Suzuki Bandit 1200 - Engine Build

Suzuki GSF1200 Performance engine build

The Bandit 1200 is Hugely popular in the UK. A quick search will find a heap of garage wizards building turbo versions, and milking every extra horse they can find out of these motors. We weren’t asked to build a turbo bandit, but we were asked to make one a lot faster.
I’ll preface this by saying we aren’t trained engineers or mechanics, but we can build the shit out of a bike.

So let’s take a look at the Purpose Built Moto approach to building a
performance Suzuki Bandit GSF1200. We started with a pretty rough, previously modified Bandit motor, and went hunting forways to increase the fun this engine would produce.

Our hit list for upgrades ended up looking like this;

Lightened crank
High Compression Big Bore kit – 9.5:1 @ 1305 CC
New conrods + main bearings
Performance cam (same as the GSX750 profile) **double check this
Ported head
Oversize performance valves
New oil pump + External oiling system
High performance clutch plates and springs
Upgraded HEL Oil cooler and high flow lines.
Mikuni RS smoothbore Carbs + DNA performance filters
Dyna 2000 Ignition and coils

Tear Down and Inspection
As we tore down the engine it was evident that this was a cowboy rebuild. Our process involves each part being checked as it’s pulled apart with damaged components catalogued for replacement, no half-measures.
On this build we found a number of warning signs including broken cam holder bolts, marred barrels and a damaged oil pump that had flow on effects with the cam and crank bearing journals experiencing wear likely due to poor oil flow.

Cleaning
Once the entire engine is pulled down, we start creating a list of upgrade parts, inspecting and measuring for replacement OEM parts, and thoroughly cleaning each piece, oiling, wrapping and sorting them into tagged trays.

This includes inspection on the gearbox shafts and disassembly of all parts for checking. All parts are assessed and necessary steps are taken to bring them back to spec. Engines stored without oil or with moisture (or sometimes water) inside may need extra attention in terms of rust stripping or blasting of the internals.

Parts that need it are vapor blasted, and prepped for re-paint or ceramic coating if requested. This particular engine was to be painted black, so the engine was thoroughly cleaned and prepped for painting at a later date.

Parts Sourcing
Once we have a thorough list of gear we need, the searching starts. Often our bikes are old, rare and hard to find parts for, but that’s not to say it can’t be done. It just takes time. We charge our customers for some sourcing time as it can take hours to find the right part combination to get these old motors singing again.

The GSF1200 bandit had the crank lightened but we made a misstep by not checking the bearing pins on the crank before this happened, they were a little worn and required some attention to clean up.
This made it very difficult to find crank bearings for.

Case repair
During our teardown and inspection, notes are taken on any damaged threads, cracked mounts, chain drag marks, broken fins etc that are noticed. Once blasted and cleaned any damage is welded up and finished back to restore the factory look of the motor. Sometimes customers opt out of going this far if only minor things are damaged. In the case of the bandit only a small cooling fin repair was needed and we were good to carry on.

Machining
As the parts arrive we get into machining. On a big bore engine this can involve a re-sleeve of the cylinders, clearancing the crank cases and making sure the sleeves don’t interfere with the crank or conrods now they’re larger.

Using the big bore kit on the bandit required a lot of work, including modification of the oiling system to include external lines to the head and cam oilers. The bandit needed all of this, and then some on the crank cases we had to take a few extra steps to gain the tolerance needed on the crank bearings which ended up sitting at 0.0023-0.0025”.

Getting here we needed to ask a few other builders with more experience when it comes to big high performance motors, and the consensus was these tolerances can be looser than factory spec, which results in more oil flow, but the balance is important and the sweet spot tends to be where we landed.
In some applications a thicker, and higher temp oil might be needed. This can be judged on a case by case basis. The head has been ported in house, fitted with new silicone bronze valve guides, and oversize valves.
Our specialist machinist partner takes care of the valve guides and seats for us as well as the cylinder re-sleeve and crank case clearance.

Once complete the heads are vacuum tested to make sure everything is sealed up and there are no hidden cracks anywhere that can often go un-noticed by eye.
With these parts complete we look to ceramic coat or paint if that’s to be done before re-assembly. The bandit however, was painted after assembly as we find it easier to maintain.

Re-building the engine
With all the parts in hand re-assembly starts, Personally I like assembling everything we can before the cases and crank are even looked at. This can include getting the gearbox together, assembling the head, with valves, rockers and dummying up the cam, Gapping and fitting the rings on the pistons, fitting conrods and bearings, and any other small parts that can be completed.

This way all of my parts are ready, clean and assembled to go together with as little stopping as possible. Gio handled the assembly of the bandit, taking care of each part as it went together piece by piece. We have a number of stop points during the process to check movement, clearance and operation to save us hassle down the line in the case of something unexpected arising which can often happen when modifying engines from original spec.

At this point the hard part is over on the bandit engine, but none the less a lot of attention is needed to make sure everything is properly clean, lubricated and tolerance to give us the best change of a trouble free start up and run in.

Engine Cycle, timing and valve clearances
With the Assembly complete the engine is cycled by hand, checking for sticking points, making sure everything is moving as it should, valve timing is correct and gears are selecting. This is done at a few points during assembly but nothing is certain until the last bolt is tightened and the whole thing is working in unison.

Our last check before closing the motor for good is valve clearances once the engine has been turned over a few times. The ignition timing is static set on the bench and set up to be plugged in and run before fine tuning.
If there’s nothing further to check and start up is close by, we will check oil cycling here as well. As expected, the bandit worked flawlessly on the bench, so it was time for install back into the frame.

First start and run in.
Sometimes we don’t get to see the first start on the motors built in the shop but the GSF1200 was for one of our custom builds so we get to run through this process ourselves. We have a list ofchecks before we hit the button for the first time including priming the oil system as best we can, and running it over on the starter motor to get that initial oil flow moving before fuel and spark are
introduced. Both with the spark plugs out (making no compression) then plugs in to listen and look for anything unexpected. The first fire up is always super exciting, especially on a custom we built. We get to hear how the exhaust we fabricated sounds and see everything work for the first time.

We run the engine, gently revving the bike with a little throttle until the cylinders are too hot to touch, then switch off and let it cool. Repeat again with a little riding involved all the while checking for sound, oil leaks and paying attention to how its all working checking plug colours between rides to see how the fuel ratio is looking. Gradually working up the rides and using the rev range a little more up for 50-60% throttle and being careful not to load up the engine too much. Checking plug

We have a blog on running in a fresh motorcycle engine, you can check out here.

Tuning
Ofcourse to ride it at all the tune has to be close, and we like to run the engines in a little before sending them for dynotune. Generally we will but 7-8 heat cycles on the motor with rides and check over everything. We usually turn to the customer to put a few hundred kms on the motor before dyno tune, but if they want it done in a hurry, our dyno guy will run the bike through a complete run in procedure and further heat cycles before dyno tuning and power runs.
This is an issue of contention with a lot of builders, so we take the info available to us and make the call ourselves.

Check out the full build here!

Check out the full bike gallery here