Do the 16 valve conrods fit the 8 valve motor?

[Shoei]

Hi Guys,

New to the K100 and I have a 1987 K100LT.
My son stripped the engine and has removed the conrods from the crank.
Having tried to find replacement bolts, it turns out they are no longer available for the 8 valve motor, but they are for the 16 valve version.
Does anyone know if the 16 valve conrods will fit the 8 valve motor?

Cheers.

[Laitch]

The way to find the answer is to consult a parts fiche like this one. I discovered that the 2V Bricks have the same conrod part numbers as the 4V Bricks. Then what was also discovered is that the conrod bolt numbers are the same and that while NA for the K1002V, they are listed as available for the K75 and the K1004V. So rather than buying new conrods, I'd buy just the K75 bolts. The parts manager at the host dealer of the fiche can also be consulted if reassurance is needed.

Omissions in parts listing from one model to the another seem to have occurred sometimes when the fiches were updated.

[Shoei]

Thanks Laitch.

Thanks mate.

Having checked the parts list, it appears the part numbers are different for the K100 2 and 4 valve versions, but appear to be the same for the K100 2 valve and the K1100 4 valve models.
I have been told that the 2 valve bolts are splined and not available any more and the 4 valve bolts are different. Maybe I should order one of the bolts and try it.

[The Mighty Gryphon]

Since there's no prohibition on reusing the connecting rod screws you might want to look at getting a set of used rods with screws.  From checking a few prices on eBay just now, not only will you have the correct screws, but you'll save a little cash on them as well.

Here's an example:

https://www.ebay.com/itm/1985-BMW-K100-K-100-Engine-Rods/190968682940?hash=item2c769ed9bc:g:2TEAAOxy3NBSelu4

[Laitch]

I'm not totally onboard with your fiche analysis but regardless of that, if rod numbers and the cap numbers are the same in 2V models, it doesn't bother me that the bolts were redesigned in 1992. I'd use them. Gryph has a much cheaper way to get the bolts so that is appealing if they're in usable condition. After all, it's only a motorcycle and not an airplane; you're much closer to the ground if the engine fails. The conservative approach of buying a pair of bolts to test their efficacy could be comforting, especially if there is no time pressure.

[Scott_]

I'll ask the obvious question.... WHY did your son take the engine apart in the 1st place.. Unless severely abused these engines are pretty bullet proof.

[Chaos]

I'll ask the obvious question.... WHY did your son take the engine apart in the 1st place.. Unless severely abused these engines are pretty bullet proof.

and it's almost always cheaper and easier to find a decent used one rather than rebuild a broken one.

[Shoei]

The bolts are stretch bolts Gryphon, so should not be reused.
They might be alright, but the build is not going to be cheap and I don't fancy throwing a rod on the dual carriageway.
There are a set of K1100 rods on eBay, which have the same part number and I can get the bolts for them.
Might order a pair of the newer bolts, which I have heard are different and see if they fit. If they do, happy days.

I have found a decent K1100 engine for sale nearby, but am wondering if the mounitngs, throttle bodies, clutch and gearbox are the same as the K100.

The engine had about 15PSI compression on 2 & 3 Scott. When we got it, it had nearly 100'000 miles on the clock and the speedo didn't work, so god knows how many miles it has actually done.
There is some wear in the cylinders, but not too much.

[Laitch]

The engine had about 15PSI compression on 2 & 3 Scott. When we got it, it had nearly 100'000 miles on the clock and the speedo didn't work, so god knows how many miles it has actually done.

What did a leak-down test indicate was the cause of low compression?

[The Mighty Gryphon]

You can spend the money on the big end screws for the rods, but there is absolutely no mention of using new screws in the BMW factory shop manual(see page 11-63).  The torque specification for them is 30nm + 80degrees.

https://www.kforum-tech.com/Manuals/BMW%20Manual%20K75-K100%202-Valve.pdf

Also, there is no mention anywhere in the BMW parts fiche that these screws are single use like the parts used in the clutch. 

[Shoei]

Unfortunately, I was unable to do a leak-down test, but did a wet test, which should give a hint of what the issue was, which appeared to be between the valves and the cylinders.

