I manage a large fleet of mostly Yale forktrucks running the 2.0L Mazda engine. We run the snot out of these things and it's not uncommon to get 30K hours out of a truck before I retire it. I've actually got a couple at 40K hours.
Anyway, the 2.0 has proven to be incredibly reliable, but I've had to get some head work done on a couple of them.
When reinstalling the head the manufacturer's recommendation is to replace the bolts. I don't understand why though. They are not torque to yield type. As long as they pass visual inspection, I see no reason not to reuse.
Last truck we did head work to had over 20K hours and we re-used the bolts. That truck was headed towards retirement, so I wasn't going to spend any more than I needed to spend.
Now I've got a truck with 11K hrs that just had excessive carbon/tar build-up on the valves and in the combustion chamber, so we're going to freshen up the head. That truck is just middle aged in my world, so I don't want to skimp, but I still can't seem to justify new head bolts.
Any thoughts or experience on the subject?
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I turned wrenches for Yale and Hyster dealers most of the 25 years I have been in the business and rebuilt these Mazda engines and did many head gasket jobs as well.
My rule of thumb on these Mazda engines was, I rarely replaced the bolts and I never had a failure of any of them that I worked on. These engines are fairly low compression, low RPM engines with a low torque value on the head bolts.
I always chased the block with a tap and chased the threads on the bolts with a die and ran the bolts through the wire wheel to make sure they were smooth and clean. I used a light grade oil about 1/3 of the way up the threads and torqued them in 3 stages through the torque pattern and used a quality gasket.
And as I stated earlier this is for these 2.0 Mazda engines and not all engines.
Well- at the risk of being the odd man out- here it goes- I've done research on this topic & I've found the following- all bolts have a "stretch" value. The question is- during assembly- does the bolt enter into this "stretch" ?
Older engines used 3/8" or 7/16" head bolts & each cylinder had 5 or 6 bolts surrounding it and the cylinder head was cast iron- the same material as the block so the expansion rate was the same. The torque values on these bolts didn't come close the the "stretch" value. New engines use only 4 bolts around a cylinder and the cylinder head is aluminum. Given the fact that the head bolt now has to deal with 2 new forces- less total clamping force per head and clamping 2 materials that expand differently, they have designed the torque value of head bolts to purposely enter into this 'stretch" zone. The head gasket, cylinder head & head bolts are now part of a matched set designed to expand & contract independently of the engine block.
The new way to torque cylinder heads gives a more uniform torque value throughout the entire head assy, that is it's purpous. It's not an indicator that the torque value is going to enter the "stretch" portion of the bolt.
With all that said- the choice to re-use or re-place head bolts lies in the person who ultimately will have to spend either the time or the money to re-do the job again.
What about doing a re-torque after it has run for a while. Is this a must do?
If I could find a set of aftermarket bolts priced to the point were it wasn't worth the time to mess with cleaning the old ones, I would. These things are expensive though... like $9 a piece.
I can get a set of head bolts for a domestic V8 automotive engine for 30 bucks.
Wonder if head bolts for a Mazda B-series truck would work. Hmm...
I have never relished the idea of arbitrarily replacing head bolts and so I have pretty much the same SOP as you for inspecting the bolts to determine reuse...........or not.
If I have 1 bolt that does not pass muster I replace them all.
I clean the bolt threads with a fine wire wheel or by hand with a wire brush prior to reuse.
I run a tap into the holes in the block and blow out debris with air afterward.
I never reuse any "torque to yield" bolts, but we only have one of those engines in our fleet now.
So it really sounds more like an issue of torque repeat-ability, not torque capacity.
If I found any significant corrosion on the bolts, I'd replace for sure. But they clean up perfect. On the previous install we chased and cleaned the holes in the block and moly lubed threads and under the bolt head. That truck is running like new 1000 hrs later.
Not cleaning up the threads in the block is probably where a lot of people go wrong.
At 65-70 ft-lbs (IIRC), the clamping force is actually pretty low when compared to other engines I've worked on.
Obviously it ultimately becomes a question of risk. In my situation, I'm the end customer and can assume all the risk. I'm just trying to assess it, and right now I see it as being really low.
I have found that replacing head bolts on aluminum headed engines is necessary. Every time i have skimped on this I've been burned.
I think so better spend few money more for new bolts and be 100% sure so all bolts have correct tightening force. When tightening the old are able to drag and next time even though the torque wrench head says something else may be tightened unevenly.
It is my opinion.
Regards
Mariusz
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