Compression Ratios, rebores and head skimming

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SteveW
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Compression Ratios, rebores and head skimming

Post by SteveW » Fri Jun 05, 2020 7:46 am

I've now had the report back from the engine shop. The cylinder bores were already +060 over sized but it now needs a rebore to +100 thou. On its own, this would result in a compression ratio of 7.74:1. However, the head will also need to be skimmed. It is already on the limit (73.58mm) so this is going to have to be a very thin skim just to clean up the surfaces. With standard cylinder bores, a head skimmed to this extent would give a compression ratio of 9.3:1 (pages 134 to 135 of TC Forever).

Will there be any issue with having +100 thou bores with a skimmed head in terms of compression ratio as this will make it higher than either of the above on their own? If so, is there anything that can be done to reduce it again (e.g. thicker head gasket). I'm wanting to avoid having to re-sleeve the bores if I can. Assuming that the compression can be tolerated, how much less than 73.58mm is it safe to go without causing other problems?

Also, the head has had work done on it before as the valves are the larger ones and there has been porting work done on the inlets. This together with other evidence, such as the front wheel hubs being drilled for cooling, lends weight to the theory that the car has been used for racing in an earlier life.

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Steve Simmons
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Re: Compression Ratios, rebores and head skimming

Post by Steve Simmons » Fri Jun 05, 2020 7:59 am

Using the tables in Wood's manual, boring from +60 to +100 increases compression ration by 2.7%, from 7.54 to 7.74. So with a starting compression ratio of 9.3, with a +100 bore you would end up with 9.55:1. That's assuming my math is correct. The only thing I would watch out for is valve to block clearance. If you want to lower the compression ratio, you can use a thicker head gasket, which will be solid copper. Or perhaps get dished pistons. And since you're buying pistons anyway, why not go +80?
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Ray White
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Re: Compression Ratios, rebores and head skimming

Post by Ray White » Fri Jun 05, 2020 8:28 am

I would be worried that +100 would risk going through to the water jacket. I believe it is possible to check the wall thickness (with the right equipment) so it might be Worth checking with your engine builder to see if he can do this. Even so, you are right at the limit. I would go for sleeving the bores. then you get to choose the size you want.

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Fri Jun 05, 2020 9:51 am

Hi Ray, I asked and they don't have the kit to measure block wall thickness. I assume that to fit the liners they would have to bore to more than +100 anyway and then going into water would not be a problem as the liner would seal it again. I guess that I could get them to try to bore to +100 and if they go through then ask them to put liners in at that point.

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Fri Jun 05, 2020 10:08 am

Hi Steve, +80 pistons seem to be a lot more expensive than +100 for some reason (at least, in the UK). Good suggestion about a solid copper gasket, this would also reduce the risk of the valves hitting the pistons due to over skimming the block. I understand that they are harder to get to seal though.

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Re: Compression Ratios, rebores and head skimming

Post by Steve Simmons » Fri Jun 05, 2020 10:10 am

I've heard mixed reviews on solid copper gaskets. Some say the racers run them without issues but others say they fail all the time. And a couple people have said you have to continuously re-torque them. Maybe this is why the racers don't have issues, since they probably change gaskets and re-torque far more often than the rest of us.
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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Fri Jun 05, 2020 12:52 pm

I've also read that people have had success using two standard head gaskets to, in effect, restore some head thickness.

How do I test to see if I have enough value to block/piston clearance?

Maybe worrying about nothing because if the head is flat enough it may only need a couple of thou taken off to clean it.

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Sat Jun 06, 2020 3:32 am

Sorry, this thread was not supposed to be my personal stream of conciousness, however, further research has suggested that cylinder head saver shims may be an option. These are steel shims (I guess the same shape as the head gasket) that you bond to the block and then put a normal gasket on top of that. In effect, they restore some of the original thickness of the head. I cannot find a source of off-the-shelf ones for XPAG engines but companies can make them to order e.g. here http://www.gosnays.co.uk/acatalog/SaverShims.html

The pre-made ones are only a little more expensive that a standard head gasket (although of course you need one of those as well) but I expect that a bespoke one will be more. I may not yet need one if my head cleans up well enough and is flat enough not to need a further skim, so I'm just exploring options for the moment.

Does anyone have any experience?

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Re: Compression Ratios, rebores and head skimming

Post by Steve Simmons » Sat Jun 06, 2020 9:31 am

I've been warned against using two standard gaskets, but I don't have any experience actually doing it.

