Head Bolts keeps vibrating loose

[KE4AVB]

This question cover various engines that one of nursery customers has. It is mostly on their 3.5-5 hp engines with aluminum blocks. The head bolts keeps vibrating loose. The spec torque is 140 in lbs and I have tried going to 150 in lbs but that didn't resolve the problem.

Can I use thread locker on these and if so which strength? I am a little hesitate to the high strength version; unless, I will be able remove the bolts latter. Another option I have is to use split lock washers on the bolts if that would work.

Also any other thoughts about this are welcome as I see this is going to be continuing problem with this customer.

[bgsengine]

Check for cooling issues, you may want to observe the engine in operation on location or determine where it is in operation and what it is being used on.

Also, make sure you run a thread chaser down the bolt holes and ensure the bolts are not bottoming out and the right length bolts in use - if you got the wrong bolt in a hole, the unthreaded shank can bottom out before the bolt head hits the head. Some measurements may be in order, determine if the head may have been modified elsewhere (A little machinist's Blue can be a helpful aid to check for contact areas) might need a new head or new bolts - compare head deck depth to the bolt shank length for example, then thread in bolt without head on and see if bolt will go in further than that dimension.

I would not use loctite on head bolts, as heat will simply melt it away anyhow so it won't solve a problem. If the problem is the head deck depth (wear from being loose before) then it may be suitable to use lockwashers which would correct that problem *temporarily* , as long as the washers are a good grade (they have their own issues when it comes to heating and cooling cycles, which is why they don't use lockwashers on heads, if you ever noticed..) or you can maybe find some hardened grade 8 flat washers (thick, slightly belled)

[KE4AVB]

Check for cooling issues, you may want to observe the engine in operation on location or determine where it is in operation and what it is being used on.

Most likely cooling is the main problem but having to this every week is getting a little frustrating.

They are being use in open nursery fields to clear between the rows and around the plants. Anything out there can be from rocks to shrub stumps. I have cleaned from under the recoil starter numerous times which may be main source of the problem. The bolts are the original ones. I post a picture of what have found under the covers in the past which is the reason why they burn up one unit. That is nothing but grass and seeds; not a mouse bed.

[wristpin]

Long shot, but are you using new, genuine head gaskets?

[Skywatcher]

Hi KE4

Yikes! that's a wad of grass. Here's where a little customer education may be in order. Teach whomever is in charge of maintenance and whomever is in charge how to clean out under the cowl and instruct them to do this on a weekly or monthly basis depending on how quickly the debris accumulates. The use of compressed air to reverse flow and clean out debris from around exhaust valve stem is also a good idea. Don't pressure wash unless the engine is actually oily, you'll only create more problems than you solve. If the engine is oily then dirt is going to stick to the oil so the oil must be pressure washed off and the source of the oil leak be addressed to prevent a repeat.

As for the head bolts, my standard practice as BG says is to run a plug tap bottoming tap down the bolt holes and clean the bolt threads on a wire wheel. If 3 bolts are longer than the rest, make sure these 3 go in the corner around the exhaust valve. I too have had head bolts come loose in the past so my procedure now is to apply anti seize compound to the lower portion of the threads (not the shank or head) then tighten bolts in sequence to 15 ft-lb in 5 ft-lb increments. Haven't had a head bolt come loose in over 9 years. Hope this gives you something to work with. All the best,

Sky

[bgsengine]

Check for cooling issues, you may want to observe the engine in operation on location or determine where it is in operation and what it is being used on.

Most likely cooling is the main problem but having to this every week is getting a little frustrating.

They are being use in open nursery fields to clear between the rows and around the plants. Anything out there can be from rocks to shrub stumps. I have cleaned from under the recoil starter numerous times which may be main source of the problem. The bolts are the original ones. I post a picture of what have found under the covers in the past which is the reason why they burn up one unit. That is nothing but grass and seeds; not a mouse bed.

If they are like the one in the picture, THOSE engines are *NOTORIOUS* for head bolts getting loose and blown head gaskets especially around the exhaust valve area - Important to use OEM head gasket there. Those 3 longer bolts, as Skywatcher points out, go around the exhaust valve corner

Back when these were most common lawnmower engines, we kept minimum stock of 10 head gaskets at all times - There were some weeks where I'd do those head gaskets as many as 6 a day. Even when there was no cooling issue, the exhaust corner is a hot spot for them and until Briggs started using the graphite layer gaskets, the metal clad gaskets we constantly blowing out and bolts getting loose

Torque spec - While the spec is 140 In-Lb, we had just done them up to 200 In-Lb dry (no lubrication on threads or bolt heads) and never had a comeback... It's been much too long ago but I do recall a Briggs school (Back in the day when everything was paper and service schools were held everywhere within an easy 1 hour drive max... Computers were a $5,000 investment and the internet was still a figment of Al Gore's Imagination.) that a presenter advised a 200 In-Lbs torque spec on 5/16" head bolts and we'd done it that way ever since with no issues.

With OHV Engines we still follow the book specs, but the L-heads, we'd always seen head gasket problems that were fixed for keeps with the 200 In-Lb spec.. And then again, *KEEP IN MIND* that was *before* the graphite layer gaskets started being used... So, I tend to lean towards Sky's 15 Ft Lb spec with new style gaskets. (Cant remember the last time we ever had to replace an L-head gasket in the last 5 years.. not too common anymore.. so I can't think of a time we have deviated from Book specs in all that time, now.. )

[KE4AVB]

Tnx, I will increase the torque to 15 ft lbs (180 in lbs) and give that try when he brings it back. They have already done knock the muffler off today. I don't get hardly muffler replacements here except on rebuilds so I got to order one in.

As for the bottom tap I will need pick one up next time in town. I don't have that one in toolbox yet. I just got a 1/4 a week ago to redo some muffler bolt threads. Probably better pick-up one those screw in muffler too. I carry an old 3.5 hp block with me for that.