Need suggestion on bolts to use in petrol engine (most likely aluminium block)

seatbeltnoob

Changed my thermostat housing and took out 3 bolts. They are rubbish allen socket bolts (softend over the years from being heated from engine), So 1 bolt stripped, and the other 2 near stripping.

I have taken those bolts out and temporarily put in bog standard stainless steel bolts from screwfix (m6x30).

My understanding is the wrong metal fastener can cause galvanic corrosion and corrode into the other metal. I can't find OEM bolts anywhere, it's a VW so I should be able to get it from TPS but TPS are complete ****es these days and tell you trade only.

What spec bolt are suitable for use in engine block?

I ordered these on ebay. I read on websites for engine block Grade 8 is what I need, Grade 8 (US) is equivalent to class 10.9 (EU) according to google searches

Metric threaded ISO 4017 Surface Treatment Zinc flake Cr6+free - ISO 10683 flZnnc Steel Class 10.9

• Driving Features: Hexagon
  
• Material: Steel
  
• Length: 30mm
  
• Head Shape: Hexagon Head
  
• Grade: 10.9
  
• Thread Direction: Right Hand
  
• Thread Size: M6
  
• Surface Treatment: ZN-Zinc Flake
  
• MPN:04116.060.030

https://www.ebay.co.uk/itm/143790233207

born_again

Give the local VW dealership a ring. Might be cheaper than you think. Or look for a local motor factor (the ones that deliver to local garages) and get them from them.

lemondrops69

10.9 are over the top for a thermostat housing, 8.8 is fine. So is s/s in ali, if you're worried change to steel, but what you already have will be ok

caprikid1

I should add that if you use plenty of copper slip grease on the bolt going back in and leave plenty on the head any problems in the future are unlikely to be yours !.

coffeehound

Aluminium and stainless react when in contact so probably best replace those

seatbeltnoob

coffeehound said:

Aluminium and stainless react when in contact so probably best replace those

Yeah that's what I heard, some of the posters don't know about galvanic corrosion and just giving incorrect advise. Does class 10.9 suitable for aluminium block?

ian16527

seatbeltnoob said:

coffeehound said:

Aluminium and stainless react when in contact so probably best replace those

Yeah that's what I heard, some of the posters don't know about galvanic corrosion and just giving incorrect advise. Does class 10.9 suitable for aluminium block?

10.9 is the strength of the bolt. 

For the thermostat housing Grade 8.8 normal steel bolts will be fine. 

daveyjp

caprikid1 said:

I should add that if you use plenty of copper slip grease on the bolt going back in and leave plenty on the head any problems in the future are unlikely to be yours !.

That's also the advice when fitting spark plugs to aluminium heads, especially now spark plugs may not be removed for years between changes.  Guaranteed as soon as you go to change them the thread will strip!

oldagetraveller1

daveyjp said:

caprikid1 said:

I should add that if you use plenty of copper slip grease on the bolt going back in and leave plenty on the head any problems in the future are unlikely to be yours !.

That's also the advice when fitting spark plugs to aluminium heads, especially now spark plugs may not be removed for years between changes.  Guaranteed as soon as you go to change them the thread will strip!

Very strange that Denso advise not to use any grease on the threads.

What do they know?

YellowCarBlueCar

Daralac is the stuff to use of you're worried about galvanic corrosion, but most engines are aluminium and use steel bolts without major problems.

lemondrops69

seatbeltnoob said:

coffeehound said:

Aluminium and stainless react when in contact so probably best replace those

Yeah that's what I heard, some of the posters don't know about galvanic corrosion and just giving incorrect advise. Does class 10.9 suitable for aluminium block?

Which posters?

I've used s/s on ali for a long time without issues, but then I dont by cheap rubbish from screwfix. As other have said use copper slip. 

As for using grease, I would on ali or the threads can pick up