Outboard Cylinder Head Bolts

[Fire99]

Hi all,

Excuse a separate thread for this but I need to get this one moving.

Currently rebuilding a Mariner 2.5 (2-stroke) and need to replace the cylinder head bolts. Is there a material / grade spec for these or is it not too specific? I have industrial fastener suppliers nearby so it seems a bit bonkers that I need order 'special' marine bolts from an outboard supplier the other side of the country.

Any info is appreciated.


Nick

 

[TQA]

Hi all,

Excuse a separate thread for this but I need to get this one moving.

Currently rebuilding a Mariner 2.5 (2-stroke) and need to replace the cylinder head bolts. Is there a material / grade spec for these or is it not too specific? I have industrial fastener suppliers nearby so it seems a bit bonkers that I need order 'special' marine bolts from an outboard supplier the other side of the country.

Any info is appreciated.


Nick

I would use a standard bolt and smear some Coppaslip or other anti seize on before fitting.

 

[elton]

For a cylinder head bolt, you need one that will apply the correct force when tightened. That means it has to be of the specified elasticity, tightened to the recommended torque. I don't know how you'd select an appropriate bolt, other than by buying the OEM part.

If you use any old bolt, tightened to arbitary torque, you risk the cyclinder head gasket blowing or snapped bolts.

 

[Hobnob]

In my old Evinrude 4 I used chromed mild steel UNF bolts. They were carefully torqued down to the correct spec, using the correct tightening pattern. No issues since, despite engine being flogged (boat needs 6hp really).

 

[Fire99]

Many thanks... I'm not being a skin-flint over the price of the bolts. Just that I'm sure the mariner bolts though no doubt are to a specific grade / type, aren't anything 'special' in the world of fasteners.

 

[SAMYL]

I think the Mariner engines used stainless bolts and they always caused trouble by corroding with the aluminium.

If you can source s/s ones then fine but otherwise H/T bolts or even mild steel if you are stuck except that they will rust. Be careful tightening them as there is a risk of the the alloy threads stripping before the bolt will shear.

Apparently you should not use copper grease with aluminium - I have forgotten the chemistry - but there is a grease/anti-sieze compound you can use but its name escapes me at the moment.

 

[chrisbitz]

It may be different with a car engine, but their cylinder head bolts are very specific use once ones, that are specifically designed to stretch on tightening, and then need to be replaced if undone, as the stretch has already been used...

It may be that outboards are more old fashioned technology?

I also think that CH bolts even traditionally are high tensile steel at the very least, due to the high torque of tightening?

 

[oldharry]

It may be different with a car engine, but their cylinder head bolts are very specific use once ones, that are specifically designed to stretch on tightening, and then need to be replaced if undone, as the stretch has already been used...

It may be that outboards are more old fashioned technology?

I also think that CH bolts even traditionally are high tensile steel at the very least, due to the high torque of tightening?

I would use HT steel by preference for a small two stroke, but many of these little 2Ts have ordinary stainless head bolts. The single use 'stretch' bolts common in bigger car engines are rarely found on low HP engines unless they are 'specials' designed for extra high revs or speed. use plenty of grease or Coppaslip to help stop them seizing in the alloy heads.

 

[Rigger Mortice]

It may be that outboards are more old fashioned technology?

It always used to amuse me that my old Mariner had a 200cc engine which produced all of 8bhp, but a 250cc Honda motorbike churns out over 25bhp.

I don't think that the cylinder head bolts on a little Mariner outboard are going to be seriously stressed. Whether the threads are the same at both ends or not, I don't know, but it may end up being easier to use OEM.

 

[CreakyDecks]

It may be different with a car engine, but their cylinder head bolts are very specific use once ones, that are specifically designed to stretch on tightening, and then need to be replaced if undone, as the stretch has already been used...

I don't think you can really compare head bolts on a modern overhead camshaft car engine with those on a little two stroke outboard. I've even rebuilt mine using the old head gasket. I wouldn't think of doing that on a car!

 

[Fire99]

I don't think you can really compare head bolts on a modern overhead camshaft car engine with those on a little two stroke outboard. I've even rebuilt mine using the old head gasket. I wouldn't think of doing that on a car!

Thanks everyone. This really is my point. My little Mariner 2.5 (well now 3.3) is about as complicated and modern as a stone tablet so was kinda reluctant to go buying 4 little bolts from the other side of the country. I like to know what (normal) bits can be used so it's easy to rebuild in an 'emergency'.