Honda GL1000 / GL1100 / GL1200 Head Gasket Details

Head gaskets on early ‘Wings are ultra reliable. The vast majority of these bikes are still running flawlessly on the original, factory-installed head gaskets which are now 30+ years old. Rare failures are usually caused by inattention and poor maintenance to the cooling system. But, a simple case of overheating can ruin your head gaskets. Warped heads are also a possibly outcome of overheating!

Head Gaskets

GL1000 Cylinder Head and OEM Honda Head Gasket

I know that a number of customers are involved in high performance projects that involve supercharging, turbos and nitrous oxide. Copper head gaskets are virtually indestructible and recommended for such high performance duty. I have used them on my own supercharged GL1000. The main drawback of copper head gaskets is that they require machining of the heads and/or block to install special stainless steel wire “orings.” These are necessary to generate the engineered clamping forces since copper does not compress appreciably and will move laterally unless it is “captured” relative to the bore. Another issue is that copper has no sealing ability, so gasket sealant must be used. Also be prepared for some nuisance coolant leaks with coper head gaskets that usually disappear.

For most customers – I strongly recommend OEM Honda head gaskets! There are some aftermarket head gaskets in the marketplace that are probably fine. But be aware that some aftermarket head gaskets are not! 

Have you ever wondered why Honda has specified so many part numbers for their head gaskets over the years:

12251-371-003 (GL1000)

12251-371-010 (GL1000)

12251-371-306 (GL1000) – current part number

12251-463-000 (GL1100)

12251-463-003 (GL1100)

12251-MG9-000 (GL1100 / GL1200)

12251-MG9-306 (GL1100 / GL1200) – current part number

Some of the explanation is due to superseded part numbers. That happens as Honda changes vendors and continuous product improvements are made. But the main reason is connected to the fact that 4 cylinder ‘Wings all have different bore dimensions (all had the same stroke at 61.4mm):

GL1000 = 72.0 mm

GL1100 = 75.0 mm

GL1200 = 76.0mm

Head gasket designers know that there is an ideal geometry and relationship of the armored crush ring aspect of the gasket and it’s proximity to the engine’s bore. Any departures from this “ideal” represent a compromise that has a negative effect on function and reliability. Honda felt the 1mm bore difference between the GL1100 and GL1200 models was close enough so that both engines could be accommodated by one part number.

However, the GL1000 was not included in this thinking and still retains its own unique Honda head gasket part number. Honda clearly feels that the 4mm bore difference between the GL1000 and GL1200 models is too great to “bridge.” I wholeheartedly agree.

Also, even though engines have perfectly round bores, the “dome” in the heads and the corresponding openings in the head gaskets are not perfectly concentric. Well-engineered head gaskets have subtle deviations from “round” to allow proper clearance for the valves.

Beware! There is a fairly prominent web vendor that offers only one aftermarket head gasket part number which it claims covers all 3 models! Heed my advice and steer clear of such head gaskets! This job is too time consuming to do twice. Use Honda head gaskets to be safe. If you insist on saving a few bucks and are set on using aftermarket head gaskets, at least be sure to pick a vendor that understands that one part number can’t cover all 3 models.

Consult the Factory Manual – Section 5 for full details on head gasket replacement. The job is straightforward, but rather time-consuming. Attention to detail is the key to this task! You must have access to the manual!

Here are some additional Head Gasket Replacement Tips:

1. The engine block and head must be surgically clean!

2. 100% of the old gasket residue must be removed without gouging the soft aluminum. Many methods will work. Be safe and use a slow, tedious method. Pro mechanics may use power tools for this step; however, that’s not recommended for amateur mechanics. Take precautions to keep debris out of the engine block openings.

3. Check the head and block for trueness using a precision straight-edge or sheet of true plate glass. It is a good practice to “surface” the head slightly with 220 grit sandpaper affixed to a large sheet of true plate glass. My method is to “color” the entire surface of the head with a Sharpie marker, then sand gently in a “figure 8” pattern until all evidence of the Sharpie marking is gone. This will reveal “high” and “low” areas or any other irregularities. The goal is not to remove lots of material…just verify trueness. Obviously, the use of sandpaper requires that the head be cleaned with warm water and detergent, then rinsed and dried thoroughly.

4. Check all critical components to make sure they are within spec: valve stems, valve guides, rocker shaft, rocker arms, valve springs, cam lobes, cam bearing journals, cam bearing ID in head,

5. The valve seats (in the heads) can be “ground” but that is rarely necessary.

6. The valves themselves have a special coating and can not be ground. Grinding will ruin them!

7. But, the valves can by gently “lapped” to the heads using fine lapping paste. Here’s a useful link to a lapping tutorial video

8. Always install new valve stem seals as part of a head job…no matter how good the old ones look.

9. Be sure the valve cotters (“keepers”) are fully seated.

10. Final step before bolting up the heads: clean both the head and block with rubbing alcohol to remove any oily residues.

11. Make sure the oil orifice passages are in place and have new orings. The “small” end of the oil orifice passage piece is oriented “in” toward the engine block. Important: each oil orifice passage piece gets 2 orings. The small oring measures: 4.1 x 1.5mm. The placement of the small oring is obvious. It goes in a “groove” on the oil orifice passage piece (small end). When installed, the small oring disappears just inside the engine block. The larger oring used here measures: 6.5 x 1.5mm. Its placement is not quite so obvious. It mounts onto the larger end of the oil orifice passage piece…filling the gap between the passage piece and the head gasket. There is no “groove” for this larger oring. Do not forget either of these vital orings! My practice: install the oil orifice passage piece with the small oring in place into the engine block. Then, install the larger oring over the oil orifice passage piece. I find this easier and less likely to create mistakes when mating the head to the block.

