1988 Force 125 stripped head bolt

[reelmasta]

So diagnosed my dying engine @ idle with the help of the furum.... Exhaust leaking and recirculating from a head bolt that came loose and got blown out.
The bolt looked like it was either cross threaded or had something get tight last time the head went on and now the block needs to be helicoiled to accept the bolt.
Compression is 135 in all 4 cylinders and ran great last time out. It's mounted on my 19' trophy and got up to 30 MPH with a full load of fuel onboard (45 gals or so).
I don't really understand how exhaust is blowing out of this hole but it is. I was hoping (with fingers crossed) that pulling the head, doing the helicoil repair and replacing the head gasket will take care of the problem.
I'm a jet engine mechanic by trade so not too scared to try this by myself....
Other than the torque sequence and retorque after running it seems to be a straight forward job... 18 bolts, make that 17 off and 18 back on... lol. Aluminum surfaces so careful prep for the install. No sealant I believe except maybe a copper spray.... thoughts?

Any words of wisdom or advice would be greatly appreciated?

 

[Frank Acampora]

Re: 1988 Force 125 stripped head bolt

Don't bother with the heli-coil. Simply retap to 3/8-16. No need to drill because a stripped 5/16 thread is the right size to retap. No need to remove the head either but you will need to carefully drill it to 3/8. Put a stop on the drill bit so you don't go into the threads.

If you do remove the head, torque to 225 inch pounds in a spiral pattern. The 3/8 can take a bit more but leaving them all the same can't hurt.

NO COPPER of any sort. Sets up a galvanic reaction with the aluminum. Nickel based anti-seize if you use anything.

 

[reelmasta]

Re: 1988 Force 125 stripped head bolt

Thanks for the quick response.
So do think it's better to just tap or take the time and helicoil?
Also.. why would exhaust be leaking into any of the head bolts?

 

[Frank Acampora]

Re: 1988 Force 125 stripped head bolt

It sounds like it is a low bolt. I forget which ones exactly because it is not important to me but: One or two of them penetrate into the exhaust chest. Thus if the bolt is missing, pressure inside the exhaust chest forces gas out through the hole.

I ended stripping about three head bolts on my 105 so I simply re-tapped all of them to 3/8 - 16 and drilled the head to clear them. Heli-coils will never strip and if you can get them for free, they are a good alternative to re-tapping BUT: There is not a lot of meat in the casting walls and heli-coils do take up more room than re-tapping.

If you do aircraft, you know that 50% thread depth will yield about 85-90% ultimate strength and since the head bolts are torqued to standard values, there is no need for heli-coils. Thus, re-tapping from 5/16 to 3/8 will be more than sufficient.

 

[reelmasta]

Re: 1988 Force 125 stripped head bolt

TY Frank...
Yes your dead on, the bolt is one of the lower ones. One of my problems is the original bolt is exactly the same length as my 3/8-16 tap (approx 2 1/4"). So looks like the head will have to come off. Plus it being the lower bolt it is inside the cowling and I would need a 90 degree drill or drill a hole in the cowling for access. Even if I use the 90 degree drill I still don't have enough throw on the tap. Maybe I can find a deeper tap? I think I'll pull the head, open it up to 3/8 and then tap the block to 3/8-16 like you suggested.
I ordered the head gasket, is there anything else to do while I'm in there, or any other parts I might need except the head gasket, new bolt and washer?