Hello everyone. Right the story goes like this. I recently ported a head from a digifant using the guide on here, and today i changed the head over from the standard one that was on there, and had compression as follows: 1: 120psi 2: 90psi 3: 40psi 4: 55psi So i bought another gasket and bolt set thinking the gasket hadnt sealed properly and the compression had got slightly worse. The confusing thing is that the car did run for afew second on first turn of the key, then cut out and wouldnt start, hence the compression test. Now my wonder is, could the valves not be seated properly and cause the loss in compresion? Or is it more likely theres some thing else wrong with the head that ive missed. It has been skimmed by a machine shop who are reputable so i doubt its unlevel. Any help greatly appreciated!
Did you lap the valves in with grinding paste? Is this a shim or hydro tappet head? It could be the valve clearances need adjusting if the seats were cut.
aye you have to watch there is a size you must mentain with hydro tappet head cam and bucket is dan i hintting must have the clearence set
Its a hydro head, and i used grinding paste to lap the valves in. Spent about 10 minutes with coarse and same again with fine on each valve.
Im going to take the head back off tomorrow and give it the duck oil test on the valves, see if they have sealed or not.
Yep, cam lobes pointing upwards on the compression phase for a particular cylinder being test, then duck oil or diesel filling the ports is a good test to check for any leakage. Did you disturb or move the guides at all?
Ahh, that may well be the issue then. Were the valve seats re-cut? If not then the valves will not be in exactly the same position on their seats as before, it only takes a tiny amount to alter their position. I am not a fan of disturbing the guides if at all possible as it ramps up the cost substantially. Fair enough if pushing the limits on a competition engine build, but for moderate road use where costs are being kept at a low level then it is borderline worthwhile IMO.
Ahh right i see, im off to get a corrado tomorrow now as a daily to keep me going while i sort this golf out.
As danster has said this definitely sounds like a potential cause to me, whenever I have replaced guides I have always had to re-cut the seat. No amount of valve lapping would get you where you need to be.....well that's not true it would but it'd take a damn long time
I would budget for 10-15 / seat ....at a rough estimate....which is cheaper than buying a set of Neway seat cutters, roughly 400-500 for a set to do three angle seats on inlet and exhaust seats!
40 for the seats...head & valves, 40 to back cut the valves. Don't underestimate the importance of the valve/valve seat seal/shape for performance...get it wrong and the flow goes to rat sh%t.
If the seat cutting is spot on you wont need to lap them after...though i normally spin the valve a couple of times to see the grey seat.
Did you see a clean grey ring round the valve seat and back of the valve when you'd lapped them? If you've got the head off, pour petrol / diesel / duck oil down the ports to check if it's sealing, or even shine a torch down the ports and look for light leakage round the valve.
Yeah had a nice smooth grey ring on the seats and valves, that's what's confusing me about it. We'll see tomorrow eh.
Well i took the head off today and look what i found Would certainly explain the loss in compression eh. Bye bye Piper cam
......ouch that's seen better days! My concern would now be that you may have more damage as a result....bent valves, damaged pistons as clearly half of it would not have been turning but crank would have been! [:^(]
