Bonneville cylinder Head

[triman65]

I have had the cylinder head skimmed and cannot get a seal with the copper gasket. I fitted it and put a smear of grease on and it leaks oil. I also had a rebore so as the rings have yet to bed in and oil gets up the bore. I wonder if the Pushrod tubes are holding the cylinder head off slightly.

Has anyone had a similar problem? any ideas, also which is the best sealant to use eg Hematite

 

[veelad]

Ok, I was bowsing the web and fell across this posting about your bonnie head

The last time I rebuilt a bonnie engine was back in 75 so I may be a bit rusty on the subject.

I am from the UK and owned and worked on bonnies for a living and without knowing the year of bike I may get things a bit mixed up

The copper head gasket MUST BE clean of all grease , oil and any other debris such as carbon deposit, as must the mating faces of the head and the barrel

The old style of doing this was to soak copper gasket in parrafin then wash in petrol and wiping with a soft non abrasive cloth.

The next stage is to anneal the copper (I dont know if you have done this or not, but will explain anyway)

I used to use my mums gas cooker (much to her displeasure), the trick is to heat it to a dull red colour, If you are doing a copper gasket as opposed to a washer, such as a cylinder head gasket, some people like to cool by quenching in water.

Drop it in edge first for quick, even quenching. Quenching isn't strictly necessary, the rate of cooling makes no difference to the resulting softness.

If the head gasket is soft, you should get a good seal, but ensure you undo and re-tighten (to correct torque) cycliner head bolts in the correct order so as not to warp the head.

You also mention a rebore?, I assume you have fitted new plus size diameter pistons and not used the old ones, if you have used the old ones this will cause knocking problems , oil leaks, loss of compression and damage the engine assuming it even fires up that is.

skimming a head will increase compression, and the bonnie has a variety of pistions.

7:1 on later models and 8:1 on earlier. however, some models were fitted with 11:1 which had a castle dome with big cut recesses to allow valve clearance, these were mainly used by speed freaks and racers and really upped the power, but also blew alot of engines (that would be me), I changed back to 8:1 after that.

To be continued.

[veelad]

This forum only allows so many words , so need to continue here

If you have the castle pistons, then I recomend going back to 8:1 so as not to bump the head due to it being skimmed.

also make sure the valve rods are seated correctly on the bowles, or your head will not fit correctly and you will do alot of damage.

as a tip, if you ever remove the barrel, be careful not to mark or scratch the push rods as that can create a weak point, a trick we used was to polish them with soft leather and a very mild abrasive so that not a mark was visible.

and last but not least, If you have had the head skimmed, did you replace the valve guides and valves, Im not sure how the leaded fuel situation is these days in the usa, but if you have gone unleaded, it may be worth finding a company that can replace the valves with a harder material valve so you can use the unleaded fuel

just a thought. hope this helps bud

Vee

[Rex]

 have had the cylinder head skimmed and cannot get a seal with the copper gasket. I fitted it and put a smear of grease on and it leaks oil.


That's unusual. You shouldn't be getting that much oil in there to show an external leak. If anything, it would be burnt. As the man says, you need to anneal and refit the gasket dry, paying particular attention that there's no raised bits around studs etc.


I also had a rebore so as the rings have yet to bed in and oil gets up the bore. I wonder if the Pushrod tubes are holding the cylinder head off slightly.
They're supposed to hold the head off 60 thou, and this ensures sufficient crush for the pushrod seals to be oil tight.

Has anyone had a similar problem? any ideas, also which is the best sealant to use eg Hematite

Shouldn't be using sealer on the head gasket, and the pushrod tubes only need a smear of oil to prevent seal inversion on fitting. For a general purpose sealer I use Blue Hylomar, and I'm surprised they even still make that Hermashite stuff....

 Hope that helps...

[L.A.B.]

Im not sure how the leaded fuel situation is these days in the usa, but if you have gone unleaded, it may be worth finding a company that can replace the valves with a harder material valve so you can use the unleaded fuel

I think triman65 actually lives in the UK.

Personally, I would (and do) use the composite flame ring head gaskets, as they tend to seal better than copper, in my opinion.

[L.A.B.]

I'm surprised they even still make that Hermashite stuff....

They don't, although it is still posssible to buy it, but it has been replaced by 'Hylotite red'.
http://motorcycleproducts.co.uk/catalogue/this-not-red-hermetite-50g-tubes-p-4196.html

Personally, I use Stag Wellseal, Blue Hylomar or Loctite 5926 (depending on the job).

[triman65]

Yes L.A.B I am in the UK

thanks chaps for all the information. answers to some of the replys are. year 1972 model, rebored with new pistons,valve guides,valves and valve springs.
 I think the problem lies with the pushrod seals. Some of the seals when fitting are to tight. If you fit them in the head, the pushrod tube will not go in. If you fit them to the pushrod tube first which you have to stretch, it will not go into the head. You twist them on the pushrod tube which allows it to enter the head but it makes the seal thicker. I have bought the gasket sets from Len Craig and Supreme Motorcycles
 
Mick