GH1985

Hi there, I have just registered with Passion Ford... I have been reading the forum for a long time now, and normally have found most answers from the forum - there is some great info on here. However, I am struggling with this one a little.

First of all... here's some info about my current project plans. I have an MK Indy kit car with a Pinto engine and I'm currently building/tuning a Cosworth engine to replace the Pinto.

I have a 1987 2wd Sierra Cosworth YB engine. I am completely rebuilding it and tuning it to what is commonly known as stage 2 (Bosch dark green 803 injectors, -31 actuator, 3 bar map sensor, ECU chip).

So here's my question...

I am unsure of which thickness to get. According to the Ford technical manual the minimum cylinder head thickness is 138.68mm, however I know that people use them past this amount and use a thicker head gasket to compensate. I have measured my cylinder head to be 138.38mm (just under the Ford tolerance limit), the head gasket that was in there was 0.051" (or 1.3mm) thick. I also note that the top of the pistons have marks on them where the intake valves have hit the top of the piston (however no serious damage has been caused to warrant new pistons/valves). What thickness gasket would you recommend?

I'm also unsure if the engine block surface has been machined or not. Could I measure the cylinder bore height to check this? I am undecided whether to get the cylinder head skimmed. Do you think I could get away with another surface cleaning machine of the surface? Is there a data chart of cylinder head thickness/block thickness against which thickness of gasket to use? For your information the pistons in the engine are slightly overbore from standard, as they are 91.775mm (does this limit my head gasket choices).

Looking forward to any useful responses.

Thanks, Graham.

GH1985

Anyone out there?

Maybe my message was a bit too long...

Does anybody know the standard block height? I'm trying to work out if mine has been surface skimmed. I know that the cylinder head has been as it is about 138.38mm.

tabetha

The limits that fords stick to/specify are likely more to do with selling customers a new head, than real world use.
The marks on the piston are 100% NORMAL, even when the head has not been skimmed.
I would have no bother using that head with a good old group a yb 0611 HG from matt lewis, 01922 692424, along with if wanted ARP stud and nut kit.
Whilst the engine is apart it is wise to get the pistons pocketed, so in future should you want to run a harier(higher lift+or longer duration cam) you can.
The piston size is not a problem, the std gaskets cope with oversize pistons.
tabetha

GH1985

Thanks for the reply Tabetha, much appreciated. I have read many posts on this forum and seen your name pop up many times... you seem to have supplied lots of useful information over the time!

So why do the valves hit the pistons then? Surely this can't be good?!

Do you think I will be able to get away with another surface clean skim of the cylinder head and block? Bearing in mind that the cylinder head is at 138.38mm and the thickness of the block is currently unknown?

The group A gasket is 1.3mm thick isn't it? Can I get a thicker one? Should I have a bit machined off the top of the piston?

Thanks again for your response

Graham.

J1mbo

iTrader: (1)

The best way to do it is measure the piston protusion on the freshly skimmed block, get them machined to a acceptable tolerance, depending on gasket this can be around 10tho out the top.

Then piston pockets, the pockets will take away the piston bowl cc lost when machining the tops of the piston, bringing the compression ratio back to 8.0:1

I would then use a grp a/ 0611 gasket

Jim

tabetha

The gasket when you measure it may well be 1.3mm, but that is not fitted, ie under compression by the head/block, so will "shrink" a little.
The usual reason for the kiss marks on the pistons is highly revved engines, unless the block and or head have been excessively skimmed it doesn't cause a problem, but in an ideal world wouldn't happen at all.
Ford tried to keep the CR up, but of course this makes things closer, as a rule always run the highest CR you can get away with, if it's an every day driver this is even more preferable, mine is around 8.2:1.
When you have stripped your engine take it to a decent machine shop/engine builders, the bore will need scoping for wear, the most wear occurs about 1/2 way down the bore on the thrust face, so is measured by a tool that goes down into the bore, get the lightest possible skim of the block done, and if necessary the head, the head will most likely need a complete rebuild anyway, guide wear is chronic on the yb and exhaust valve wear occurs quite bad as well in some, and some grind the shit out of the valves cutting new seats when they should be renewed, the edges can then glow and wreck the engine.
tabetha

GH1985

Sorry for the delayed reply... been doing a bit of research and calculations.

Thanks again for your responses. Some useful information there.

I think I have worked out the thickness of gasket required...

I measured the protusion of the pistons above the block, seemed to be about 0.35mm at most.

The cylinder block is 0.3mm below the Ford limit.

I have researched that the recommended minimum gasket thickness above the top of the piston should be 0.75mm

The standard gasket is 1.3mm.

Lets assume I want to skim 0.05mm of both the cylinder head and the cylinder block (so 0.1mm in total).

I reckon I should use a .070" (1.778mm) thickness gasket. This will compensate for the 0.35mm that the cylinder head is below limit (with the new skimmed head), accomodate for the piston protrusion of the 0.4mm (with the new skimmed block) and then give a clearence of 1.028mm, which is 0.278mm above the recommended minimum so won't alter the compression ratio too much (and should compensate for any gasket 'shrink' under compression of the bolts).

What do you think guys, any problems with that? Then I guess I could leave the pistons as they are.

Looking forward to your thoughts.

Graham.

tabetha

It will work.
tabetha

GH1985

Perfect, thanks.

Looks like I'll be taking a trip to the engine machinists on Monday then

Have a good weekend.

steven8489

Hi, I have just rebuilt my top end and fitted a new head gasket. When I stripped the engine it had a multilayer head gasket on which was total shite. My pistons were protruding the block by .70mm hence the reason for this gasket. I got the piston skimmed and re pocketed and they sat between 0.35-0.40 above the block. I took Tabethas advice and got the yYB0611 head gasket and it works better than ever. Never missed a beat sionce the rebuild.

Good Luck

GH1985

Hi Steven,

Thanks for the info... definitely going to go for a YB0611 head gasket.

I took the block to the engine builders today for a skim... so looking forward to getting that back after a reface!

GH1985

Been looking at head gaskets... ideally need something a bit thicker than the .051" (1.3mm) group A gasket. I don't think they do this in anything thicker... ideally need .070"?

The only ones I can see that do the thicker ones are the Cometic ones... heard they are quite fussy about the mating surfaces being perfectly flat. Apart from that, any thoughts people?