Engine Mount Bolt Length - Big One Into The Rubber

[revilla]

Can anyone tell me the length of the big 1/2" UNF bolts that go through the engine mounting arms into the rubbers? Car is Imperial K Series.

I can measure them when I get home but I'd like to order some new ones today if I can. From the fastener pack info I'm thinking 3½" or 4" in Class 10.9 (or Imperial Grade 8).

Searched the archives and managed to find info on all the other engine mounting bolts and the thread size but not the length of the one I need!

Thanks,

Andrew

[CharlesElliott]

1/2" x 2.5" caphead.

Caterham do not explicitly call them out as Grade 8/10.9.

 

[Jonathan Kay]

0.5" UNF x 2.5" according to a 1998 Assembly Guide.

Jonathan (Crossed with Charles')

[revilla]

Thank you gentlemen ... I do have the assembly guide and looked through it before asking but must have missed it.

[revilla]

Hmmmm ... are we sure about this?

My engine mounts came back from Arch and the bolts also arrived today from Namrick. 1/2" UNF x 2½" as suggested above. I can now see that the tubes on the engine mounts are 2" long so I'm only getting about 4 threads through which is clearly wrong!

Of course I had them delivered to the office and the original bolts are in my garage at home, so I still can't measure them (I didn't both measuring when I got home last time as I had already gone with consensus on here).

In my original post I mentioned that I was thinking 3½" or 4" and looking at the fastener pack contents lists, if they're not 2½" they must be one of those - but still not sure which. Will have to measure up tonight.

[Jonathan Kay]

2.5" in this thread, too. But if they're not long enough for your brackets they're not long enough. 

Jonathan

PS: Have you got still a bit to go there on the gap between the bottom of the head and the washer... ?

[revilla]

Yes a Google search unearthed a few more references and they all agreed on 2½".

The "gap" is because it is a spring washer, so it will compress a little and allow a bit more thread engagement but I'm sure the original ones fitted well down into the rubbers. Any just from memory of seeing them on the garage floor when I took them out, I'm sure they were longer than that.

I may of course be wrong. Time for actual measurements though. Will check tonight.

[CharlesElliott]

I haven't measured mine as the car is in bits and the bolts aren't to hand, but everything I have says 2.5". Once you bolt it up and the spring washer compresses it should be fine.

You didn't go with Grade 8 then? (nor caphead!)

[revilla]

Charles,

I didn't go with capheads because the ones I took out were not capheads, neither were the ones I've seen on any of the other cars I've worked on. Was really wanting to replace like with like. Not sure what the particular advantages and disadvantages are so played safe.

Wanted Grade 8 but I just couldn't find anywhere that supplied them. Metric in 10.9 yes but places seem to list UNF bolts in these sizes up to Grade 5 / 8.8 only, or what they refer to as Grade 'S' (which looks roughly similar to Grade 5 and may even me a misstype). Tried all my usual suppliers.

Since then I've found these that look better. They are listed as UNF bolts but to a metric steel Class 10.9 rather than Imperial Grade 8 which is probably why I was missing them when looking before. For safety I will probably order these but I'm going to measure the ones I took out first because it still looks to me like a lot less thread engagement than I would expect.

I took the spring washers out and just slipped them under the head of the bolt from the outside if you what I mean to see how much thread will protrude when the spring washer is fully compressed and it's only about 9mm if thread on a bolt that is 1/2" (12.7mm) in diameter. I thought that general good engineering practice called for 1.5 times the diameter so I was expecting more like 19mm and that feels more like what I remember when I removed the old ones.

Just out of interest and for my education, how could you tell from my photograph that the bolts were not Grade 8 as the head markings were not visible? The only reasons I can think of are:

• Maybe they don't usually do Grade 8 in BZP (but I've used plenty of Metric 10.9 BZP fasteners).
• I mentioned they came from Namrick and you know they don't sell them in Grade 8!

Is there any particular reason why capheads would be preferred in this application?

Thanks,

Andrew

[CharlesElliott]

I know that Namrick don't do Grade 8 (at least, not on their website). I use Margnor for those that I care about, like the front suspension. To be honest, I wouldn't be too worried about them for the mount.

My fastener pack (2003) came with capheads and so that's what I've always used. I don't know why, but in that situation where it goes into the mount tube a caphead just seems more appropriate, not sure why.

I understand what you mean on the length and would also want to double check. Is the mount slightly more than 2in, as with no washer you should have at least 12mm showing?

I just looked at the packs that Caterham are currently selling for imperial, and they are 1/2" x 2.5".

[Steve_K]

I'm not sure about the "1.5 times the diameter" standard but I have come across having a minimum of 1.5 threads protruding from the nut to show its in good engagement. This is from aviation practice.

Steve

[revilla]

I was making allowance for the thickness of the fully-compressed spring washer as shown below:

That's what I was trying to explain when I said "I took the spring washers out and just slipped them under the head of the bolt from the outside if you what I mean" in my previous post but I admit I wasn't very clear.

