stainless fasteners in aluminum engine

[petertha]

Just starting to do some assembly on my radial. I mostly have the typical black oxide cap screws in my metric collection. But I happened to grab some stainless ones for a particular size & kind of liked the look of it. I don't know if its 304 or 306 or what dark back alley source I even purchased from. Is there some corrosion or thread sticking issue with mixing these alloys? I cant recall where I thought I came across it but hoping its an outside, harsh environment or saltwater thing? OTOH every methanol RC engine I see usually has black oxide fasteners. But maybe that's a cost thing?

Also, some of my stainless cap screws have a smooth shoulder as opposed to the vertical knurl lines. Is there a fastener standard that defines this?

[derekwarner]

Peter...

Working backwards, universally Cap Screws [SHCS] and their cousins are marketed by Grade or tensile strength

For the Steel Industry [in Australia] for impuslive applications Grade 12.9 is specified, for lower stressed applications Grade 10.9.. then to General bolts of Grade 8.8 these are produced from varying steel melts of differing chemical composition

300 Series stainless steels have higher Nickle content so therefore corrosion resistance is markedly increased, however at the loss of tensile properties

So no, using high tensile carbon steel [blackened Oxided] SHCS of Grade 10 9 or 12.9 is due to a requirement of strength, ......in that a specified size SHCS torqued to the recommended value will maintain the induced torque without stretching or failure

A 300 Series stainless steel SHCS to attain the lockdown strength requirement of the application would need to be of larger diameter

There are Engineering Standards to define the Grade of carbon steel fastner be they hex head or SHCS....these are by numeral or line marking in a pattern and stamped into the head of each and every bolt

I am not aware of any universal Engineering Standards used to define by physical marking by Grade of stainless steels

So in an earlier life  I spent many years supervising electro/hydraulic maintenance of US built Gun Mounts & Missile Launchers for our Royal Australian Navy.........the subcomponents in the hydraulic systems were secured with High Tensile SHCS with a gold or black phosphated surface finish........this surface treatment was post product heat treatment and to improve corrosion resistance......

It was not unknown for smallish ~~~~5/16"x32 UNF SHCS to shear the heads literally with the sound of pistol shots when a component was pressurised........yes the secondary process of phosphating with an incorrect heat value negated the previously attained mechanical strength by the initial heat & quench process   

Derek

 

[Jo]

Stainless Fastners are defined by the standard ISO 3506-1 : http://www.bsf-sh.com/upload/file/20160601/20160601090581068106.pdf

Jo

[Roger B]

I have used a mixture of stainless and plated steel fasteners on aluminium engines without problems up to now. The cylinder head bolts on this engine are in contact with the cooling water and can be undone. The choice is often based on which material the sizes I want are available in in sensible numbers. If I want 1000 off I may have a choice.

In most cases I use caphead screws because I like them not because I need the mechanical properties. Even the big end bolts on the diesel don't need to be 8.8 quality.

[MJM460]

Hi Petertha, aluminium and stainless are along way apart on the electrochemical series and in the presence of an electrolyte, the aluminium corrodes to protect the stainless.  This effect is used deliberately to protect steel, stainless steel and bronze propellors by attaching aluminium alloy anodes.  Immersion is deadly.  I have seen a mast on a yacht fold in half due to severe corrosion behind a stainless fitting attached. That should definitely not be submerged.

 But even in marine application, the combination is used and the corrosion prevented by using an insulating layer of paint or similar between the materials, and using a special paste on threads of fasteners.  I don’t know whether the paste actually electrically insulates the components or just excludes water.  But it works.  And even in that environment different results are found in different environments such salt content and water origin.  And slightly different alloys offer different degree of protection.

There is also the question of whether the stainless steel itself is subject to corrosion.  I have found that on boats some special alloys are excellent, 316 is normally good and does not appear to corrode, but 304 will result in rust stains running down from the fastener head after a relatively short time.  But again, boats are not normally very far from the sea.  And I don’t have specific experience of boating on far inland freshwater lakes.  The rust stains may or may not be the result of salt water.

I don’t know if your engine application would be considered normally wet and so subject to galvanic corrosion.  Certainly Rogers experience is encouraging.

As Derek says, stainless is lower strength that the high tensile steel used in high quality bolts, but again, it is not certain that your application requires the full strength that his example required.

If you search the web for Unbrako cap screws your will find a very good technical manual which has an excellent engineering data section with material strength  and corrosion resistance.  Of course your screws from unknown sources probably do not meet the standards specified in that manual, but the information is still useful and authoritative.  Useful for relative properties of bolt materials, and a very good description on corrosion issues.

None of this really answers your question.  I suspect the answer is to follow the practice set by others who have built similar engines. 

I hope that is some help.

MJM460