Penetrating oils - how do they compare?
The classic car fraternity is often faced with bolts that are difficult to release with the consequent fear that increasing effort will result in the bolt shearing which then leaves many DIY enthusiasts with a major difficulty - extracting a thread. So using a rust release agent or penetrating oil to help free a seized bolt thread is a sensible option, but which product is likely to be the most effective? The modest additional cost of a release agent that performs well in releasing a seized thread is well worth bearing but also you need patience and restraint so you do not apply too much force in attempts to free a thread.

RV8 enthusiasts trying to replace their bump stops often find removing the worn unit is not straightforward because the thread is often seized. It's very easy to shear that thread by applying increasing force. The wise approach was highlighted in a V8BB thread by Angus Munro in 2012 who touched on the risk of shearing saying "with the RV8 bump stops, particular care is required with these items. They should simply screw out, the stud staying with the bump stop and leaving a nice clean threaded hole for the new item. However, the stud is pretty likely to have corroded in position and when you apply what you consider to be a very light amount amount of torque, the stud, being made of British Standard Cheese, will shear leaving you with a very difficult extraction. To help the matter soak the area of the bump stop metal plate to chassis location with WD-40 and keep doing it for as long as you can wait before starting the job. Then proceed very gently. Once the old bump stop is out put the new bump stop in place with Copper Ease on the thread and only nip it hand tight. It does not need to be tight at all really."

WD-40 is a big-selling and incredibly versatile product found in most workshops and readily available in many shops. For many DIY maintenance jobs it is a valued aid but when it comes to dealing with seriously seized threads then dedicated rust release products can often perform better. Some of those products have extraordinary penetrating and release abilities and some more sophisticated products include a severe cooling or freezing action to "crack" the seized area by thermal movement. So a combination of the "cracking" effect together with the penetrating action on the seized section of the thread is able to release the bolt so it can be undone without excessive effort and consequent risk of shearing components.

Nic Houslip agrees "the cost is likely to be the least consideration if it is going to take four or five hours to get a corroded stud, nut or bolt off. In difficult cases I rely on PlusGas, which is the most penetrating compound I have found. I had to overhaul two Johnson outboard motors that had extension shafts on and I needed to dismantle the lower parts that had been exposed to sea water for some years. I managed to remove all the bolts - if I recall about 30 of them - that were screwed into aluminium housings without breaking a single one, but it took more than a week of patiently applying PlusGas around the bolt heads using Plasticine to make a berm to contain the fluid, then attempting daily just to move all the bolts until I got some movement, adding more PlusGas then trying again later that day. Eventually they free up sufficiently to withdraw completely."

Nic Houslip added "the most useful tools in these cases however are patience, restraint and enough time to complete the job. A copper based anti-seize compound is useful to prevent recurrence of corrosion when re-assembling, but be aware that dry assembly of some nuts and bolts is often stipulated. This is particularly true of modern wheel bolts, where torqueing of a bolt to a particular figure may not leave the bolt in its correct tension state if lubricated. The theory of fasteners requires that the bolt is stretched so that should movement occur in the joint the bolt can still “give” and not be stretched past its elastic limit. When the force that moves the joint is removed the bolt will still apply the correct clamping force. If you lubricate the bolt, the threads can slide more easily and the bolt might be stretched more than it should be in order to reach the desired torque. In the case of dry assembly, I find it satisfactory to apply a spray of WD-40 after assembly to the finished parts to prevent corrosion occurring later. There was a lot of discussion on the Internet last year about a home brew mixture of Automatic Transmission Fluid (ATF) and Acetone being the most effective penetrant, but I cannot vouch for this and besides acetone is highly flammable, so great care is needed."