Head bolt stripped

[John Seybold]

Hi,

I have a K&B 7.5 outboard that has a stripped out head bolt and was wondering what’s the best way to fix this? Thanks for any help.

 

[Steve White]

Heli-Coil or equivalent thread insert. Easy peasy.

 

[Wasted wages]

Oooorrrrr..... if you can sneak a nylock nut under the fins of the block, you can use a longer bolt and tighten it that way.. may have to do some grinding with a Dremel to get the nut in place.

May not be the best repair, but will work until you get the proper heli coil and tap to make a permanent repair..

 

[Rod Fonda]

I'm looking at an early 7.5 case,Pretty hard to fix or helicoil if its an early 7,5 as the head bolt holes are very close to the liner wall,,about .010"-.015",even the liner usually shows 6 wear marks where the bolt threads expand under tension and heat,the later square fin case/red head has more room,,As Frank mentioned,,maybe a stainless nut ground down flat very close to the threads and some dremeling,,but it looks almost impossible,,if its the bolt hole in the exhaust port,there's .100" more meat you can drill deeper and tap

 

[Wasted wages]

if it's metric sometimes you can just thread in the next size up SAE tap and use a different bolt.... or if it's SAE, try the next size up metric tap...and bolt..

 

[Rod Fonda]

looks like 4-40 unc from memory,,around .110" dia,,about 2.7mm using my vernier, so maybe 3 mm bolt might just do it,,but you'd have to be carefull when tightening as its a finer thread ,,glad I have a few cases lying around,,I usually use that ARP bolt grease on the bolts and carefully remove burrs when I cut down allen head bolts to suit,,I usually cut more threads in Kand Bs and use longer bolts than stock

 

[Mike Hughes]

Fill with JB Weld and re tap it. You c an also go to a 5-40 bolt.
Mikr

 

[Charles Perdue]

One other option, if you have enough material for the screw boss, is to drill the hole deeper in the case and then tap it for more threads using a longer head bolt.

I also found when doing this that using deeper threads and longer bolts on all head bolts, spread the load of the head bolt in the case over a longer distance and did not distort the case and sleeve, as much up high, with the same clamping force, where the engine makes its most compression.

Just one of the small things that can be done to possibly help the engine run better.

Charles

 

[dwilfong]

4-40 Helicoil bin there done that........still have the kit in my tool box. Works like a charm on a K&B 7.5

 

[John Seybold]

Thanks everyone,it is an early case and it’s the top bolt so no extra meat on the case.

Maybe Helicoil or jb weld or tap next size up from 4-40

 

[Mark Anderson]

On those cases, I have used helicoil and 5-40 bolts. I preferred the helicoil.

 

[John Seybold]

I see two different types of Helicoils? Which one have you guys used? Any special tool need to insert them?

 

[Wasted wages]

i think they sell a kit that has everything in it..
drill bit,tap, insert tool, and coils.
Or you can buy the coils by themselves.

the insert tool is just a little plastic thing to wind the insert in the new threads.

 

[Steve White]

It will look just like this: Amazon.com: AB Tools M14 x 1.5mm Thread Repair kit/helicoil 9pc Set Damaged Thread 15pc AN025: Automotive

Only for 5-40 thread size. You may need to go to an industrial supply site to find 5-40, they are out there - I have them in my shop.

Bit, Tap, Z handle insertion tool and the straight tool to break off the tang on the coil once it's screwed into place.

On my engine builds I purposely drill out the cases and heli-coil everything as a normal part of the build. Art Hammand advised me on that years ago when dealing with exhaust throttles falling off.

Consider drilling out all the head bolt threads and heli-coil in them - that will ensure even torque on the head and prevent any further issues.

 

[Mark Anderson]

If going 5-40 no helicoil needed.

 

[Rod Fonda]

After you repair it,I would also file a small flat on the liner OD, corresponding to the bolt hole area around TDC ,as I have often seen 6 polish/witness marks on the liner bore at TDC,due to the bolts being so close to the thin liner,just with the stock bolts,,I actually file 6 small flats on the liner, and have noticed less liner distortion
Gees,,you guys stripping threads must be heavy handed,,Kand Bs are delicate

 

[Charles Perdue]

Back in the day, some of the "Big Boys" would machine a ring to fit where the liner lip was, torque it down and then lap the case bore just slightly to remove the bulges that were caused by the head bolts.

Yes, I know that the same process was used on many good running automotive engines also....

Charles

 

[Rod Fonda]

Back in the day, some of the "Big Boys" would machine a ring to fit where the liner lip was, torque it down and then lap the case bore just slightly to remove the bulges that were caused by the head bolts.

Yes, I know that the same process was used on many good running automotive engines also....

Charles

Yeah,,interesting Charles,,I never thought about that years ago when I had access to lathes ,until I read about using torque plates years later,,I suppose the better engines have a better bolt placement and thicker liners, which helps with the issue and the Kand B 7.5 basically being a .40 originally,then bored out to a .45 ,with a thin liner and reduced transfer passages,,I can see how it made a good .40 pylon engine as Bill W designed it.
I made sure ,the machine shop used one on my small block car engine a while back,,It's surprising how stud/bolt tension distorts even cast iron blocks,especially some blocks with thin cyl walls and/or core shift issues

 

[scottnewkirk]

I always drill and tap K&B holes deeper and use at least 1/8" longer screws. As you already know the cases are a tad soft

 

[Charles Perdue]

Rod and Scott isn't it amazing how great minds think alike.....