32 replies to this topic

[Cooperman]

I seem to recall this as a problem from long ago when oversize valves were fitted to a 998 race Cooper. The block had to locally relieved I think, but that was over 50 years ago. I don't recall the details as it wasn't my engine.

[Daz1968]

Does sound like valves contacting block, I have a 12g295 ported and with oversize valves but on trial fit I need small pockets for the exhaust valves. Not an easy job with engine in situ

[DeadSquare]

Does sound like valves contacting block, I have a 12g295 ported and with oversize valves but on trial fit I need small pockets for the exhaust valves. Not an easy job with engine in situ

 

 

Not an easy job with the engine in situ?

 

This is where a burnt exhaust valve comes into its own.

 

Step forward with your oxy acetylene and braze a small tungsten carbide tool tip in the burnt hole and grind it to give about .050"to the side and below the valve head.

 

Insert the valve and grip the stem with your Black and Decker.

 

Replace the head and secure with a couple of nuts and mark the valve stem for the desired depth.

 

With Henry sucking from the exhaust port, carefully machine a valve pocket in the block.

 

I have done it many times.

[m.angus0]

Ah I hadnt thought about the valves fouling the block. That may very well be the issue. I will check tonight. The machine shop did change the valves but annoyingly I cant find the old ones to see if they have put larger ones in. What a pain thats going to be if thats the case.

[carbon]

If you find that the exhaust valves have been touching the block then you need to:

- take the head off and check all four exhaust valves for any signs of being bent (you may see witness marks on the block)

- bin the other exhaust rockers, as they may also have been stressed. Are they A+ sintered type?

- think about checking the exhaust cam followers for any distress, this is relatively easy on a 998

- roll the exhaust pushrods along a flat surface (piece of plate glass or similar) and look for any signs of them being bent

 

If the exhaust valves have been kissing the block then you have a few options, including cutouts in the block, a different head with deeper chambers and/or standard size exhaust valve, or change to a lower lift cam such as the MD266.

[m.angus0]

Right problem found. I whipped the head off tonight and sure enough the exhaust valves have been hitting the block. I will strip the head down tomorrow to see if its damaged the valves. What to do now? I would preferably pocket the block but is this something a machine shop can do with the engine in as I really dont want to take it out again? Otherwise its a cam change which again I dont really want to do as they are not cheap and have only just bought it.

[DeadSquare]

Right problem found. I whipped the head off tonight and sure enough the exhaust valves have been hitting the block. I will strip the head down tomorrow to see if its damaged the valves. What to do now? I would preferably pocket the block but is this something a machine shop can do with the engine in as I really dont want to take it out again? Otherwise its a cam change which again I dont really want to do as they are not cheap and have only just bought it.

 

 

Oh dear, this is a classic case of why Downton wouldn't sell heads, only prepare the whole engine.

 

The simple answer is to take it all to a good engine builder, and take out a mortgage.

 

The cheapest thing to do yourself is to change the cam for one with less exhaust lift, like the old BMC 731, and for peace of mind, take Carbon's advice and have new cam followers, push rods, at least cast rockers and new exhaust valves.

 

The BMC 731 cam, which has a 268 inlet period very close to the 270 one you have chosen, only had a cam lift of .250";  your 280 exhaust period has the same .315" lift as a full race BMC 648 cam.  This will be hard on the exhaust valve train, which may be acceptable for sixty, flat out, 30 minute races, but at a hard, sporty 1/10 of the wear rate, it is only 300 hours cam life,  unless you are not going to drive it hard.

 

Do you need such a warm / hot cam?  Although Kent say it will fit 850 - 1300, a 731 cam was not nice in an 850.  In a 998 it would pull a ton, but there was sod all bottom end, I couldn't pull away on a snowy slope because "getting it on the cam" made the wheels spin.  OK, we don't have snow these days and you have another car, Lol

 

As I say in another post, valve pockets can be cut in situ;  I used to do it for friends who thought it would be nice to have a S head. 

 

Does anyone here know of a machine shop that would do it?

[timmy850]

Or you could get new exhaust seats and install smaller/stock exhaust valves? I'm guessing they're larger than standard?