Without the washer yes I get about ½" of thread protruding. Measurements are also rather crude as I'm sitting at my desk at work and the most sophisticated measuring device available to me is a shatterproof plastic ruler.

I wasn't aware of Margnor before but I will make a note of them for "serious" bolts in future.

Thanks for all the help.

[Jonathan Kay]

"1.5 times the diameter" is pretty widely used, it's just a reasonably safe rule of thumb. 

Biggest risk of inadequate penetration of nuts is probably some variety of thread stripping when the first bit of thread is damaged and that hasn't been recognised. For that failure mode 1.5 threads protruding probably serves the same purpose.

Jonathan

PS: Two pages and no personal derogatory jokes? :-)

 

[Jonathan Kay]

... is a shatterproof plastic ruler.

I always treat that as a challenge. And into how many pieces do plastic rulers break? It's worth destroying one just for the fun of finding out. Spaghetti* is cheaper. And YouTube not as satisfying.

Jonathan

* Uncooked.

[revilla]

@Jonathan - We are all gentlemen (today!).

@Steve: In this application the bolt goes into a threaded bush rather than a nut. However, the standard thickness of a ½" UNF nut is 7/16" or 11.11mm. The thread pitch in 1/20" so 1.5 threads would be 1.90mm. Total thread required to be equivalent to 1.5 threads through the nut would therefore be 13.00mm. On that basis it is still a little short.

It may of course just be that it is screwed into a blind holes and the the next standard size up would be too long. My VVC engine is rated at 174Nm peak torque and guessing (as I'm at work and can't measure) the engine mounts rubbers are about 70cm apart. So if the torque of the engine tried to pull one mount out of the rubber it would exert a maximum pull of 258N or about 26kg. The weight of the engine alone will be exerting a downforce greater than that so it wouldn't even lift up under torque reaction if the bolt was missing. A Grade 5, ½" UNF bolt is rated at 18000lbs in tension and 15000lbs in shear (Grade 8 would take that to 23000lbs and 18000lbs respectively). I think the bolts will probably be more highly loaded by the inertia of the engine over bumps and bends but still I don't think those bolts are actually particularly stressed!

I'll measure the old ones when I get home, although the evidence seems to be stacking up that I'm likely to find that they are 2½" long and I'm worrying about nothing.

Thanks again.

[revilla]

Well I got home and measured the old bolts and they were ... 2 1/2" as everyone said. But they didn't have spring washers, which I think is what created the impression that they were longer. And they weren't Grade 8 / 10.9 either, they had the three radial lines on the head to signify Grade 5, i.e exactly the same as the ones I bought. I'm still surprised that they don't fit longer bolts as peering into the rubber mounts I can see a fully threaded metal bush that must be 20mm deep. Well the old ones have done just fine for 13 years so I will fit the ones I've bought, but with spring washers as they should be. Thanks as always for the help and support.

[Jonathan Kay]

... support.

:-)

Jonathan

PS: Those safety factors are interesting. And their size is the main reason I'd be most worried about other modes of failure.

[Shaun_E]

I've added a bolt list to the guides section here

Please email any additions/changes to me and I'll keep it up to date.

[revilla]

That's great thanks Shaun, will try to update you as I come across things.

[Paul Deslandes]

Fastener pack contents are listed here although quite a few are 'miscellaneous' which doesn't help much when working out which one you need.

The 95 vintage build manual, which is quite informative re fixing sizes, and other info here.

Paul

[revilla]

Thanks Paul

[TomB]

Related thread resurrection!

Ive been advised to alter the spacers on my engine mounting, so am looking into this now. I have ~5mm of washers between the chassis and the engine mount and the two attachments so lifting the engine higher to give more clearance between the steering column and DS scavenge pulley. 

As such the underside of the mounting rubber is suspended between the two sets of washers, and so isn't sat on the rail and there could be risk of failure of the metal base. So I need to take the spacers out, lower the mount flush with the chassis rail, and put 5mm of washers between the top of the rubber and the mounting bracket. With a split washer and 5mm of washers, the 2.5in bolt doesn't engage with the threaded bush, so I need a longer bolt. 

Looks like plenty of bolts on eBay, 2 3/4in & 3in unf, I think 3in will be best to give a decent amount of thread.

 

 

 

 

 

 

 

 

 

[Paul McKenzie]

Tom,

 

Your washers should be on top of the engine mount as you are about to do. I have used 2 and 3/4" bolts on mine since almost new as the rubbers compress a little and eventually the carbs snag the cutout - (heavy VX lump).  Just make sure that when you measure up the engine mounts etc to get the correct bolt length you don't go too long or they'll bottom in the mounts and rip the rubbers.

[TomB]

With a split lock washer under the head and 5mm of washers under the mount on top of the rubber, a 3in bolt was perfect.