Hydraulic Lifter Clearance Questions

[Richie-Van-GTi 74 2 Cars]

First off, how much clearance or depression should there be with hydraulic lifters at TDC with 0 degree advance on the cam? Ive had mixed responces so need an actual answer. Ive been told between .1mm clearance and .2mm depression. Next question is how far does the lifter press before opening the valve?

really need definate answers ASAP so some head work can commence.

thanks

Richie

[pip470 61]

my immediate thought is there isnt supposed to be a clearance, i thought they were always in contact with the cam when they have oil pressure. Thats the whole point of them they adjust automaticly with engine wear. Have i misunderstood the question?

[Richie-Van-GTi 74 2 Cars]

no you are correct AFAIK there should be zero clearance but I have been told there should be .1 clearance and also that they should actually be depressed by .2mm so its all getting a bit confusing. I know haynes says zero clearance but that doesnt mean theres no depression of the lifter.

Im putting a head in to have the valve seats cut back roughly .5mm so that it takes up the gap between cam and lifter. I would ideally like to go slightly further back and gain and extra .2mm if possible, well more would be nice but I dont know if the hydraulics will allow for this or if I will end up with a closure issue on the valves. At the moment I have about .405mm gap between cam and lifter. I need to obviously zero this gap but if I can go a bit further back I will as I need just a fraction more space between valve and piston to get more advance on the cam.

[DrSarty 90 1 Cars]

my immediate thought is there isnt supposed to be a clearance, i thought they were always in contact with the cam when they have oil pressure. Thats the whole point of them they adjust automaticly with engine wear. Have i misunderstood the question?

 

I agree with PIP470 on this one, although you know I'm no expert. As if there were any clearance at all, it would be a specification you could check and perhaps adjust. There is no spec listed and no adjustment possible, as the hydraulic cam followers do it all for you. They're nice fellows like that.

 

Hope that's released the stranglehold on your work.

 

DrS

[pugrallye 0]

they shoud permantently contact, and each one should have circular patterns on the contact surface, if you have any straight wear marks, there is a problem, but.. you can shim up hydraulic tappets

Edited June 6, 2007 by pugrallye

[Richie-Van-GTi 74 2 Cars]

theres no issues with wear etc, Ijust need a definate answer to the above for the head work.

[Mattsav 1]

They should always be in contact with the cam.

 

Dont run them at maximum extension or you run the risk of them rattling.

Most followers have approx 3mm travel and the manufacturers design the head so they run at mid point. Except for the Cit/Pug 1600 16v engine which run within 0.3mm of full extension (and means the base circle of the cam needs to be spot on or they rattle).

 

Hydraulic followers actually compress approx 0.01mm as they open and then pump up to contact the base circle when the valve is shut. This enables them to alter their height to match the expansion of the valves/head etc and still keep zero clearance.

This is fine unless you get valve float and then the follower expands to try and take up the clearance and ends up holding the valves off the seat.

7500rpm is the generally taken as the max continious rpm for hydraulics although they seem to be OK for very short periods up to 7800. Above this and your into dangerous territory.

 

Personally I'd just fit a cam with the correct base circle and save yourself a lot of bother.

[Richie-Van-GTi 74 2 Cars]

Thats excellent info thanks Matt, youve answered everything I needed. So if Im reading you right I can get it set to 1.5mm despression of the follower at TDC and zero advance. If thats the case then I am a very happy person right now

I guess the other 1.5mm is to allow for theraml expansion etc.

 

I hear what your saying about using a different cam but as Ive now got the cam and the head is ready to have the work done its not really much more hassle, and it isnt costing me whcih is a bonus

[Richie-Van-GTi 74 2 Cars]

Can anyone confirm I have read matts info correctly?

[Mattsav 1]

As long as the follower is compressed at least 0.5mm when its on the base circle of the cam then you shouldn't have any problems.