[Spider]

 

Right problem found. I whipped the head off tonight and sure enough the exhaust valves have been hitting the block. I will strip the head down tomorrow to see if its damaged the valves. What to do now? I would preferably pocket the block but is this something a machine shop can do with the engine in as I really dont want to take it out again? Otherwise its a cam change which again I dont really want to do as they are not cheap and have only just bought it.

 

 

Oh dear, this is a classic case of why Downton wouldn't sell heads, only prepare the whole engine.

 

The simple answer is to take it all to a good engine builder, and take out a mortgage.

 

The cheapest thing to do yourself is to change the cam for one with less exhaust lift, like the old BMC 731, and for peace of mind, take Carbon's advice and have new cam followers, push rods, at least cast rockers and new exhaust valves.

 

The BMC 731 cam, which has a 268 inlet period very close to the 270 one you have chosen, only had a cam lift of .250";  your 280 exhaust period has the same .315" lift as a full race BMC 648 cam.  This will be hard on the exhaust valve train, which may be acceptable for sixty, flat out, 30 minute races, but at a hard, sporty 1/10 of the wear rate, it is only 300 hours cam life,  unless you are not going to drive it hard.

 

Do you need such a warm / hot cam?  Although Kent say it will fit 850 - 1300, a 731 cam was not nice in an 850.  In a 998 it would pull a ton, but there was sod all bottom end, I couldn't pull away on a snowy slope because "getting it on the cam" made the wheels spin.  OK, we don't have snow these days and you have another car, Lol

 

As I say in another post, valve pockets can be cut in situ;  I used to do it for friends who thought it would be nice to have a S head. 

 

Does anyone here know of a machine shop that would do it?

 

 

Sorry for going off topic here.

 

Deadsquare - not sure if you're aware or doing this on purpose, however your type font in this thread has been big to huge, it kinda gives an impression of trying to talk over all else, probably not what your trying to do though.

 

Can you kindly check and reset your font size to 14 please - thanks.

 

OK - resume topic

[m.angus0]

Ok a cam change may be the answer. How much performance will I lose if I put the standard 998 A+ cam back in? I dont really want to spend £200 on another cam to find the same issue.

[DeadSquare]

 

 

 

Sorry for going off topic here.

 

Deadsquare - not sure if you're aware or doing this on purpose, however your type font in this thread has been big to huge, it kinda gives an impression of trying to talk over all else, probably not what your trying to do though.

 

Can you kindly check and reset your font size to 14 please - thanks.

 

OK - resume topic

 

 

Sorry  about the type.

 

I have links with a group of poor sighted teens, and my Dragon speech thingy is set up to print large and very large.

 

I'll try to remember to reset it.

Edited by DeadSquare, 06 May 2018 - 08:34 AM.

[DeadSquare]

Ok a cam change may be the answer. How much performance will I lose if I put the standard 998 A+ cam back in? I dont really want to spend £200 on another cam to find the same issue.

 

Are the valves bent?

 

Answer to you question:-  You will loose most of the performance and remember that you have a cam to sell.

 

 

 

Let's start from the scratch:

 

Presumably this is not your only car?  so you have time to put your time into it?

 

What was your aim when you started?  overhaul?  general go faster?

 

What are you going to use the car for?. competition?  shopping?

 

What have you actually paid someone to do to the engine beside the skim?  head polished ?  crank rods and pistons balanced?

 

Any other info ................................

[m.angus0]

Ok so it only has 13000 miles from new so I wanted some extra poke and thought it be a shame to change out the original low mileage engine so I would tune it. I want it to be useful for the odd sunny day blast and shows so I dont mind it being cammy which is why I went for the 276. Ive spend a lot on this tune up so far so I dont want to spend much more so as long as it wont be disappointing with the standard cam Im happy to swap it back because Ive still got a stage one kit and a port & polished 12g295 head with bigger valves. As long as it will make some sort of difference and its still drivable Im good

[Daz1968]

Easiest option would be a standard valve sized 12g295, post up some pics of the chambers without valves, as they may have just fitted larger valves because it’s easier and 1300 valves are cheaper, the actual head may not have been opened up behind the valve, so may be able to fit new seats and standard valves,

[racingbob]

what about selling you cam and getting a mg metro cam for about £70 I'm sure they work well in a 998