Cylinder stripped thread experiment

[ncsteve]

When I took the top end off the 2008 Keeway Matrix 50 I bought recently, I found the worst thread damage I have seen in a long time. Someone pulled the cylinder stud out of the aluminum case threads, put in a helicoil, and then pulled the helicoil out of the case too! This left a hole almost 3/8" in the crankcase! So here is what I decided to try: At Grainger I found a stud 1/4"-20 by 5" long and an EZ Lok thread insert threaded 1/4"-20 internally and 7/16-14 externally. I drilled out the large hole to 23/64 and ran a 7/16 by 14 tap in deep enough for the EZ Lok. Then I drilled the bottom about 5/8" deeper with a 13/64 bit and ran a 1/4-20 tap in. I installed the EZ Lok with a little JB Weld for good measure, then put more JB on the stud and threaded it in until it is about the same height as the other studs. Tomorrow I will install the new piston, cylinder, and cylinder head. When I put the torque wrench to it, we will see what happens! L hope you never see a mess like this, but I will be the guinea pig to see if this fix works. I will let you know!

[repherence2]

I've ripped threads out of my Chinarelli case from torqing the head fasteners before. repaired it with helicoils. when I I took apart the block to rebuild it for a mid-race bore I removed the helicoils (2 of them) and redrilled all 4 existing stud holes, tapped it, and installed Keensert 1/4-20 threaded inserts. the o.d. of the 1/4-20 bolts I'm using is smaller than the o.d. of the original m7 studs. when fitting up the head to the bore, there was a lot of slop/misalignment because of the smaller o.d. of the bolts. I ended up machining some bolts (3/8 x 1 1/4 if I recall correctly). bored through the bolt, to fit the o.d. of the 1/4-20 bolts. machined off the threads and machined the o.d. of the bolts to fit the i.d. of the stud holes on the bore and head. the machined bolts act as alignment dowels to center/align the head to the bore.

I did all four stud holes even though 2 of the existing holes were okay. I did it because I didn't want mixed materials clamping it down. I wanted to make sure all the hardware doing the clamping had the same thermal an tinsile/stretch properties to make sure the clamping stayed consistent when the bore heats up to 350°.

[190mech]

Gotta 'love' those folks pulling out case studs as they are "diesel mechanics" used to using a 3/4" drive breaker bar for torquing their Cummins heads...

[90GTVert]

These things can be more fragile than they should be, so you don't necessarily have to be a diesel mechanic to strip them. I hate torquing down cylinders because I've pulled the threads out of a few Chinarellis using a torque wrench. I used to go 10-15ft-lbs or 120-180in-lbs. Now I stick with 10-12.5ft-lbs or 120-150in-lbs; usually 144in-lbs. I've done this a lot and still every time I'm thinking "please don't strip, please don't strip, please..." as I torque them.

I don't like the 4Ts either. I haven't spent the time on them that I have with the 2Ts, but I have had cylinder studs snap (they use pretty thin studs that are larger at the threads) while torquing with a torque wrench. Only once IIRC, and of course it had to be when I was working on someone else's scooter, but it was enough that I don't like doing them either.

I don't think that this stuff is the norm. Most people would probably never see a stud or threads fail when using a torque wrench or even a reasonably sized ratchet. I think that Chinese stuff is almost unlike any other platform. Usually you get more comfortable as you do it more. The more I work on and the more I see fail, the more I worry. I was prob more confident tightening fasteners the first time that I ever installed a BBK than I am now, but maybe that's just me.

Maybe the moral of that story is to buy a Yamaha or Honda so you're less likely to have nightmares about that feeling when threads let go.



EDIT : Also, I have to ask... is that paint or did someone go buck wild with copper gasket spray?

[ncsteve]

Yeah, it's copper spray-everywhere! You may also notice a chunk of the crankcase broken out and filled in with epoxy. It gets more interesting all the time!

[ncsteve]

Today I put the engine back together, and it runs! I gradually torqued my way up to 13 Nm and all was well. When I tried for 15 Nm, the repaired stud held fine, but one of the other studs gave me that funny feeling, so I quit there. If you've been working on engines for awhile, you know the feeling I mean. It's in the pit of your stomach and tells you that if you keep tightening, something is going to give way! Anyway, I'm calling the stud repair a success, and if I end up having to do it to another stud, so be it.

[Jwhood]

Yeah the dreaded "event horizon" of head nut torquing I've been there,not knowing if a nano mm more and "SNAP" lol

[ryan_ott]

I’ve never use a torque wrench on these small studs. I just use a 1/4” drive socket and I don’t think I’ve had a failure - knocks on wood! It’s important to make sure the studs are screwed in all the way first!

[Jwhood]

Yeah I don't trust a torq wrench my self no matter how well calibrated it is not saying they don't work well I just get sketched out using one on such a small stud and nut I used 1/4 drive and just re toeq as needed if rather take scroud off multiple times then to deal with headache of Serts and drilling old stud out

[ThaiGyro]

Being the sage mechanical dude, I will offer a few things to juggle around in your heads, but recall or note, if you are so inclined.

