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New 0.060" Pistons - High or Low Comp?

Posted by gregwni

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New 0.060" Pistons - High or Low Comp? #1
gregwni Greg W Belfast, Northern Ireland, United Kingdom	Posted: July 01, 2012 06:08AM Registered: 1 year ago Posts: 240

My engine is in the machine shop and its looking like we will have to go to 0.060" oversize pistons. It was already bored out to 0.030" and there has been quite a bit of wear such that 0.040" might be a bit too close for comfort.

My question is - hsould i be getting high comp or low comp pistons?

My car is a 76 (Early RB) 18v 5 bearing engine.

Also, are the ones that moss europe list ok for the job?

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Re: New 0.060" Pistons - High or Low Comp? #2
sws615 Stephen Struck Grand Haven, MI, USA 1967 MG MGB GT 1975 MG MGB "Commission # G23N124120" 2007 Acura TL Type-S	Posted: July 01, 2012 06:16AM Registered: 5 years ago Posts: 3,557

Greg,

I'll let the engine experts chime in but my first thought is the answer depends on what else you're doing or not doing to the rest of the engine. Rebuilding an engine is a bit of a balancing act in that things need to match up. For example, the exhaust ports in the head are the weak link in the B's breathing. Absent having the head ported properly, there's not a lot to be gained in doing other engine mods. Hap, Kelvin, Dick and others are much more knowledgable.

Steve

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Re: New 0.060" Pistons - High or Low Comp? #3
Speedracer Hap Waldrop Greenville, SC, USA 1967 MG MGB "The Biscuit"	Posted: July 01, 2012 06:39AM Registered: 7 years ago Posts: 20,339

Greg, here in the US, for good long while, you could't find the 8.0 to 1 piston, they started to resurface now with people doing supercharging, the HC pistons, are not really that high, at 8.8 to 1, and here in the US, thats doesn't even regulate them to run fuel octanes above regular unleaded, many folls will alos shave the block and/or head with the 8.8 to 1 piston go even higher. Long story short, with your later car, the 8.8 to 1 HC pistons would be a good choice for you, and restore some needed horsepower. Hope this helps.

Hap Waldrop
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Re: New 0.060" Pistons - High or Low Comp? #4
gregwni Greg W Belfast, Northern Ireland, United Kingdom	Posted: July 01, 2012 06:57AM Registered: 1 year ago Posts: 240

Thanks Steve and Hap!

Hap - the head i have has an unleaded conversion done, but everythign else in the engine will be pretty standard (apart from a very small skim of the head just to make sure its straight).

Im not going to be doing anything too major on the car engine other than put those pistons in, so if having the high comp pistons means that car can benefit from a wee bit of extra power, then that sounds good!

I presume it doesnt mean i can or cant run particular petrol grades here in the uk ? (i know, this is most likely a stupid question!)

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Re: New 0.060" Pistons - High or Low Comp? #5
gregwni Greg W Belfast, Northern Ireland, United Kingdom	Posted: July 01, 2012 07:20AM Registered: 1 year ago Posts: 240

One more question (sorry) : Which of the items on this page is it i need?

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=188

Would it be 12H5163H/60

and how would i be sure that that would be compitable with the con rods i have in the engine?

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Re: New 0.060" Pistons - High or Low Comp? #6
Speedracer Hap Waldrop Greenville, SC, USA 1967 MG MGB "The Biscuit"	Posted: July 01, 2012 07:27AM Registered: 7 years ago Posts: 20,339

In reply to # 2108079 by gregwni One more question (sorry) : Which of the items on this page is it i need?

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=188

Would it be 12H5163H/60

and how would i be sure that that would be compitable with the con rods i have in the engine?

Yep, that is them, 8.75 to 1 for the later press fit pin connecting rods.

Hap Waldrop
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Re: New 0.060" Pistons - High or Low Comp? #7
dickmoritz Dick Moritz Philly 'burbs, PA, USA	Posted: July 01, 2012 08:36AM Registered: 3 years ago Posts: 7,285

Hi, Greg. I agree, there's pretty much no down side to using the 8.8 pistons, since, as Hap correctly points out, that's still pretty low on the compression scale and should run just fine on 87/89 octane (R+M/2...) fuel. I'd also suggest switching to the double roller timing set, as that retards cam timing about 4 degrees, moving the torque curve up just a little higher in the rpm range, making for a more pleasant and rewarding driving experience...

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: New 0.060" Pistons - High or Low Comp? #8
gregwni Greg W Belfast, Northern Ireland, United Kingdom	Posted: July 01, 2012 10:07AM Registered: 1 year ago Posts: 240

Thanks Dick - there are actually two timing chains on my engine already - im guessing someone has most likely done that upgrade already?

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Re: New 0.060" Pistons - High or Low Comp? #9
dickmoritz Dick Moritz Philly 'burbs, PA, USA	Posted: July 01, 2012 10:42AM Registered: 3 years ago Posts: 7,285

Greg,

If you have two timing chains on your engine, then it's gotta be a Twin Cam... grinning smiley

Actually it's one chain, but a double roller design. With the chain off, hold it extended in a horizontal position and look for sagging. If there's any noticeable sagging replace it. Practically never necessary to replace sprockets unless you can see visible wear. Regardless, replace the timing chain tensioner assembly, and be sure to use a gasket between the tensioner and the front engine plate.

Good luck and keep us posted...

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: New 0.060" Pistons - High or Low Comp? #10
j-blueridge Jamie Bourland C'ville VA, USA 1968 MG MGB GT "Antichrist"	Posted: July 01, 2012 11:11AM Registered: 8 years ago Posts: 664

I'm not an expert but wht about the head? Don't some MGB heads have a smaller combustion chamber thus effectively raising compression? Maybe you already took that into account.

