Golf Rod Bolt fitting, shaft interference, thread standards.

[indiatango]

I'm trying to buy uprated con rod bolts for a 1996 Golf GTI engine (2.0 8V AGG) from ARP but the listing in their catalogue is a bit vague and not 100% sure if they will fit.

After much searching I've found the details of the con rod bolt specified for an OEM one matching the original part number:

Outer thread [mm]: M8 x 1
Length: 59.5mm
Diameter: 13.2mm
Thread Length: 25mm
Shaft Length: 29mm

ARP do sell a con rod bolt listed as '1.8 & 2.0 VW watercooled' which my supplier thinks will probably fit but isn't sure. This bolt is UNF 5/16 x 24, which is obviously very close to the M8 fine pitch in size.

What concerns me is that the shaft is obviously an interference fit and presumably there could be some small difference in shaft diameter if the bolts are cut to different standards?

So... would you generally expect both bolts to use the same standard of interference fit?

And on that note is there actually a 'standard' relationship between the nominal size of a given thread and the tolerated range of sizes for a press fit?

Thanks in advance for anyone who knows the answer to this!

[GreenV8S]

I'm not familiar with that engine, but if it's an interference rod bolt then it will have a specifically designed profile. You can't swap it for another bolt with vaguely similar thread and expect that to work. To work, it would need the same profile. It seems unlikely that the engine manufacturer provided metric and imperial alternatives for the engine so I would assume you need to replace with the identical part unless told otherwise by somebody you trust. I assume you'd be replacing the nut and bolt as a pair so the fact the threads are different diameters and pitches might not be a deal breaker.

[indiatango]

Yeah that's pretty much what I figured. I suppose in principle it's possible that the locating section of the shaft could be correct but for some reason they have applied a different thread standard. Can't really see why they would do that though.

The specs for rod bolts usually just seem to provide the thread size, not the diameter of the actual locating section, which makes me think that there is a relatively standard relationship between the two but then that would support that theory that it's the wrong bolt!

Ultimately it's not a very powerful engine and I won't be revving it to oblivion so I guess the £1.20 FEBI bolts will be alright...

[stevieturbo]

indiatango said:

I'm trying to buy uprated con rod bolts for a 1996 Golf GTI engine (2.0 8V AGG) from ARP but the listing in their catalogue is a bit vague and not 100% sure if they will fit.

After much searching I've found the details of the con rod bolt specified for an OEM one matching the original part number:

Outer thread [mm]: M8 x 1
Length: 59.5mm
Diameter: 13.2mm
Thread Length: 25mm
Shaft Length: 29mm

ARP do sell a con rod bolt listed as '1.8 & 2.0 VW watercooled' which my supplier thinks will probably fit but isn't sure. This bolt is UNF 5/16 x 24, which is obviously very close to the M8 fine pitch in size.

What concerns me is that the shaft is obviously an interference fit and presumably there could be some small difference in shaft diameter if the bolts are cut to different standards?

So... would you generally expect both bolts to use the same standard of interference fit?

And on that note is there actually a 'standard' relationship between the nominal size of a given thread and the tolerated range of sizes for a press fit?

Thanks in advance for anyone who knows the answer to this!

Are you seriously asking if you should use an imperial bolt, in a metric hole ?

[E-bmw]

stevieturbo said:

indiatango said:

I'm trying to buy uprated con rod bolts for a 1996 Golf GTI engine (2.0 8V AGG) from ARP but the listing in their catalogue is a bit vague and not 100% sure if they will fit.

After much searching I've found the details of the con rod bolt specified for an OEM one matching the original part number:

Outer thread [mm]: M8 x 1
Length: 59.5mm
Diameter: 13.2mm
Thread Length: 25mm
Shaft Length: 29mm

ARP do sell a con rod bolt listed as '1.8 & 2.0 VW watercooled' which my supplier thinks will probably fit but isn't sure. This bolt is UNF 5/16 x 24, which is obviously very close to the M8 fine pitch in size.

What concerns me is that the shaft is obviously an interference fit and presumably there could be some small difference in shaft diameter if the bolts are cut to different standards?

So... would you generally expect both bolts to use the same standard of interference fit?

And on that note is there actually a 'standard' relationship between the nominal size of a given thread and the tolerated range of sizes for a press fit?

