While Rumble is a Yordle and very tiny in size he packs a punch in that armored suite of scrap parts. Once you get a few kills he can start to carry a team with his Ultimate's massive amount of DPS it outputs. Detailed below is my build for The Juggernaut version of rumble who is tanky and deals a ton of damage at the same time.

Passive- Junkyard Titan
Rumble uses a heat system as you well know if you have ever looked at the champion spotlight and gains power for maintaining a certain heat range. The danger zone starts as early as 50 heat and goes up to 95 heat so if times correctly you can use all 3 powers in the danger zone and still gain the overheat damage if you hover at 55 heat.
Pay very close attention to your heat gauge in laning phase. Keeping your harass damage to a maximum is ideal and your shield gives you a larger speed boost in the danger zone.

Heat dissipation is slow and not nearly as hard to keep up as you may think. flamespitter has a 4 second cool down however is cast for 3 of the 4 seconds. while a power is active heat is not dissipated and is held through the duration of the cast. simply having flamespitter running at all times will keep your heat at that 55 heat sweet spot and keep you moving.
[Q] - Flamespitter
This is rumbles main move for AOE damage in every situation. while not your main power it is a very good tool for stopping minion waves, farming and dealing good early game harass to enemy champions.
While other skills are better for their utility and damage always take flamespitter first as you ca enter the danger zone from level 1 whilst spamming this move.
[W] - Scrap Shield
This is your main defensive move and assistant harass move. With scrap shield you gain a sheild to soak up damage and a move speed increase for a few seconds. It is a great escape mechanic and chasing mechanic as it gives you some protection while making you run faster.
Combo this with flamespitter for good harass damage and farming. when coupled with Harpoon harass this is an excellent closing or escaping mechanic as the harpoon slows while the shield speeds you up.
[E] - Electro-Harpoon
This is your favorite move after the first few levels as it does lots of damage. On hit by the harpoon it delivers a decent amount of damage and also slows the targets move speed considerably. Spamming electro-harpoon is the best way to set yourself up for early game kills. if you hit an enemy champion with one harpoon, fire the other at the same target. ***ming both connected run at them with Flamespitter and Scrap Shield for good chasing, AOE damage and also at this point you should have overheated and be smashing their face in with auto attacks.
These harpoons are also best fired when a team fight is about to happen at a single enemy champion and then drop your ult on as many targets as possible.
[R] - The Equalizer
This move utterly destroys a team fight as it deals an unholy **** load of damage if the fight happens over the equalizers AOE. dealing near 400 damage on landing it then proceeds to deal 200 to 300 damage a second to all who are in the blast zone.
Once you drop your ultimate be sure to run in and overheat attacking a squishy target with your overheat damage to drop them fast.

Pro's
- Good at harass
- Tanky
- More mobile than you would expect
- good at escaping and chasing
- excellent 1v1 bruiser with Junkyard Titan bonus
- Bruiser ability can carry into team fights as you nearly always overheat in a team fight
- Good AOE damage in team fights

Con's
- Short range
- easily aggro's tower
- not the easiest skills to use effectively in a team fight (particularly Electro-Harpoon)

9-21-0 Masteries are taken, putting points in dodge, specializing your exhaust for a more potent slow. Magic pen and reduced damage are what rumble needs most as you will most likely be the target of most of the harass while you harass the enemy team.

For runes I take
Marks- Magic penetration
Seals- Flat Armor
Glyphs - Flat Magic Resist
Quints - Flat Ability Power

The seals and glyphs will make you extremely tanky early game and hard to kill with your defense spec and 33% reduced damage from resistances.

The marks and quints are used to give you some good harass damage early game as you will eb punching through most of the enemies magic resist and doing extra damage with flamespitter and your electro-harpoon.

Take a point inFlamespitter first as this power will allow you to enter the danger zone and activate your Junkyard titan power if needed. take a point in every power to start followed by an additional point in both electro-harpoon and flamespitter before your ultimate is ready. You want to rush the hell out of a Guinsoo's Rageblade as it does lots of damage early game and you can keep the stacks up with sheild and harpoon very easily. After you have the rageblade build either sorcerers shoes or merc treads depending on if the enemy team has a lot of Crowd control or not. Next rush a hextech revolver as you have no healing mechanics so you need the spell vamp from your flamespitter and harpoons.

While in laning phase use the harpoon, shield, flamespitter combo to harass the enemy champions away from the minions. try your best to keep your heat gauge from exceeding 95 heat at all costs until you are chasing down a low health champion. Preemptive overheats are not good as you have to spam more powers to regain the bonus damage from danger zone for being at above 50 heat.

Once team game has initiated you should be near the largest concentration of champions on the map or at least in range to drop your ult should it start. The equalizer has a much greater range than most think as you will probably like i once did think you have to aim it inside the circle. The start point of his click and paint ult must be placed in the circle after the start has been placed yo can fire it in any direction even if it is aimed outside the circle.

Item terms go for a abyssal scepter and a crystal scepter as these will put out decent crowd control and also make your damage and any other casters damage more potent because of the reduced magic resistance.

continue to harass as usual and use your ult to initiate if the harassed target is nearly dead and your team is right behind you chasing them. Make sure to paint the best blast zone to pick up the kill and damage the rest of the team defending. overheating during a team fight is acceptable as you will do loads of extra damage to a single target and then be back up and casting for either a retreat or a chase to kill more champions.

Finish off your build with a gunblade and Sunfire cape to add to your damage and tankyness to continue to be a major threat that the enemy must focus if they don't want to die. Team fight as usual and make sure to pick off anyone straying too far from the enemy team. Electro-harpoon into exhaust is a nearly instant kill as they will be slowed for forever and be unable to escape.

Ragebalde. no alternate choice this is a must. Rageblade is a very good item for anyone with spammable abilities and rumble fits the bill, plus he can make use of the extra AD. with 8 stack Rageblade gives 32% attack speed, 96 Ability power, and 35 attack damage for 2350 gold. Tell me this isn't a great item to start with for damage.

Boots

Sorcerer shoes if they either A. only have one to two hard Crowd control mechanics, or B. have a lot of physical damage champions as the additional MR would be uselessMerc treads if the enemy team is CC heavy or has a lot of magic damage champions.

Sunfire cape can be switched out for Thornmail if the enemy team is auto attacker heavy and has at least one person focusing you.

The only other variation is order. if the enemies are doing lots of physical damage to you get armor faster than Magic resist and vice-versa