My take on cylinder head resurfacing in the mill is to use a single point tool - a flycutter. Multitooth cutters can speed things up, but if one cutter edge gets chipped you´re in trouble. Flycutters take longer time to use since only one cutting edge is doing all the work, but if you have a mishap you only waste one insert. And swapping from cast iron inserts to aluminium inserts is quick, you change the one insert and go to town.
Now, I seldom do heads more than a few times a month. If all I did was cylinder heads you can be sure I had a big multitooth cutter or even a cylinder head resurfacer, but since I don´t I haven´t. Thats why I use what I already have - a vertical mill and a flycutter.
If the head is warped, which it probably is if you intend to resurface it, you have (at least) three options; pull it flat against the table, shim the blocks to fit the twisted head or straighten it before resurfacing. The easiest of course is to just tighten it down and start cutting. If there is only slight twist this may be the best approach. Shimming might be necessary if the head is too warped to pull flat. But if the head is so bowed or twisted that the cam bearings are out of line, maybe even causing the cam(s) to bind, it really should be straightened first. Or junked.
Straightening typically involves twisting or bowing the head the opposite way it's already warped, and then baking it in heat to let it "take a set" and end up straight. The bearing bores, that is.
While this sounds easy in theory it may be tricky in reality. After the head is straight 'nuff, the gasket surface can be resurfaced.
If the cam bearings are out of line, perhaps you could also resurface the head as is and then grind some off the bearing caps making the bore smaller and then hone them out again. Obviously this only works if there are removeable bearing caps. The end goal is to have the cam(s) sit straight and parallel to the gasket surface.
For a typical 4-6 cylinder head, I usually place the head on ground blocks, tighten it down just a little and feel if any blocks are loose - you want the head bearing on all corners. Watch out for burrs, dents, grit and gasket remains. After any needed shimming I continue to gradually tighten the hold downs until its firm and secure. I then touch off, snug the table locks (don´t lock the feed axis) and take a skim cut of 0,001" or so. This lets me see if any corner or side sits high or low. Add shims as necessary to average out the differences. Newsprint works well for small adjustments. When you are satisfied, dial in and take a cut. I typically run 269 rpm, 0,004" depth or so and 6-7 ipm while roughing. Adjust the cutting depth according to how unflat the head is. When it's almost flat I take another 0,0015" or so at 2 ipm to finish it off, hopefully cleaning up all areas.
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| Rover head starting to clean up - iPhone 4s |
Note in the above picture how the o-ring grooves are starting to disappear at the ends, but are still there in the middle - this was a banana for sure, and the grooves seem to be cut crooked aswell.
Now, the above procedure assumes the previous gasket surface is factory original or at least resurfaced parallel with the valve cover surface. This is important as you are averaging the cut over all four corners. If the head is banana shaped you want to hit the ends equally. If it is twisted you want to hit diagonal corners equally. If it is both banana and corkscrew, well.... you get the idea. But if the head has been cut wedge shaped by the guy before you, and you also cut more on one end than the other, you´ll have a head that sits tilted when its done. Perhaps not by much, but still.
Of course you could touch off the skim cut, find the lowest point, dial in one roughing cut to remove all warp and then take one finishing cut. But fiddling around finding the low point may take just as long as just cutting away until things start to look flat. I tend to engage a cut and then do something else while the mill hums along.
How do you measure the removed amount? I like to touch off, set zero, cut until its clean and then see how much was removed from touch-off. You could also mark a spot that you measure with a micrometer before and after cutting and see how much you removed. If you measure the middle of a concave head you could end up with 0,001" although you removed say 0,01" at the ends. I feel this is misleading and thats why I prefer to measure from touch-off. That gets you the max that was removed.
If you don´t have readouts you could set up a dial indicator on a magnetic base as a poor man´s readout.
Could I get your opinion on a job I am doing? I have a volvo 5 cyl that overheated. With patience and sometimes ether, it could be started and it would run pretty even and idle after it burned out some coolant in the middle cylinders. After disassembling, I found the head is warped about .018". The cams will turn by hand if the tappet/lifters are removed. They turn without much resistance. An old experienced mechanic tells me to flatten the bottom of the head and don't worry about straigtening it. He also says that if you only take some of the warp out when resurfacing (say .013", it will help with the cam/top warp because when the head is bolted onto the block, it will flatten out some. I'm seriously considering doing this since it would run beforehand and I am not building a race engine...its just a 10yr old car used for commuting.
ReplyDeleteHi Alex!
DeleteWhile I may not be an authority on the subject, this is how it sounds to me: If the cams turn without resistance that to me says the top is not warped enough to disturb the cam bearings alignment. You could try bolting the head to the mill using parallells, one block in each corner. Thats how I mount the Volvo heads, and most other, for resurfacing. When you have it tightened to the mill table, try turning the cams again. If they move freely I'd say the head is straight enough. If you know which corner is warped otherwise you could try to put some shims to counteract that, you could use the blades from a cheap feeler gauge or common household aluminium foil, and try torquing it into straightness against the mill table. Try moving each parallell after tightening the hold downs to make sure the head is secure. Then mill the head until you have a new clean surface. I try touching off the cutter on both ends. You simply put the mill in neutral by setting the gearbox between two gears, and move the cutter by hand while slowly cranking up the table. When it touches on one end, swing the cutter past the head (if you use a fly cutter as I do) and move the table so you get to the other end. Then try touching the cutter against the head on this other end. Be careful so you don’t put a deep gouge in the other end because sometimes the head sits higher on one end, and if you aren’t aware of that its easy to cut a deep grove when touching off on the high end. Yep, I’ve done that. You may have to go back and forth a bit and shim and retighten the head to get an even cut on both ends and also side to side. When you are happy about the alignment of the head on the mill, take light cuts until you clean up the entire head, then finish with a light cut with fine feed so you get a nice gasket surface. You could also touch off at the ends and measure the gap in the center between the cutter and head to know how much you have to remove. Then take almost that leaving some for the finish cut to clean up.
I personally agree with the mechanic that you probably can ignore the straightness/warp issue altogether. But I also think that you should cut a new nice surface over the entire head so you get a good seal. The head will flatten some when you bolt it back on but its hard to know how much, so I feel that if you have it on the mill I would go ahead and cut the whole surface of the head.
Also, make sure the head is well trammed on the mill. I try to tilt it just enough so the trailing end of the cutter doesn’t engage in the cut. It would engage a little when cutting with faster feed since the grooves left have higher peaks left between them, and the cutter would lop the tips off these peaks, but when taking the finish cut with fine feed the cutter would clear at the trailing end. If you tilt the head too much you get a concave surface and that could give you poor gasket sealing between the cylinders. Try to cut as flat as possible.
Hope this helps!
Kind regards, J
Thank you for your advice. I wasn't really looking for milling info though. I repair engines. So, most machiners are telling me to straighten this head and then resurface it. I did some experiments tonight. I heated the head and bolted to the block with some spacers/shims at the ends under it and tightened down in the middle to flatten it back out. Amazingly, this brought the top where the cam journals are back true and flat while I measured it bolted down. However, after letting it cool, I removed the head and measured that it returned to its warped state. I wonder the consequences of bolting a heated, warped head like this permanently? Would it not need planed on the mating surface at all? Would it be flat on the bottom if the top cam journals are flat again? My 2 cents would tell me that a warped head could be tightened back to flat as you mentioned especially if it is heated. I just don't know if this could lead to problems. Thanks so much for your reply
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