11 replies to this topic

[henryS]

hi,
Hopefully a fairly simple question....

How can you tell (when stripping down a cylinder head) whether the valves require simply relapping or a full regrind and lap?

Have been told by a machine shop that sometimes lapping isn't enough and that a regrind can be much better. (3 angle grind I believe)

I'm starting to rebuild a cylinder head and want to work out what work it will require.

Thanks

[nicklouse]

grinding and Lapping are the same to me.

cutting is what happens when they talk about the three angles.

what is needed depends on what it is like now.

if you are changing valve size it will need re cutting and then lapping (grinding) in.

[Spider]

Generally if the Valve Guides haven't been disturbed (ie replaced) and the actual seats look OK, flat, not pitted then they can be Lapped. Anything more than that and they need to be Re-cut and then Lapped. If the seats don't 'clean up' after about 10 turns of the valve, then they should be re-cut. Sure, continuing to Lap, especially with coarser paste, you may get a seal but the seats themselves will unlikely to be flat.

 

If the guides are worn, then usually the seats in the head will also be worn to a point that they will need re-cutting.

 

If replacing Valve Guides and / or Valves, it's best to have the seats Re-cut.

 

In short, it's not too often that they will only need a Lap without re-cutting first.

 

<Edit: If the engine that the head came off was burning oil, even for a short time, the Exhaust Seats in the head will be pitted for sure.

 

@ Nick - I see them as different (Lapping vs Grinding). lapping is done with the stick / suction cap on the Valve head by hand with Lapping Paste, Grinding (or cutting as I usually do them) is done with separate tools that are far more aggressive to correct the shape of the seats. It's not just the Seats in the Head that are done this way, but also the Valves too. >

Edited by Moke Spider, 15 June 2015 - 09:12 AM.

[henryS]

Thanks for the quick and detailed replies.

I'll take a look at the valves when I can strip the head and as you say MokeSpider, I shall perhaps try a quick lap (if they don't clean up I'll look at re-cutting)

I'll get some photos on here when I can.

--
On a side note - is it worth increasing the valve size on a 998 head whilst getting them re-cut? i.e. for the small additional cost, are the larger valves worth the HP increase?
Also is it worth the £80-£90 to get the head ported? I know this may be opening a bit of a can of worms regarding how much power can be gained from a 998 but whilst it's at the machine shop, it may be worth getting it done...?

[cal844]

Thanks for the quick and detailed replies.I'll take a look at the valves when I can strip the head and as you say MokeSpider, I shall perhaps try a quick lap (if they don't clean up I'll look at re-cutting)I'll get some photos on here when I can.--On a side note - is it worth increasing the valve size on a 998 head whilst getting them re-cut? i.e. for the small additional cost, are the larger valves worth the HP increase?Also is it worth the £80-£90 to get the head ported? I know this may be opening a bit of a can of worms regarding how much power can be gained from a 998 but whilst it's at the machine shop, it may be worth getting it done...?

The restrictions on A series enghines is the heads, id say open the ports up and machine/fit bigger valves

[Spider]

As Cal844 has said, all the Power of these Engines is locked up in the Head.

 

If you can get a half decent Port Job done for 80 - 90 quid, then sure, but given how labour intensive it is, I'd have to question what you'd get for that (I think in another recent thread, someone was suggesting a straight head service was more than that figure). In these parts a fairly OK Port job on these heads is around a Grand (about 500 quid).

 

Certainly increasing the Inlet Valve Size will help a little, but given the shrouding that the very enclosed chamber has, don't expect any big improvement.

 

I would suggest finding a 12G202 casting head (as fitted to the 997 Cooper and nearly all 1100 engines) first. Just overhauling one of these and having it skimmed to the the CR to at least where yours currently is - or better - will make a big improvement. Then if you wanted a Port Job, again, this is a better head to work from as some of the work is already done.

 

Just one other thing I'd suggest, if your head (or a new one) hasn't already been fitted with hardened exhaust valve seats, I would suggest having them fitted (or finding one that has them).

[henryS]

Thanks again both. I agree that the head is where some good power will come from. I guess the difficult decision to make is how much to spend to get a reasonable increase.

I'm not looking to make loads of power, after all it's only a little 998. Long term I would look to get a 1275 or similar so for now I think making a few small mods to the 998 head (already unleaded but thanks for the suggestion).

I figured that getting it ported to match uprated inlet and exhuast manifolds plus with re-cutting the valves or larger valves would be a reasonable place to start.

I will have to investigate the costs some more and see what they think that level of porting would achieeve - I may need to spend more in which case it may not be worth it for the 998.

I'll take a look for the alternative heads but again they may be a bit too expensvie for the gains?

[Spider]

I haven't read the whole advert but at a quick glance, this does look to be something worth putting a bid on

 

http://www.ebay.co.u...=item3cfbfcb6da

 

It does seem that 12G202 heads in the UK are still reasonably priced, much more so than any port & chamber job that you'd do to a 998 head to bring it to around the same as the 12G202 head.

 

A better head again is the 12G295, but for the price that these now sell for, you'd be better off going straight to a 1275 engine.

[henryS]

Thanks MokeSpider. that's really helpful - I did just take a look for some.

So if I stuck with my unleaded 998 head, the most I should spend money on doing to it is probably just clean it, lap the valves (or re-cut if required) and use as it is.

Otherwise going for the 12G202 is the way to go, before moving up to a 1275...?

Thanks

[Carlos W]

I paid a lot more than 90 quid for a rebuilt head to my engine spec, and it's a work of art. It's currently stored under the bed as the garage is too cold for such a thing of beauty. 

 

If you're planning on keeping your 998 then a decent head will give increased power gains, especially with a better cam, all properly set up.

 

If you are planning a 1275 transplant in the near future then keep your money for that.

 

Some people love the way a 998 drives, so don't think you have to go to 1275

[Spider]

Thanks MokeSpider. that's really helpful - I did just take a look for some.

So if I stuck with my unleaded 998 head, the most I should spend money on doing to it is probably just clean it, lap the valves (or re-cut if required) and use as it is.

Otherwise going for the 12G202 is the way to go, before moving up to a 1275...?

Thanks

 

Do your own homework for sure, but more or less, yes - not withstanding Carlos's comment.

[henryS]

Thank you Carlos, some good food for thought there..I may 'refresh' the current head ,clean and tidy, lap valves or re-cut if necessary but leave the major work for now until I can decide what to do regarding 998 or 1275.

As I have the original engine for the Mini, I do like the idea of keeping the engine, whether or not I put a larger engine in the car for a while I don't know....

Thanks for all of the comments above, I had better do some thinking and if I do go for the 998, perhaps a 12G202 or 295 head would be a better one to start with as discussed.