First, most scooters and motorbikes do not have aluminum/aluminium cases. They are cast alloys of whodafukknosium. Meaning...magnesium mixed with hmmm, melted hyundais and toyotas and chevies and such.

Now some schooling, if I may. The base part in any design should have thread robustness, equal to or greater than it's fastening hardware. Meaning, that really cheap ass alloy cases should have cheap ass alloy studs and bolts. However, that is not reality in assembly.

Case in Point: A discovery whilst an oil refinery operator was too many leaking valves...plugged valves! Used as vents (top) or drains. (btm). OK...normal carbon steel bull plugs will succumb to operators who use to much torque...galling the threads on both valve and plug.

Now step into my world, where dissimilar metals can often shake hands. In the above study, operators used 304 or higher stainless to cure their leaking, when they should have replaced the valve. Bad leaks lead to nothing good in a refinery. SEEE? the Stainless thread are stronger and more slippery than the carbon steel. The stainless won't gall, but the carbon will, and quickly. Even a few in/out torques can cut a thread to below tolerances.

So...back to the scooters: They come with mild steel studs, bolts and whatever. HARDER than your cases! That is why we have issues that will never go away...unless you treat each threaded contact like foreplay of the best intent.

I will replace case bolts with softer materials if I can afford it. (Remember Mohs scale?) I even use nylon or other plastics when no heat or stress is present. Haha! PPL use titanium fasteners! MX'ers use them, probably road racers...super light, but Rockwell 6+, closer to hardened steel and tungsten. OK...can be done!  Do 'ya think they use OEM torque values?  NOPE!  Less than OEM and Locktite Blue.

The bottom line for cases is extreme preparation and caution. Every threaded hole needs proper foreplay. Ensure it will be happy with your intended ummm, insertion. Clean, verified thread engagement. (Using a proper tap by hand)

We do a bit of case welding...it sucks! Cannot know the base metals. If you chose a repair with JB or other hard epoxy? Good! Do not however think you must fill in the hole...WOW, what a waste! In an average case of fk'ed up threads, you want good threads! Right? I will first use a cotton bot...hmm, ear swab in America, trimmed to be flat-ish on on side. Dip the epoxy goo a little...enough to get a start at the furthest point of threads. Then carefully work it out, turning,  as if just coating the insides. A boroscope helps a bit. Apply, lather, repeat, but be patient and wait for initial curing each time. THEN, retap...by hand. If you catch a bad thread hole by caring...it will last many years.

MInd you...I have used Yamabond in a case hole, but you cannot fill it up!  Fill past the ugly thread and drill pilot, retap.  THe above method is much more effective.

[jackrides]

I learned the hard way that torque wrenches can be quite inaccurate at the low end of their scale.

[GrumpyUnk]

When I re-assembled a 2T liquid cooled, the case threads looked as if they had been dipped in rusty water and left to dry. I used a tap to clean the threads, manually starting, and turning slowly, backing off after partial turns. Threads seemed to clean up nicely. Then used a die on the stud(bolt in this case) threads, and they also cleaned up. When re-assembling, I oiled the threads before running them down and tightening. I did use a torque wrench, 1/4" drive, set to ~12 ft/lb, the low end of the conversion from nm values. I was leery all the way into tightening, but it seemed to tighten up nicely.
I have had some studs on 4Ts that apparently were bent by previous owners/techs. Bent studs make it difficult to assemble, and add various levels of resistance to tightening that makes proper torquing questionable(to me). Those things should(!) just slide into place without the need to lean on the studs in any way. Getting them to fit past the alignment ferrules and fall into place is a PITA when the studs have been 'worked on' by gorilla techs.
I really don't have a lot of faith in the consistency of the metal used in China scooters. Coors vs Budweiser vs Vega and VW blocks. I'd trust a mix made of VW blocks. Less the Vega.(Ha, no one knows of what I speak)
tom

[geoffh]

Congrats on a good cheap repair,but something tom said about oiling a stud before assy reminded me of a problem I remember from 40 years ago,cyl heads on diesel engines kept coming loose after rebuilds,it turned out that by oiling the cyl studs the oil would fill the bottom of the stud hole and gave a false torque reading at the top,mind on that's what I think I remember(no notes were taken in the 80s)

Geoff

[GrumpyUnk]

Just watched a bunch of 'Bus Monkey Mechanic' videos on youtube. When installing the head bolts on a GMC Detroit Diesel, the manual instructs that the threads be coated, filling the thread gap, with 'peanut butter' looking grease. Totally covered. Filling the threads.
When I lubed the studs, I did not pour any lube down the threaded bores, just on the threads of the studs. They were then screwed into place by hand, so I could 'feel' if any sort of hydrolock occurred. They were given enough time(I think...) for any trapped gas or lube to work its way backward up the threads, between the two 'unloaded' thread walls. There is always a gap between the threads, especially when in tension.
tom