No Compression After Head Change

[dartman 4 1 Cars]

Changed to a longman head and pt28 Kent cam but after reassembly there is no compression in any of the cylinders. I'm no a mechanic and am a relative novice at engine building.

 

The one thing that crossed my mind was if the cam was fully allowing the valves to close. The head has had a huge skim and the valve trian is standard except for double valve springs, all the shims etc are standard, this has made me think that the valves may not be compleatly closed causingbthevair leak and therefore lack of compression. The cam said that it should have valve clearances of 0.25 for inlet and 0.30mm for exhaust, is this the gap between the cam followers and the cam lobe? If so I do not have a gap there. I then changed back to original cam and there still wasn't a gap. If anyone understands what I'm saying or can help please do!

 

Many thanks.

[welshpug 1,655]

you need to set the valve clearances by the sound of things, got a haynes workshop manual? its very well documented in there.

[dartman 4 1 Cars]

Thanks for the reply Yes I do have a manual. Did what I say make any sense? Would that sound like a reason for there to be no compression? Is it adjusted by shims?

Edited January 31, 2015 by dartman

[welshpug 1,655]

it is a plausible explanation especially if there's zero clearance with the cam lobe pointing upwards.

 

clearances are indeed altered by different thickness shims.

[Miles 331 1 Cars]

I guess the cam and head are from different places? If so they need adjusting, the right springs for the cam and then piston to valve clearance checked, No idea on the spec of 8v cams as it's been a long long time since I did a spec'ed one

[dartman 4 1 Cars]

They both were bought from the same guy ( very experienced and trusted) the head is a genuine longman and the cam is a pt28 Kent cam, the valve train is standard with standard shims etc but the correct double valve springs for the cam, should I buy the Kent cam followers as recommended by there website as well? I am new to head building and to be honest am a bit over my head...

[Miles 331 1 Cars]

a follower is a follower so no need for that, Shimming can be done in a few ways, but something I always get my engineering shop to do as it's more cost effective and a s**t job

[dartman 4 1 Cars]

The cap that sits in between the follower and the top of the spring (looks like a watch battery) is that ok to remove, the reason I ask is because it has one standard shim which sits between the spring and the head and if I remove that it still wouldn't be enough to get my valve clearance, the way I could do it would be removing the spring cap ( watch battery) and hen pack out the difference with the correct shims, on a slightly different note, if the cam wasn't timed up correctly would that cause no compression, the cam is in and it turns over without hitting pistons but I haven't fine tuned it yet, wondered if it would be out enough to cause no compression. Sorry for all the questions, I'm heading up the workshop now so will read rough the Haynes regarding valve clearance.

[welshpug 1,655]

you dont touch the springs or caps, just lift the tappet and change the shim.

[dartman 4 1 Cars]

You've lost me with tappets? The shims that are on it sit below the springs against the head, but there is only one there and it's very thin, if I remove it, it wouldn't lower enough to get the clearance to the cam, i need to go and have a look now because I'm not 100% if what I'm saying is making sense...

[petert 585]

If the head has been reconditioned, the valve(s) will be sitting higher in the head, reducing the clearance between the top of the bucket and the cam lobe. This clearance needs to corrected by fitting a thinner shim(s). ie the discs approx. 14mm in diameter and 2-3mm thick which sits on top of the valve. What you're calling a watch battery.

 

I normally aim for 0.009-0.010" clearance for the inlet and 0.012-0.013" for the exhaust, AFTER the head has been torqued down. I stress this point, because the clearances close up by approx. 0.0015". Thus if you shimmed up the head on the bench with 0.008" on the inlet, it would finish up with 0.0065" when torqued down. So long as you allow for this, it's not a problem. In any case, this wouldn't be your direct problem, as you have less than zero clearance by the sound of it.

 

Your only course of action is to remove the cam and buckets and fit shims approx. 0.010" thinner than what you have to begin with. This should at least give you something to measure.

Edited February 1, 2015 by petert

[mikeyd 13]

sounds like you need to take it to a machine shop for shimming before it all goes badly wrong ---- done properly - done once !! if you dont know how its done get a professional or wreck the top end!!

[dartman 4 1 Cars]

Taken out the original shim (3mm) and put the caps back on to measure the distance, they were all 0.30mm which is recommended for the exhaust but can you buy shims of 0.05m? Seems bloody small to me however with the clearance at 0.3mm there is compression in the engine!! So big step forward. I say again with no shims watsover the gap is 0.30m so a shim of 0.05 for the inlet valves should do it.

[welshpug 1,655]

the shim sits on the top of the valve not under the spring.

[dartman 4 1 Cars]

Yer sorry I was confusing myself, like I said above though, I have no shims in now and all the clearances are at 0.30mm means it's had a hell of a skim on the head...

[Tom Fenton 1,541]

Head skim makes no difference to valve clearances.

I would recommend you get an engine machine shop to reshim the valve clearances for you.

[dartman 4 1 Cars]

Sorry yer the skim won't make a difference, the thing is there nothing to reshim? With no shims the clearance is 0.30mm and I can't see anybody making 0.05mm shims even at a machine shop. The cam figures say the exhaust should be 0.30mm so that's fine, it's just the inlet that needs to go down tom0.25mm can't think of any way to do it...

[Tom Fenton 1,541]

You can't leave the follower bucket directly on the top cap. Suitable "watch battery" shims are needed.

[welshpug 1,655]

sounds like the valve seats have been re-cut rather deeply, might need the top of the valve stem grinding down a bit if there's enough space to the collets.

[dartman 4 1 Cars]

Just out of interest why can't you run the followers on the valve retainer without shims? Would it damage components? Looking more and more like a machine shop job

[Tom Fenton 1,541]

Because you would then be pushing on the top cap not the valve stem. But not to be an arse if you need to ask why not then you aren't really grasping what's required and you are better off getting someone with some experience to do this job for you.

[dartman 4 1 Cars]

Yer I got that, sorry my wording wasn't great I meant as in to run a shim but hat compleatly flush so it wouldn't push the follower up or would that not be possible, like u said I'm not a mechanic I'm just trying to learn to do things for myself probs not e best way but ill never learn if I keep sending it to the shop.

 

Cheers for the advice

[Tom Fenton 1,541]

As Miles said above, shimming is not something most folk do themself, I've built lots of 8v engines and always pay the machine shop to shim the heads for me.

[Reebmit 51 4 Cars]

I shim all mine myself but it takes time and there are no short cuts. Check and re-check is key but from what I've read above you really do need to get a professional to do it. Maybe ask if you can watch them then next time you can have a try

[mrfirepro 127 1 Cars]

I followed Damirgti's advice from this thread on my first engine, but used sharpening stones instead of sanding sheets. Took an age and had sore things for a week

 

http://forum.205gtidrivers.com/index.php?showtopic=158338&hl=

 

Second engine I gave to a machine shop which took them less than an hour, would never do them myself again.