Thanks in advance for anyone who knows the answer to this!

Are you seriously asking if you should use an imperial bolt, in a metric hole ?

It looks like it!

[TekoTime]

stevieturbo said:

Are you seriously asking if you should use an imperial bolt, in a metric hole ?

A rough calculation says the pistons for that engine will have around 1,600kg of upward inertia force at 5,000 rpm.

Imagine hanging a Passat off a couple of 5/16-24 bolts mashed into an 8x1.0 thread.

[InitialDave]

I would go direct to ARP and ask them, based on your engine code and their part number.

If the design of the assembly is a toleranced fit of the bolt shank into the conrod, then ARP will almost certainly be grinding their shanks to an appropriate size to achieve that - the fact it's a UNF thread on the end isn't especially relevant.

[stevieturbo]

ColourRestorer said:

Perhaps all those rushing to judge the OP should have considered his use of the words “… the shaft is obviously an interference fit…” and considered the significance.

The con-rod bolts are in fact supplied as a stud and matching nut, so there is no suggestion that the OP is trying to mix and match threads, simply that he is wondering whether a stud with different thread form would be acceptable, and whether its shank will be of the correct diameter to form that interference fit in the con rod.

It’s a shame that so many PHers don’t read properly, or stop to think, before they start to type.

Nothing is obvious...he could equally mean screwing the wrong bolt into a thread is "obviously" an interference fit....but that is because it is wrong. Not by design.

If con rod "bolts" are a stud and nut.....then they are not really "bolts"...they are a stud and nut and should be stated as such.

Yes, do read and think before you type, and be crystal clear and specific especially if intending to do something odd.

You are equally making assumptions when multiple times it has been stated as a bolt...not a stud and nut.

And the OP has disappeared ?

[indiatango]

Haha, sorry I've just seen all the replies! I didn't see any notification about new posts...

To clarify then, no I'm absolutely not talking about trying to force a a female metric thread onto male imperial or vice versa! That would of course be ridiculous, so apologies if my post was misleading.

The locating mechanism of the rod bolt is supplied by the 'gripping' (unthreaded) section of shaft which is pressed into the con rod. The threaded portion of the bolt serves no locating function, so provided it is mated with the appropriate nut It seems safe to assume that the exact thread doesn't matter as long as the interfering section of the bolt mates to the rod within the specified tolerances of the fit.

As everything else in the engine is metric I would assume that this section is toleranced according to the ISO standard (I.e. H7/p6 etc). I believe imperial units use the ANSI standard which I'm not really familiar with but they are not interchangeable.

It seems wierd to me that ARP would create a rod bolt which used a metric standard for the grip section and imperial for the thread, though of course there is no physical reason why you couldn't.

So I guess the first part of my question what whether there is typically a 'standard' relationship between the nominal thread size and the nominal size of the gripping section of the shaft in a rod bolt. For example, would it be expected that an M8 con rod bolt always have a gripping section with a nominal size of 8, cut to the standard ISO tolerances for the fit. If this is the case then the ARP bolt is completely ruled out as we could expect the grip section to be toleranced according to the ANSI fitting standard.

If the bolts were cheaper or returnable I could just order them and measure with a mike to see whether the grip section was within accpetable tolerances for the shaft, but they're £150 with no refunds and I don't know exactly what interference fit is specified!

I emailed ARP prior to the original posting and also called them but they were extremely unhelpful and just insisted that the bolt would fit all VW 1.8 and 2.0 litre engines which seems unlikely. They could not (or would not) check it against my engine code or even just my model of car, nor could they provide me with the exact spec of the grip section.

[InitialDave]

Yes, there are standard relationships between shank size and thread size (standards are great, always loads to choose from!), but for a proprietary bolt like this, that's not really a factor.

If they're unhelpful and won't supply the technical specifics of the product, I wouldn't give them my business.

[stevieturbo]

Unless you are using huge RPM's over standard...I doubt there is any need to use anything but OEM fasteners.

Although ARP can often mix n match stuff so nothing with them can be taken for granted.

They might offer a fastener with correct fitment, but in a different thread, as being US based they can do odd things. Or often they might offer a metric fastener...that requires an imperial socket, again this seems more aimed at US buyers as they have imperial tools. Even if it is clearly the wrong way to do things

[indiatango]

stevieturbo said:

Unless you are using huge RPM's over standard...I doubt there is any need to use anything but OEM fasteners.