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Re: New 0.060" Pistons - High or Low Comp? #11
gregwni Greg W Belfast, Northern Ireland, United Kingdom	Posted: July 03, 2012 04:38PM Registered: 1 year ago Posts: 240

In reply to # 2108229 by j-blueridge I'm not an expert but wht about the head? Don't some MGB heads have a smaller combustion chamber thus effectively raising compression? Maybe you already took that into account.

I really dont know about this,

Dick - my car is actually a pretty unique concept car twin cam (ok - its not - but i will check the double roller chain for sag ) thanks!

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Re: New 0.060" Pistons - High or Low Comp? #12
Ogre1 Allan Ogilvie Melbourne Victoria, Australia 1977 MG MGB "Greenb"	Posted: July 03, 2012 06:06PM Registered: 2 years ago Posts: 1,343

Greg, if your car is a euro RHD MGB then you require HC pistons as you have a HC head with 42cc combustion chambers.
Only our US counterparts suffered from Low compression engines, all uk engines are HC unless they have come back from the US.

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Re: New 0.060" Pistons - High or Low Comp? #13
ingoldsb Terry Ingoldsby Calgary, Alberta, Canada 1971 MG MGB	Posted: July 03, 2012 10:10PM Registered: 9 years ago Posts: 9,767

Just make sure you calculate your volumes (and compression ratios) with your head combustion chamber volume in mind.

As odd as it may seem, the low compression (18V) engine had a smaller combustion chamber than the high compression engine.

So - whether a piston is HC or LC depends on what head it is mated to. It is easy to get higher or lower compression than you anticipate.

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: New 0.060" Pistons - High or Low Comp? #14
Dave Braun Minnesota, USA 1952 MG TD "Tommy" 1970 MG MGB "Maggie" 1974 Triumph Spitfire 1500 "Sammy"	Posted: July 03, 2012 10:59PM Registered: 1 year ago Posts: 2,005

Terry is correct.

Also, the cam is the achilles heel of the engine. And they aren't that expensive. Even the stock cam will restore a lot of performance if the existing cam has worn lobes.

warmly,
dave

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Re: New 0.060" Pistons - High or Low Comp? #15
ingoldsb Terry Ingoldsby Calgary, Alberta, Canada 1971 MG MGB	Posted: July 03, 2012 11:15PM Registered: 9 years ago Posts: 9,767

Just to be clear - I'm not sure what the status of the Euro-spec cars were - the North American 18V engines were LC. Make sure you know what head was used on the European engines.

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: New 0.060" Pistons - High or Low Comp? #16
Ogre1 Allan Ogilvie Melbourne Victoria, Australia 1977 MG MGB "Greenb"	Posted: July 03, 2012 11:28PM Registered: 2 years ago Posts: 1,343

In reply to # 2110540 by ingoldsb Just to be clear - I'm not sure what the status of the Euro-spec cars were - the North American 18V engines were LC. Make sure you know what head was used on the European engines.

If it has no air injection ports, it will most likely be 42cc HC head. If it has air injection ports it will most likely be a US spec 37cc head. Either way your ok with HC. You will end up with 8.8:1 or higher. What you should not do is put LC pistons with an HC head as you will end up with a comp of less than 8. Not sure on the casting numbers for UK but the late model RB head had the extra casting for the air injection ports without them being drilled into the head.

Edit - Late b UK head is 12h4735

Edited 1 time(s). Last edit at 07/03/2012 11:45PM by Ogre1.

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Re: New 0.060" Pistons - High or Low Comp? #17
DB Wood Daniel Wood Bend, OR, USA 1969 MG MGB GT "Clyde" 1970 MG MGB GT	Posted: July 04, 2012 12:40AM Registered: 7 years ago Posts: 3,328

I have a head that is 37cc and no air injection ports. Unfortunately, someone ground the numbers off of it and so it is a mystery what it was on. That, on a .060 over block gave me almost 10:1 compression. I didn't know that all of the Euro heads are 42cc like the US market ones before 1972. That would give you a little higher than the 8.8to 1 because of the larger bore but only a few tenths of a point.

Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
69BGT project (V-6?)
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP)

Lucas= Loose
Unsoldered
Connections
And
Splices

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Re: New 0.060" Pistons - High or Low Comp? #18
Ralph 7h Ralph Siebenhaar Viersen, Germany	Posted: July 04, 2012 02:12AM Registered: 4 years ago Posts: 1,774

Greg,

some years ago, I had the same question and decided to use County flat top pistons with .060 over. It boosts the CR to 9.5:1 on an EC specs engine. The result was perfect and I love it so much, I would do it again with this same pistons on the next engine.
More crisp, less fuel consumption and very noticeable increase of power.

Ralph

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Re: New 0.060" Pistons - High or Low Comp? #19
Speedracer Hap Waldrop Greenville, SC, USA 1967 MG MGB "The Biscuit"	Posted: July 04, 2012 06:15AM Registered: 7 years ago Posts: 20,339

Allan, I got a local buddy that I'm helping with engine build, he has #4735, just exactly how you describe, air pump bosses are there, but not drilled.

Ralph, I can get the flat top County piston right now for the MGB engine, but they cost more than 3x the cost of the 8.8 to 1 pistons, so less expensive to just resurface the head to get to 9.5-10.0 to 1 compression ratio, also if you use the flat top piston you have to try to mantain stock piston to bock deck height, I never ran the CR numbers on a flat top at stock deck height (aprox. .035-.040" winking smiley

but a flat top at near zero deck height with a 38cc head, with only a clean cut of less than .010" would put you at 11.0 to 1, that one I do know.

Hap Waldrop
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