In this session we’re dealing with the most common problem encountered by TAC inspectors – lack of clearance. 

Adequate clearance between fixed and moving parts is critical for the proper operation of a motor vehicle and is mostly common-sense but hot rod builders still need to think about it more carefully and, for new builders, it’s something to now be more aware of. Vibration, knocking, rubbing and steering stiffness are brought about by lack of clearance and take the fun out of driving your new project, so it’s important to get right for your enjoyment and safety.

Let’s summarise the areas of concern and talk about how to deal with them.


The engine and gearbox are mounted on flexible mounts and therefore move and vibrate. Make sure there is at least a finger clearance (10-15mm) between them and everything else – especially steering shafts. Put all of the accessories on the engine during set up to avoid clearance problems later.


The exhaust moves with the engine so it needs clearance. Clearance to the chassis, crossmembers and rear suspension must allow for exhaust movement. At the rear, the suspension travel has to be taken into account during the design of the exhaust.


Steering shafts and universal joints pass by the engine, exhaust system and chassis members and must have clearance to rotate freely. Steering adaption should be done at the same time as engine location and exhaust fabrication. All too often it is clearly an afterthought (which is strange due to its importance) and results in too many universal joints and bearings to get around obstacles that shouldn’t be there.

Finger clearance again is the rule. Remember that there’s nothing more important than steering and brakes, so treat them that way. Steering is more important than exhaust! If the steering doesn’t clear the standard headers, maybe a custom set will be needed.


Suspension members go up and down and turn for cornering. It’s therefore a good idea to keep other elements from contacting them. Exhaust systems often suffer from lack of differential clearance. Incorrectly fitted brake hoses can rub on moving and turning parts. Put the suspension and steering through its full travel and check for fouling and rubbing during the set up process.

Springs are often not set up at right angles to front and rear axles. The result is that they can then rub on either the axle housing or the crossmember supporting them. Make sure that front beam axles and rear differentials are set up at ride height during the build to avoid this kind of issue.

Building a hot rod is a sequential process but it’s one where you have to be thinking three steps ahead at all times. Experienced builders know that mocking up your project as fully as possible is the key to avoiding mistakes that come from after thoughts. In a production car factory, new vehicles are designed and drawn on computers which simulate the full vehicle operation and the relationship between fixed and moving parts. Hot rod builders don’t get that luxury. Experienced builders understand that full mock up is how garage builders do the same thing and only then do they fully weld brackets and finalise their “design”.

As the engine rocks on this hot rod, the alternator fouls on the chassis.
Engine/transmission clearance to the firewall is important to avoid knocking during operation.
The exhaust pipes are sitting on the crossmember and will give a constant vibration through the vehicle.
Lack of header to engine mount clearance can be seen here.
Make sure that brake hoses cannot rub during the operation of steering and suspension travel. 
The rub mark on the intermediate steering shaft shows that there’s not enough clearance with the exhaust.