and Timing side bearing selection

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Front view

 

Flywheels

 
 
 

Standard Offset Crankshafts

Stroker Crankshafts

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