A friend has one of those shims, about 1/8" thick. I was under the impression that you used a head gasket on either side. It was made by a local machinist. It should be an easy part to make, but would likely need to be surface ground smooth.
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Re: Compression Ratios, rebores and head skimming

Post by Ray White » Sat Jun 06, 2020 2:46 pm

SteveW wrote:
Fri Jun 05, 2020 9:51 am
Hi Ray, I asked and they don't have the kit to measure block wall thickness. I assume that to fit the liners they would have to bore to more than +100 anyway and then going into water would not be a problem as the liner would seal it again. I guess that I could get them to try to bore to +100 and if they go through then ask them to put liners in at that point.
The wall thickness would be right at the limit. It wouldn't allow for any bore wear... just imagine what might happen after a few thousand miles... :wtf:

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Mon Jun 08, 2020 12:20 am

As I understand it there are quite a lot of XPAGs out there running on +100 bores (+100 pistons are an off the shelf item), but you are right, there are also stories of the rebore going into the water jacket. My engine man says that, whilst he cannot measure the wall thickness, he can get a good idea if it's thick enough from the sound of that the boring machine makes as it's cutting.

With regard to cylinder head saver shims, I've just spoken to Gosnay's, who are the go to people in the UK for these. They do not have an off the shelf one for the XPAG but they can make a bespoke one for £150. The unit price would come down a lot for more than one as most of the cost is in the set up, so if my head needs to be skimmed beyond the limit I could see if anyone else needs one.

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Re: Compression Ratios, rebores and head skimming

Post by Steve Simmons » Mon Jun 08, 2020 7:13 am

For £150 you could also buy a new head.
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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Mon Jun 08, 2020 9:46 am

I know, but it would be a shame to waste mine, particularly as the previous owner went to the trouble of having it ported and cut for larger valves.

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Re: Compression Ratios, rebores and head skimming

Post by Keith Beningfield » Tue Jun 09, 2020 11:21 am

I run a supercharged XPAG bored +140 with a Laystall cylinder head. To keep the compression ratio under 8.1 i use an aluminium decompression plate 0.9mm thick together with a standard copper head gasket. I know aluminium against an ally head is OK not sure about iron. These decompression plates are made by Ferriday Engineering in the UK, they have the drawings for 1250cc and 1500cc . So far they have performed faultlessly, I have also bored XPAG and XPAW blocks plus .120 and plus .140 without any problems. Depend on the valve lift as to whether you need cutouts in the block.

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Wed Jun 10, 2020 12:35 am

Hi Keith, Thanks for the information and the advice. I've spoken to Mike at Ferriday Engineering and he was really helpful - including giving advice that might mean that I don't need to but anything off him. He has offered a solution that will work for me should I need it if I have to over skim the head.

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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Thu Jul 09, 2020 3:50 am

I've now had my head and block back from the engine shop (took a while because their local post office had a COVID-19 outbreak and went into lockdown with some of my rebuild parts in it). The block was successfully bored to +.100 and the minimum skim on the head to make it flat has put it 8 thou (0.2mm) under the minimum thickness stated in TC Forever. From a rough measurement I'm ok on valve clearance and my maths (math if your American) puts the compression ratio at just over 10:1. If I put a 1mm cylinder head saver on (effectively an additional solid gasket) then the compression will drop to about 8.7:1

I know that some will say 'get a new head' but the previous owner did a lot of work on this one by having it ported for better gas flow and also installing larger valves so I want to keep it.

Do you think that I can live with 10:1 or is it too high?

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Re: Compression Ratios, rebores and head skimming

Post by Tom Lange, MGT Repair » Thu Jul 09, 2020 5:14 am

Well, let's think for a moment about the down side of a high compression ratio.

1) need premium fuel
2) will have to advance timing to avoid pinging (plinking or pre-detonation); Distributor rebuild and advance re-curve will be beneficial
3) high-compression engines tend to run better at higher RPMs, and worse at low RPMs
4) MUCH more heat generated
5) much more stress on internals – more likely to damage pistons, crank and rods
6) Everything works in tandem – you should probably add headers and H4 carbs
7) I would want a different cam with a high CR
8) Harder to start and stop – tendency to run on

Yes you will get more power with a higher CR, but as with life, everything is a compromise and a balance. If it were I, I would reduce the compression ratio to no more than 8.7:1.

Tom Lange
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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Thu Jul 09, 2020 8:28 am

Hi Tom,
Thanks, I think that you have confirmed what I probably already really knew. Sometimes it takes someone else to say it, which I guess is one of the benefits of this forum.

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Re: Compression Ratios, rebores and head skimming

Post by Steve Simmons » Thu Jul 09, 2020 8:31 am

Maybe a thick head gasket would be enough? I'm running 9.1:1 with no issues. Good driveability and runs on mid or high test fuel equally well in most situations. Solid copper gaskets come in double thickness. That would give you back just a little over 1mm of thickness back.
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Re: Compression Ratios, rebores and head skimming

Post by SteveW » Sat Jul 25, 2020 12:25 pm

I've now acquired a second head as a back up. It's never been skimmed so is full thickness. Is it worth cutting in the larger valves and doing some work on the inlet ports or is it best left stock? I'll be replacing the guides as a minimum.

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