Marinucci: Finesse and patience prevent thread damage

Practical precautions and old-fashioned patience often prevent costly thread damage during fastener removal.

Careless technicians may turn routine bolt removal into a nightmarish repair by rushing the job or overlooking signs of thread trouble, sources report.

By ignoring critical clues, hurried techs may strip threads on an engine, transmission or other components. What's more, repairing the threads may be tedious because they are situated in an awkward location.

Carelessness, therefore, may delay a repair job, cost the business money and tarnish its reputation.

I also discussed avoidable mistakes in my last column (Aug. 28): I reminded readers that seemingly innocent steps could strip out a threaded blind hole or crack the casting surrounding it.

Many owners and managers of automotive repair businesses have not turned wrenches — have not performed the nitty-gritty work. Nonetheless, I hope that bosses learn to assess mistakes in a reasonable manner.

When a casting cracks or threads strip out, was the tech truly unlucky? Or was the tech simply displaying careless work habits that a boss should address?

Exposed thread issues

Corroded, exposed threads may not occur on every vehicle — let alone cause trouble when they are present — but this corrosion, which is easily overlooked, may destroy adjacent threads.

A classic example is Honda and Acura vehicles equipped with a Mitsuba-brand OEM starter. On these applications, the lower starter bolt may thread into an area of the aluminum transmission case called the bellhousing.

Sometimes, a starter bolt threads into a blind hole in the engine block or the transmission bellhousing. Because the blind hole shelters the bolt threads from the elements, the threads do not corrode; the bolt comes out easily.

The hole on these Honda and Acura vehicles, however, is drilled completely through the transmission bellhousing. Experience has shown that the factory-installed bolt may protrude through the bellhousing, exposing one or more of its threads to road splash, snow and road salt.

Understandably, vehicles operated in snow-and-salt regions of the country are much more vulnerable to thread corrosion than other vehicles are.

However, a normally cautious tech may not notice the exposed threads — let alone dirty, corroded threads — on the Mitsuba's lower mounting bolt. What's more, this bolt may come out fairly easily in spite of some dirt or corrosion on its exposed threads.

Corroded bolts usually offer considerably more resistance than healthy ones. An ambitious tech may continue applying force to the bolt until it comes out. Unfortunately, the corroded bolt threads may tear the threads out of the transmission bellhousing.

Hopefully, prudent techs proceed cautiously when they feel excessive turning resistance in a fastener. For example, they examine the work area and locate a place to spray penetrating lubricant into the fastener's threads.

Here, spraying lubricant into the outer end of the threaded bellhousing hole suffices. Then techs slowly turn the lower starter bolt inward and outward a little bit at a time — adding lubricant each time — until the bolt is free again.

I have stressed in previous columns that a patient visual inspect prior to a repair is an invaluable step. Suppose a tech discovers exposed threads such as those on the Mitsuba's lower mounting bolt. If that is the case, patiently pre-soaking those threads with penetrating lubricant is a sensible step.

Techs also should remember to clean and lightly lubricate those starter bolts before installing the replacement starter.

Unusual thread type

Some techs habitually recheck a threaded hole by manually spinning a clean bolt into it. When they cannot turn a bolt easily with their fingers, they usually clean up the threaded hole with a restoration device called a thread "chaser." (A thread chaser resembles a traditional thread tap.)

Verifying the bolt's actual thread type with an appropriate thread gauge is a smart — but often overlooked — precaution. The Mitsuba starter's bolts resemble common 12-mm bolts. However the thread pitch is an atypical 1.25 dimension.

Furthermore, a kit of traditional taps or thread chasers may have only 12-mm sizes in the more-common 1.50 or 1.75 thread pitch. (To grossly simplify, pitch relates to the relative fineness or coarseness of a fastener's threads.)

An inattentive tech may destroy the threads in the bellhousing hole by trying to restore them with the incorrect-pitch rethreading tool.

The key takeaway here is that an ounce of prevention is always worth a pound of cure. Patient, cautious steps are the keys to a stress-free starter replacement or other task.