Oil O-ringLarger o-ring for oil orifice passage piece goes here

12. Each head has 2 locating dowels. I recommend that these be replaced with new dowels. Old pitted dowels can prevent the head from seating fully “home” and cause the whole job to fail! The corresponding holes in the head and block should be cleaned thoroughly. I use a small brass brush on a Dremel.

13. Section 5, page 19 of the official GL1000 workshop manual recommends the use of “liquid sealer” but does not get any more specific. Plus, that information is 30+ years old. My practice: I install OEM Honda head gaskets “dry.” They are impregnated with heat and pressure activated agents to insure a proper seal for the oil and coolant passages. Adding any “gasket sealer” to the equation might compromise Honda’s engineering and invite disaster. Decide for yourself, but if you ever want to test this and waste $40.00, torque down a new Honda head gasket. Then come back the next day (without firing the engine) and remove the head. You will be amazed at how well bonded the head gasket is without the benefit of a heat cycle! Note: this “experiment” will destroy the new head gasket!

14. Both gaskets are identical, but they are not symmetrical! Be sure the lower oddly-shaped aspect of the gasket matches the corresponding shape on the block.

15. Unlike modern car engines, the head bolts on these engines can be reused. They should be carefully inspected and the threads cleaned. Be sure the threads in the block are absolutely clean as well. If the threads aren’t clean and perfect, torque values will be corrupted!

16. Use a very light application of molybdenum disulfide grease (“Moly” paste) on the head bolt threads and under the heads of the bolts to ensure accurate torquing to full engineered clamping pressure.

17. Torque carefully in the sequence and stages outlined in the Factory Manual. Use an accurate torque wrench and proper technique. Torque the six 10mm head bolts first, then torque the small 6mm bolt last. I make 3 passes in the recommended sequence to arrive at the final recommended torque value (50%, 75% and 100% passes).

18. Be aware that in 1978, Honda changed the torque spec for all GL1000 cylinder heads to: 5.8 – 6.2 KG-M (42-45 ft-lb).

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19. I recommend that all new head gaskets be put through a heat cycle and then re-torqued. I use 100% distilled water for this step. The heat cycle should be done with no pressure in the cooling system (radiator cap off). Let the engine cool completely and then re-torque. For this final re-torquing, I recommend that each head bolt be cracked slightly loose and then re-tightened to specification following the recommended torque sequence pattern.

20. Always use the correct coolant. It should be a “non-silicate” type. There are many suitable brands available. I use the Honda brand of premixed coolant. There is some evidence that links the use of old-fashioned antifreeze containing silicates with “scouring” of the head gaskets and premature failure.

201. Final Tip: The normal procedure is to remove the carbs and coolant tubes to do a head gasket job. I’ve found that that is not really necessary – especially if you are certain the coolant orings don’t need replacement. They are a hassle to replace correctly and they can be left alone if they are not leaking. If you are doing only one head gasket, you can leave the carbs in place. Support the side you are working on by gently wedging suitable blocks of wood between the engine and carb assembly. BEFORE you remove the head bolts, remove the intake manifold bolts and the two screws at the coolant tube flange. Then you can CAREFULLY separate the intakes and coolant tube flange with a slim, sharp-edged wedge tool. Important: you only want to separate the seals…not move the parts any appreciable distance! Then, proceed according to the normal procedure. You will still want to renew the intake orings and the coolant flange gasket. Refitting the refurbed cylinder head is slightly trickier but not bad considering how much work you just saved! This also applies if your doing both head gaskets. In that case leave the carbs in place and support with bungee cords.

Head GasketPhoto shows head gasket repair without removing carbs or coolant tubes 

 If the tips above are followed carefully, you will have many years and miles of trouble free service!

Comments

  1. will GL1000 heads work on a GL1100 and which head gaskets should i use.

    • Norton Muzzone says:

      Ken,
      To answer your question directly no, they are not interchangeable. They have two very different cylinder dimensions.
      Thank you for your inquiry,
      Randakk Customer Service Team

  2. Mariano Vico says:

    HI friends, do you know where can I find those oil orifices (part 12238-371-000) ? Thanks !

  3. Hello,

    I have a 1976 gl1000 and she start up fine but I’ve notice a little drip of oil dripping from the timing belt cover and from the left cylinder. Do I need a new Head Gasket?

    thanks

  4. L. T. Damgaard says:

    You recommend the cylinder bolts re-tightened after a non pressure warm up. Do i have to slightly loose one bolt and re-tighten one after one, or loosen all bolts and re-tighten after Warp up sequence?

  5. steve white says:

    looking at your site for head gasket replacement, great information. the last picture in the tips shows the head gasket a copper color was it sprayed with copper coat? and do you recomend doing that . thanks for the help.

    • No that’s a custom head gasket I had made out of sheet copper for a supercharged bike. Not recommended due to minor nuisance cooling leaks and the difficulty in installing metal “orings”in the head /block. Orings are mandatory when using copper head gaskets. I’ve since reverted to OEM Honda head gaskets on that bike.

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