Shaved P79 on Flat-Top L28, Piston to Valve clearance?

[Guest mpcapps]

so when you shave the head, you throw off the distance between the crank and cam sprockets. why not compensate this by adjusting the valve timing on the valve sprocket? the cam sprocket fits on the camshaft 3 different ways, allowing less-than-whole-chain-link valve timing adjustments. why not compensate for the shaved head with that kind of adjustment?

Was this already talked about and ruled out and i didn't notice? Or is this a viable alternative to shimming almost everything on a shaved head?

[SleeperZ]

Datsun Dude, if the head is shaved 0.080" and the cam towers are not shimmed too, the chain doesn't have plenty of tension; it has plenty of slack...too much slack to be compensated for by tensioner, guides, etc. Either the towers have to be shimmed 0.080" (which means thicker lash pads or longer valves with shimmed valve springs), or the chain needs to be shortened a specific amount (this isn't a popular approach) and I'm saying that if that amount might equal 1/2 link that it might be possible to save a lot of work to accomplish the same goal. Adjusting/compensating for the incurred cam retard due to shaving the head is good until about 0.020" or so, any more than that and you've got to deal with the excess chain slapping around. DAW

 

The chain tensioner easily has 1/2" of travel, if not more. You are saying it cannot tolerate 0.080" of extra length in the chain? I bet the chain "stretches" that much from old to new.

[JMortensen]

.080 shaved would be .160 extra chain FWIW. SleeperZ, you would think that the tensioner has enough throw to keep everything in line, but the engines I saw with milled heads that did not have shims did just what DAW described. Not sure why it happens, but I've seen it.

 

Jon

[SleeperZ]

.080 shaved would be .160 extra chain FWIW. SleeperZ' date=' you would think that the tensioner has enough throw to keep everything in line, but the engines I saw with milled heads that did not have shims did just what DAW described. Not sure why it happens, but I've seen it.

 

Jon[/quote']

 

You are right, 0.160". Wow, I wouldn't have guessed. Sounds like shimming the cam towers and using the longer valves is the best way to go.

[Guest BigWhyteDude]

ok dumb question from oe who has never build an engine in his life.. yet.. how do you do the play-doe test????

[Guest hf240z]

hi DAw,

did i get it right. i can cut the head(P90) by 0.08 and i dont need to shim the cam tower, and all i need to do is replace the valve with one coming from an N42 or N47 head? what about the spring, will the spring from the same head work?

thanks,

hf240z@yahoo.com

 

There is a way to do a 1/2 link shorten' date=' it involves removal of one roller only; not two, by using a special side plates that are wide at the end attaching to the outside of one end of the chain, and narrow at the end with the roller which attaches to the other end of the chain. You would remove a full link from the chain and install a 1/2 link. I'm not saying that this is sure to work, but I've not seen the idea discussed before.

 

However, the 1/2 link approach may still be too much distance to remove from the chain...I haven't measured to see. The needed chain length change would not equal 0.080" of course, the easiest way would be to check a stock chain on a shaved head with the cam in place but without the tower shims in place.

 

Tensioner modifications don't help because the stock chain is just too long if the towers aren't shimmed.

 

BTW, you don't need to buy new lash pads if you do the 0.080" shave/shim technique as long as you swap in N42/47 valves, their added length is the same as the added thickness of lash pad you'd need if keeping the P79/90 valves. DAW[/quote']