Although ARP can often mix n match stuff so nothing with them can be taken for granted.

They might offer a fastener with correct fitment, but in a different thread, as being US based they can do odd things. Or often they might offer a metric fastener...that requires an imperial socket, again this seems more aimed at US buyers as they have imperial tools. Even if it is clearly the wrong way to do things

How weird! That certainly makes sense of it though. Re. the need for it, I'm totally with you on that - my golf is a lovely little car but it's definitely not a rocket ship and I won't be revving it to oblivion.

My main reason for interest in the ARP is that, a) the dealership would charge the same price for standard bolts as ARP do for uprated ones, and b) the dealer has none in stock anyway. The only OEM ones available are from FEBI at £1.20 each. I suppose they will be fine, I just haven't used that brand before and it seems extremely cheap to me.

If the opinion on here is generally that £1.20 FEBI bolts will be perfectly fine and durable for normal-ish driving then I'll just forget all this nonsense and go with them!

Still, it's been interesting learning more about this!

[stevieturbo]

FEBI are a god brand.

I would still prefer actual OEM from the VW dealer where possible.

FEBI whether good or not are aftermarket and may not be the same as OEM. ( might be worse, might be better...but no way to know. )

[indiatango]

Thats reassuring. Yeah unfortunately the dealership is telling me that there are none listed anywhere in europe and they have no idea if/when there might be some available so It's pretty much a choice between the FEBI ones which should definitely fit and may or may not be good quality, and the ARP which are definitely good quality and may or may not fit!

Thinking I may just take a gamble, order the ARP ones and if they don't fit will just have to stick them on Ebay and hope someone else wants them.

[Camaro]

One word of warning I would give you OP, is that from my experience with ARP Fastners and engines, you need to double check your bearing clearences once torqued to ARP specs.

Their specs will not match OEM and because of that, you can have more deformation on the mains and journals. Will ARP stud gear on a different engine, you have to torque to spec and then varify you still have your bearing clearence. If not, you have to buy larger bearings.

[stevieturbo]

I'm sure the FEBI would be fine.

Surely there are companies in the UK who deal with these engines and can supply the correct part ?

[Boosted LS1]

Camaro said:

One word of warning I would give you OP, is that from my experience with ARP Fastners and engines, you need to double check your bearing clearences once torqued to ARP specs.

Their specs will not match OEM and because of that, you can have more deformation on the mains and journals. Will ARP stud gear on a different engine, you have to torque to spec and then varify you still have your bearing clearence. If not, you have to buy larger bearings.

Their literature always advises that you check for tighter clearances but I've built a number of engines without any issues. I suspect the increased clamp/torques aren't capaple of crushing a bearing cap. I agree that it's best to check though :-)

[indiatango]

Camaro said:

One word of warning I would give you OP, is that from my experience with ARP Fastners and engines, you need to double check your bearing clearences once torqued to ARP specs.

Their specs will not match OEM and because of that, you can have more deformation on the mains and journals. Will ARP stud gear on a different engine, you have to torque to spec and then varify you still have your bearing clearence. If not, you have to buy larger bearings.

I've read various things about increased load equalling potential for more deformation - I've spoken to a few experienced engine builders about it who all had very different opinions on this! It's interesting that ARP don't really mention it anywhere that I've seen - you would think that if it were a common issue then they would warn you about it... maybe try to sell you a nice £400 ARP branded clearance/roundness checker!

[indiatango]

Boosted LS1 said:

Their literature always advises that you check for tighter clearances but I've built a number of engines without any issues. I suspect the increased clamp/torques aren't capaple of crushing a bearing cap. I agree that it's best to check though :-)

Ahh... I completely missed that line when I was reading through their website!

[indiatango]

stevieturbo said:

I'm sure the FEBI would be fine.

Surely there are companies in the UK who deal with these engines and can supply the correct part ?

You would think so! I already contacted VW heritage who couldn't supply it. Dealership have no idea when they might be able to get any, if ever. I've used used engine parts uk ltd before and found them pretty good but in this instance they couldn't confirm the fit.

If the general opinion on FEBI is that they are fine then may as well get on with it. I can put the £130 I'll be saving towards my next worthless old banger.