Head Torque Sequence

[Fred M.]

I would like to re-torque the head bolts on my Generac generator engine. It is the GN-190 (Generac-Nagano) engine, about 14 years old.

A shop manual downloaded from the Generac web site, covers several versions of the GN-series engines. The only information about a head torquing sequence is for one that has 4 head bolts. Numbering the bolts clockwise from the top left corner, the 4-bolt sequence is 2-4-1-3.

My engine has 5 head bolts arranged in a circle. Would it be better to follow the manual's sequence and then do the odd bolt last? That would be 2-5-1-3-4. Instinct tells me that the sequence should be 2-4-1-3-5, but there could be some rationale used by engine designers that I'm not aware of!

Fred

[BTPost]

The "Conventional Wisdom" says you hand tighten ALL the Bolts, first, then slowly
tighten in a Diagonal Pattern, across the Head, Starting with the Head Bolt nearest
the Exhaust Manifold. Final Torque Spec would be dictated by the Grade and Thread
of the Head Bolts in question. This is for when NO Other Data is available. YMMV

[Andrew Mackey]

Where you start on an odd number circular pattern is not that important. If you are not replacing the gasket, loosen all bolts. Find torque spec for the bolts. Lightly coat threads with anti-seize, and install finger tight. Tighten every other bolt to 1/3 of the final value, until all bolts are tight. With the first bolt you tighten, the pattern will be - 1, 3, 5, 2, 4. Repeat to 2/3 of the value, using the same starting point. Finally, torque to the final value, again using the same starting point, every other bolt, until all bolts are done.. Torquing in 3 stages will prevent stress fractures in the head, by reducing load stresses made by full value torquing at 1 time. There is one exception to this. If you replace the head gasket, at about 5 hours run time, remove the head bolts one at a time, re-lube, and retorque to final spec torque. This will take up any loss generated by gasket compression. The initial re-torque, after new gasket installation, is the only time I do not stage head bolt tightening, and it is a 1 time deal. Torque head bolts at room temperature - approximately 70 degrees F. If the engine was in a cold area, warm to 70 for 24 hours, to let the block and head stabilize temperature.
Andrew

[Fred M.]

Bruce-

Since the manual's 4-bolt sequence starts with one away from the exhaust manifold, the designer may not have thought it mattered.

Andrew-

You answered my other question. The torquing of the rocker arm cover starts with the one closest to the exhaust manifold, 1-3-2-4, but specifically states using a two-step process. Since the rocker arm cover is torqued in two stages, I wondered if the same would apply to the head bolts. Perhaps this is understood by experienced people, so they didn't mention it in the shop manual!

Fred

[BTPost]

In my post, I assumed that the reader would torque the bolts in multiple steps, as
that is how all Torquing should be done, so I didn't mention that. With most engines,
the Exhaust Manifold side of the engine heats first, as the engine comes up to temp.
It is therefor, a good idea to start there, as that will have more stress, than the other
side of the engine. If the Tech Writer even considers that, when writing the Specs for
Head Bolt Torquing, is a matter for the OEM, to decide. Torque Values are determined
by the Bolt Grade, Thread Spec, and Block Material.

[Andrew Mackey]

Fred,
I 3 stage my torque tightening, as that is what I was taught early in my rebuilding career. I have been working on engines for about 47 years now, and have spent more than 45 rebuilding engines, from 5-S B&S to The American La-France Junior V-12 (527 CID). I have yet to crack a head or have a head gasket I installed blow out. Believe me, on a V-8 or the V-12 engines, it takes a long time to tighten all the bolts, (almost 11/2 hrs on each head of the ALF) but if you are careful, and take your time, it is time well spent. As a point of interest, I do not use a 'click off' torque wrench either. I use the pointer rod and scale type wrench. The one used on the small engines is graduated in inch pounds, and the automotive is in foot pounds. The inch pound wrench is used on automatic trans work as well! I usually check the readings on the wrenches once a year - a friend has a calibration tool that reads how much torque the head of the 'bolt' has on it, and I check the torque wrench reading against it. Both of my wrenches are Craftsman built, and in over 40 years of service on the ones I own, I have only had to replace the foot pound wrench once, and that is because I made a mistake I won't repeat: I lent it to a friend who was a 'mechanic', and when I got it back, the torque pointer was 5 pounds off center. It turned out that he was using the torque wrench as a breaker bar, in order to remove frozen lug nuts on an old ford truck. The torque wrench had a longer handle and arm then the T handle lug wrenches he had (he had folded 2 of them before he borrowed my wrench). He ended up shearing off 5 of the 8 lugs, but actually bent my wrench in the process. Sears Craftsman made good on the replacement, they do stand behind their tools, but that was not the point. I do not lend them out any more. If you want the wrench, you get the mechanic with it!
On an air cooled engine, it makes sense to start near the exhaust valves, as posted earlier, but in the end, the entire head must be torqued to the same value, so start point is not that critical on the 4 and 5 bolt heads. On larger heads, due to the larger area of the areas to be joined, start point and sequence does become a critical factor. This is especially true of multi-cylinder engines with a common head. Another note of interest, on aluminum block and heads, I usually make sure the surfaces are clean and true. On heads, a piece of fine sandpaper (350 grit or finer), laid over a piece of 1/4" plate glass, will remove high spots created by warpage, and gasket collapse. The same goes for the block deck on small engines.- a light dressing with a fine flat file will usually take off any thread pulls, without major work. Dress lightly, lay the file parallel with the longest side of the contact area, and only remove major high spots. I usually stuff a kerosene dampened rag in the cylinder, in order to stop any shavings to enter the piston clearance area, and I also use compressed air to claer out blind bore bolt holes. EYE PROTECTION IS A MUST> On larger or multi cylinder engines, if there are thread pulls (the raising of the aluminum metal around the tapped holes in the block) that are major, then a professional re-decking may be required, as well as a thicker head gasket, to prevent clearance problems. There are usually 2 reasons for thread pulls - 1), over torquing, or 2), extreme metal fatigue usually brought on by: a), mechanical problem like over heating (running too lean due to mis-adjusted or worn carb, or intake manifold air leak, b), cooling system faults - no water or blocked cooling fins), or c), running for extended time with a burned head gasket or exhaust valve. If the temper on the aluminum metal is compromised, even a Heli Coil repair may not help, and on larger engines, it may lead to fatal damage to the block.
Andrew