The following are useful tips for technicians andengine specialists who spend any time sealing up engines.
1. Make sure all the head bolts are in perfect condition with clean,undamaged threads. Dirty or damaged threads can give false torquereadings as well as decrease a bolt’s clamping force by as much as 50%!Wire brush all bolt threads, carefully inspect each one, and replaceany that are nicked, deformed or worn. If a bolt doesn’t thread into ahole smoothly with finger force alone, there is a problem.
On aftermarket studs or bolts, never use a die to recut the threads.Most have “rolled” threads, which are stronger than standard threads.Use a thread chaser to clean up the threads.
2. Dirty or deformed hole threads in the engine block can reduceclamping force the same as dirty or damaged threads on the bolts. Run abottoming tap down each bolt hole in the block. The tops of the holesshould also be chamfered so the uppermost threads won’t pull above thedeck surface when the bolts are tightened.
Finally, clean all holes toremove any debris. If the block deck has been resurfaced and you areusing studs rather than head bolts, chamfer the top of each hole
3. For head bolts that screw into blind holes, lightly lubricate thebolt threads as well as the underside of the bolt heads with engineoil. Make sure the bolts do not bottom out or hydrolock because of oiltrapped in the blind hole.
For head bolts that extend into a coolant jacket, coat the threads witha flexible sealer. Failure to coat the threads may allow coolant toleak past the bolt.
4. Because TTY head bolts are permanently deformed once they aretightened, they should not be reused. Reusing TTY bolts will cause themto stretch further, which increases the risk of breakage. A stretchedbolt also will not hold the same torque load as before, which may causea loss of clamping force resulting in head gasket leakage.
5.Check bolt lengths. Make sure you have the correct length bolts for theapplication and for each hole location (some holes require longer orshorter bolts than others). If a bolt is too short and only engages afew threads in the block, it may pull the threads out of the block.
Bolts should also be measured or compared to one another to check forstretch. Any bolt found to be stretched must be replaced because 1) itmay be dangerously weak, 2) it won’t hold torque properly, and 3) itmay bottom out when installed in a blind hole.
6. When installing head bolts in aluminum cylinder heads, hardenedsteel washers must be used under the bolt heads to prevent galling ofthe soft aluminum and to help distribute the load. Make sure thewashers are positioned with their rounded or chamfered side up, andthat there is no debris or burrs under the washers.
7. Resurfacing a cylinder head decreases its overall height, so be sureto check bolt lengths to make sure they won’t bottom out in blindholes. If a bolt bottoms out, it will apply little or no clamping forceon the head, which may allow the gasket to leak.
If a head has beenmilled and one or more head bolts may be dangerously close to bottomingout, the problem can be corrected by either using hardened steelwashers under the bolts to raise them up, or by using a copper headgasket shim in conjunction with the new head gasket to restore properhead height.
8. Always look up the specified tightening sequence and recommendedhead bolt torque values for an engine before installing the headgasket. Never guess. Complete cylinder head torque specifications fordomestic and import vehicles can be found in service manuals and torquetables published by gasket manufacturers.
9. Use an accurate torque wrench to tighten standard-type head bolts inthree to five incremental steps following the recommended sequence andtorque specs for the application. Tightening the bolts down graduallycreates an even clamping force on the gasket and reduces headdistortion. It’s a good idea to double check the final torque readingson each head bolt to make sure none have been missed and that the boltsare retaining torque normally.
If a bolt is not coming up to normal torque or is not holding areading, it means trouble. Either the bolt is stretching or the threadsare pulling out of the block.
With TTY head bolts, use a “Torque-To-Angle Indicator” gauge with atorque wrench to achieve proper bolt loading. Don’t guess on the angle.Be exact.
10. If a head gasket requires retorquing (most do not), run the engineuntil it reaches normal operating temperature (usually 10 to 15minutes), then shut it off. Retighten each head bolt in the samesequence as before while the engine is still warm. If the engine has analuminum cylinder head or block, however, don’t retorque the head boltsuntil the engine has cooled back down to room temperature.
On some applications with retorque-style head gaskets, it may benecessary to retorque the head a third time after a specified time ormileage interval due to the design of the engine. Follow the vehiclemanufacturer’s recommendations.
Did You Know? In an engine with 4” cylinderbores and peak combustion pressures of around 1,100 psi, each cylinderexerts about 13,827 lbs. of pressure against the cylinder head at fullthrottle. In fact, head bolts may have to handle loads of more than 5tons per bolt at wide-open throttle!
Latest posts by Larry Carley (see all)
- New Engines Force New Pistons - Feb 27, 2015
- Choosing the Right Camshaft for a Performance Engine - Feb 6, 2015
- Picking a Racing Oil - Jan 16, 2015