Any math majors here? OHC head milled.....

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pishta

pishta

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Got a question about an OHC motor that has had the head milled. If the cam sprocket is 109mm in diameter (54.5 radius) and the cam moved 2mm closer to the crank driven by a chain, how many degrees has the cam rotated by being 2mm closer to the crank? It is going to 'roll' down the drive (R) side of the chain CCW and retard the cam timing some degrees. I have an idea how to figure this but I may be dead wrong: 342mm circumference / 360 degrees would give me .95 degrees per mm and if you dropped that sprocket 2mm, would that be 2x.95 or ~1.90 degrees retarded..minus the rotation angle? I would have to cut a new keyway only if this is accurate.
 
thats why thay make off set keys and shorter than standerd chains,..same as if ya line bore a block and move crank center line!!
 
This isnt a performance motor so no aftermarket support. A shorter chain would solve it for sure. Offset key would work too, but its an inch long, they make em in metric? Mazda motor.
 
What type tensioner does it have?
 
Was it milled to clean up the gasket surface? If so, then just get a cylinder head shim to make up the difference.
 
idk if thay make them for your app but cant be to hard to make ether.... now if ya put it in bassackwards itll be advanced insteda retarded or visa versa!
 
If a shim is not available, I would just put it back together and run it. I would think the tensioner would get rid of most, if not all, of the slack.
 
The slack is not the issue as its on the trailing side, the tension side does not change. Shims are available but they are only .020". The mill limit on these heads is only .20mm! This machine shop took 1.7mm off this head...Great for a compression boost, not so great if it barely passed NOx smog once before with the uncut head. IIRC the stock compression ratio is 8.6:1, not that high to begin with.
 
Then stack some shims together.
 
Your math is correct, the cam will be retarded by about 2 degrees (1.62 for a 1.7mm cut). Typically there are adjustable cam sprockets available for the engines that have a "performance" following. Not sure which Mazda engine you have, but I know they are available for the engines used in the MX5/Miata.

A shorter chain/belt won't fix the problem here, unless the tooth pitch is reduced on the chain and sprockets. Yes, the chain is about 4mm too long now, but the tensioner should be able to compensate. Cam timing is controlled by the tension side, the tensioner is on the slack side to reduce whip (noise).

Why on earth did they take off that much?!? If the head was that warped, it is most likely junk now. With most modern OHC heads, Japanese, German etc..., the cam bearings are part of the head casting. Wear isn't a big deal usually, but head warpage is. If the head deck surface warped significantly, so did the cam bore centerline. If the deck surface is corrected, the cam bores might not line back up once the head is torqued back down. Knew a guy a few years back who kept breaking cams in his Honda. Misaligned cam bores due to a previous overheat was the problem. Might not be an issue in your case, but food for thought
 
excellent info! I guess I found the chord measurement but close enough. The head was "cleaned up' in his words to take care of some surface pitting. the PO must have run straight water as there were some pretty good corrosion pits that the guy had to take 1.7mm off to 'clean up'. I was new to OHC motors back then when I got it resurfaced. I knew this guy from a few telephone jobs I did in his shop so he was doing this for peanuts. IIRC I only paid $80 to get the head rebuilt: tanked, 8 new guides and seals, all the valves cleaned (2 exhaust were replaced!) seats stoned and decked. Probably used it to teach someone how to do a head as there was no warpage issues prior to this, it was just having smog related issues and I was hoping it was a valve sealing issue. Teh shims are just plate AL (or steel) that look like a head gasket. They set on top of block with only Indian head shellac and then the head gasket goes on top. Who would have thought thy would seal like this? One guy on the Ford Courier forum said he cut his head .030 and runs 2 head gaskets and it is still holding after 7 years! :eek:
 
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If it's failing due to NOX does it have an EGR system? If so, have you checked the EGR passages, valve & controls?

Lotta modern engines run virtually emission free & have compression ratios above 10-1.. Just gotta get the emission control systems right..
 
yes, the EGR was functional, sort of. I made a bunch of adjustments on it as it was opening on idle (?) and F'n up the curb idle that had to be changed to get it to run right, then I found the EGR had an adjustment on it that could tighten up the pintle delaying its opening so I had to go back and fix the curb idle again once the EGR would stay shut. It was a lesson in early 80's carbed mechanical smog devices that walked the tightrope of passing the ever tightening smog regulations. Then EFI came and thankfully retired most of this mechanical vacuum timed crap.
 
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