As they are stretch bolts, I am really wary about putting them back in. The stretch built into the bolt is designed to hold it under torque whilst in moting, so I couldn't trust re-torques stretch bolts personally.
Have always used new stretch bolts on all the engines I have done in the past.

[daveson]

I have full respect for your diligent, go by the book, attitude. But if it doesn't say single use, I spose used would still be by the book. I would measure to see if any are stretched. Check if they look like torque to yield bolts.

One of the good things about bricks is the ridiculously easy access to the crank. I'd use the bolts if they're not stretched, you could check after 1000 miles if any got loose.

I also have respect for some mechanics who have confided to me that they re use torque to yield bolts without issue.

Its just one of those things that makes you go um and ah.

Another thing that makes you go um and ah is that using k1100 bolts in a k100 is not going by the book, I spose.

[Shoei]

You have a good point there, the crank certainly is easy to get to.
I think it was in the Haynes manual that it stated that the conrod bolts should not be reused.
Would you suggest tightening them up to torque and not the extra 80 degrees as they have already been tightened and then turned through the extra 80 degrees?

Technically, using the 1100 bolts would be by the book as they have the same part number.
Although, I understand that BMW did have and issue with some of thier parts numbers, so who knows if they would work.

[daveson]

Yep, plus 80 degrees. I almost always think going by the book is the way to go. I've got three brick manuals, I trust Haynes the least, but I use it heaps.

[The Mighty Gryphon]

Yeah, go by the Haynes book instead of the OEM factory manual.  That's the ticket.

Wasn't it the Haynes Manual that specified using some sort of special tool to tighten the nuts on the big end bearings?  A tool that tightened both nuts simultaneously or some such.

I gave my Haynes Manual away a long time ago.  Too many weird things in it.  Clymer and the OEM manual is where I go.

[Laitch]

The bolts are stretch bolts Gryphon, so should not be reused.
Might order a pair of the newer bolts, which I have heard are different and see if they fit. If they do, happy days.

The history of BMW Motorrad is one of continual modification over each model's lifespan with attention given to backward compatibility. That the K100 2V crank bolts are listed as NA is not surprising considering the K100 2V was discontinued entirely in 1991; however, crank bolts were redesigned after that. The same bolts are used in the K75, the K100RS 4V and the K1100 regardless of conrod part number. Any of those bolts would be suitable for your relic, Shoei, if you want to be conservative in your rebuilding approach, because they're all the same bolt spec.

BMW tends to be assiduous in indicating when parts should be used and when they should be replaced. Whether that's driven by profit or by engineering data is in most cases left for the consumer to determine, so Gryphon's observation about reuse seems valid.

[Shoei]

Thanks for your replies guys.
Makes me feel a bit better about reusing the bolts.
The gaskets are on order, so time to start cleaning bits ready for the rebuid.

[The Mighty Gryphon]

Keep us posted on your progress.  You're going to have a nice machine when you're done.

[Shoei]

Will do fella, although, I am busy working on the house at the moment and then have a kitchen to rip out and replace, all while trying to run a business, so it's not going to be a quick build.
Not sure if you guys are all purist Motobrickers and will be offended by a cafe racer build?
If not, I will stick some pics on when it is done.

[Laitch]

Not sure if you guys are all purist Motobrickers and will be offended by a cafe racer build

During the frenetic pace of your life, you probably couldn't help but overlook the Project Custom Motobrick section below the Motobrick Workshop. The Project Custom section is the home of Cafés and other queered-out creations and is awaiting your photos and descriptions of your creation with an open-door welcome. 

[Shoei]

Ah, so there is.
It's not just the fact I'm too busy, it's also about being a bloke and not looking properly. 

[Laitch]

It's not just the fact I'm too busy, it's also about being a bloke and not looking properly. 

Start your project thread soon. Your blokehood will undoubtedly make it long-term reading for some of the idle blokes around here, if they ever find it. 

[koshak]

if rod numbers and the cap numbers are the same in 2V models, it doesn't bother me that the bolts were redesigned in 1992. I'd use them.

post-1992 bolts have different part number.
post-1992 bolts do not fit pre-1992 conrods directly as they are something like 0.1mm thicker.

[Laitch]

post-1992 bolts do not fit pre-1992 conrods directly as they are something like 0.1mm thicker.

How much did it cost you to determine that?