| Author | Message | ||
     Bob uk Unregistered guest Posted From: 94.197.122.74 |
This posting is for the amateurs who are new to cylinder heads. Skimming. Only ever skim a head if it is warped. It is not standard practice to skim a head just because it's off the engine. On vee engines this may cause misalignment of the inlet manifold bolt holes, and the ports. On non vee engines no such problem. If a head is skimmed there is a risk of breaking through into the water jacket. On a head with cam bearings (ohc) if the head is warped then the brgs will be out of line and the cam could snap. Hydraulic lifters can end up trying to work with too longer push rod. Rocker shafts don't rotate so will accept a bit of misalignment with no problems. Valves before removal I like to put kerosene in the ports to check for leaks. This gives a good clue as to the state of the seats. If a finger is placed over the valve guide hole and the valve pulled out a good valve guide and valve stem will have vacuum and pop when fully pulled out ( no valve seal fitted or oil) Valve guides if worn should be changed which is not really a diy job. Because the seats will have to be reground. Diy valve seat cutters will chatter on the seat and there is only so much metal and regrinding after can cause pocketed valves and wrong rocker angles. Combined with a head skim and a big problem may arise needing shorter push rods. If the seat is a bit tatty then using emery cloth between the valve and seat will save lapping. To clean valves place in chuck and polish the head with emery cloth. Do not linger on the stem just a two second polish. Using a flat file for support wrap emery cloth around the file and touch the seat up. If the valve does not clean up then get them refaced by a shop. Lap the valves in until a thin grey unbroken line is apparent in the middle of seat. Do not think that by lapping more that the end result is better, it won't be. If the grey line is not roughly in the middle then put the valve back in chuck and touch up with a bit of bias to slightly alter the angle. Then lap again. Once correct that valve is matched to that hole and must not be mixed up. If done properly the lapping should take 30 seconds a valve. Rocker shafts wear on the underside. Some rockers have no brgs And are usually aluminum alloy. Often it is found that the rocker brg is unworn and a good fit on the New shaft. But the old shaft has forced fine particles of steel in to the brg ready to rip up the new shaft. Rocker pads. These are where the rocker pushes the valve. They wear. If it's a feeler gauge adjustment then getting the clearance right is difficult. It is ok to stone these to tidy them up. Anything more than 3 thou is a bit naughty. Rocker angle. Some are case hardened and soft inside. The springs between the rockers are not critical and seldom need replacing. Polishing ports makes more power because of metal removed by polishing not because it's shinny. To remove carbon either scrap off or try aluminum wheel cleaner to soften and then scrap of. Do not dig divits in the head. If using media blasting ( turn the telly volume down) Remember to check oil ways for media. Lotus twin cams have been wrecked by media trapped in a corner oil way. In the good old days when engines had wider tolerances mechanics had to keep valves and push rods in order to fit them back in the original place. Modern engines such as the RR V8 are made much more precise. So mixing all valves and pushrods up makes no difference. The traditional thinking is that a push rod cup will wear to match the ball and so on. If the wear is that much then the part is worn. Once the valves have been lapped then the order must be kept. By all means push push rods through cardboard but if they get mixed up then it's no worry. Note on some engines with bucket and shim often a valve lap will close up loose clearances to the correct figure so keep those in order example Jag XK engines. Valve springs are checked by free length and or by clamping lightly in a vice with a new spring when the vice is tightened the old spring if u/s will not be as strong and be as long. Springs end to end in vice. Terrys springs are part of AE and probably the oem for RR. Always check all screw threads before refitting. Now's the time to helicoil not when the heads fitted to the car. For the rest read the Work shop manual. (Message approved by david_gore) | ||
| Jeff Young Prolific User Username: jeyjey Post Number: 199 Registered: 10-2010 |
Hi Bob, While I'm not new to this stuff, I do often find a gem or two in your posts. So keep them coming. Old habits die hard. I don't think I could handle allowing the valvetrain parts to get mixed up. ;) Cheers, Jeff. | ||
| Bob uk Unregistered guest Posted From: 94.197.122.72 |
I know, what you mean Jeff. But by the time a few new bits have been added it starts to seem pointless. I have fitted bits from another engine which makes complete nonsense of the idea. (Message approved by david_gore) |
