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Finding Top Dead Center

Posted by HobieTrax 
HobieTrax Avatar
David Cohen
Westport, CT, USA   usa
1972 MG MGB "Cliff"

I know if finding TDC was this simple someone else would have thought of it by now, but couldn't you use a compression gauge to find TDC.


David
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Jeff Schlemmer
Minnesota, USA   usa
1950 Willys Jeep Pickup "Ratrod"
1955 Chevrolet Thriftmaster
1958 MG magnette
1971 MG MGB "POS"
1973 MG Midget MkII   → more

Plain and simple, no. That'll only tell you when the exhaust valve opens.

Summit racing sells a piston stop for about $8. Install it in the spark plug hole. Rotate the engine by hand CW then CW, marking the crank pulley where it stopped each time. Split the difference and you have your exact TDC.



jeff@advanceddistributors.com
HobieTrax Avatar
David Cohen
Westport, CT, USA   usa
1972 MG MGB "Cliff"

The light bulb went on, I get it now, duh. Thanks.

David
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Greg M
North of the City, Ontario, Canada   can
1970 MG MGB
1972 MG MGB GT

Finding TDC seemed really straightforward to me after watching John Twist use some fluid in a clear tube attached to a drilled out spark plug.

I chucked up a drill in the press and clamped an old plug down in the vise. Broke off two drill bits and shattered a bunch of old plugs trying to core out the center!

I bring this up because my next thought was to use a compression gauge... then realizing that wouldn't work I was going to hack off the plug end of the gauge to use instead of the hollow plug. Then I gave my head a proverbial shake.

Went down to the local speed shop and got a 14mm piston stop. Oh so much easier... The simplest tools work best.



__________________________
1970 MGB Black Label OD, APTVP12, Fidanza Flywheel, Flowspeed Head, Hilton AUD405 HS4 SU's, Schlemmerized Distributor, Pertronix Ignition, Saturn Alternator, 15" Dayton Wire Wheels, Falken ZE 912's, Falcon Big Bore
1972 MGB GT Blue Label OD, HIF SU's, 123 Distributor, Peco Exaust, APTVP14, Fidanza Flywheel, Flowspeed Head, Saturn Alternator, 15" Rota RB Wheels, Dunlop Direzza, Frontline Costello front valance

Jim Smith
CASSIDY, BC, Canada   can

To get real close you could use a 1/4 inch dowel in the spark plug hole.

When I messed about with motorcycles we used a dial gauge that threaded into the spark plug hole. All you did was turn the engine over by hand until the gauge needle changed direction. Once this happened we found the highest piston position and marked it as TDC on whichever stroke the engine was on.

We also used this dial gauge method to verify ignition timing marks as there is a relationship between the degrees of rotation and the piston on travel in a complete turn.



Jim...Nanaimo, BC
1971 MGB collector
1975 MGB driver
1975 MGB in pieces waiting for V6
2005 Mini Cooper

A wise man overhauls his brakes before his engine.
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bills Avatar
Bill Spohn
W. Vancouver, BC, Canada   can

Jim1971 Wrote:
Quote: When I messed about with motorcycles we used a dial gauge that threaded into the spark plug hole. All you did was turn the engine over by hand until the gauge needle changed direction. Once this happened we found the highest piston position and marked it as TDC on whichever stroke the engine was on.

That works for Twin Cams too, but for pushrod engines, you can approximate it with a piece of bent wire - and get very, very close.



Bill Spohn www.rhodo.citymax.com/carstuff.html
1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1969 MGC roadster (175 bhp), 1957 Jamaican MGA (200 bhp)
1965 Jensen CV8 (375 bhp), 1971 Jensen Interceptor (350 bhp)
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
2009 Pontiac Solstice GXP Coupe (350 bhp)
West Vancouver BC

MrMarty51 Avatar
Martin Draper
Miles City,Montana, USA   usa
1978 MG MGB "" Little Red Crapper Car""

As staated,The piston stop method is the most accurate.
When the piston reaches TDC,using a dial indicator or any other method You still have that slight turn of the crankshaft where the piston has reached TDC and then when it starts back down again.I`ve never figured out how to make an accurate al;lowance for this bit of movement except by using the piston stop method,Then You know exactly where TDC is located on the balancer.
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Hap Waldrop
Greenville, SC, USA   usa
1967 MG MGB "The Biscuit"

If you do this stuff long enough, a screw-in piston stop will one of the tools in your box, I got mine at CV products.



Hap Waldrop
Acme Speed Shop
864-370-3000
Performance Street/Race engines- modified heads, and DIY engine rebuilt kits
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Jeff Schlemmer
Minnesota, USA   usa
1950 Willys Jeep Pickup "Ratrod"
1955 Chevrolet Thriftmaster
1958 MG magnette
1971 MG MGB "POS"
1973 MG Midget MkII   → more

If you try to "eyeball" TDC, its a +/- 5 degree proposition. The piston stays at TDC for a while. Why take the time if you're just going to guess anyway, and the tool is almost always under $10.



jeff@advanceddistributors.com
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Hap Waldrop
Greenville, SC, USA   usa
1967 MG MGB "The Biscuit"

What I do I do when using the TDC piston tools, is mark on the pulley, thet stop point in both directions, then cut a strip of paper the aprox width od the part of the pulley where you are going to put the mark, I trim the strip of paper down with scissors until I get it the perfect length form mark to mark, then use a precision scale ruler, that goes down to 32ths or 64th, to find absolute center, mark that on the strip of paper, then pal;ce it back over the pulley, use a hammer and chisel to mark the new TDC mark with right thru the the piece of paper on the pulley, then use a small touch up paint brush and opposing color paint to the pulley to mark the the TDC mark I made with the chisel.



Hap Waldrop
Acme Speed Shop
864-370-3000
Performance Street/Race engines- modified heads, and DIY engine rebuilt kits
New alloy wheels options for MGBs, see vendors forum for details.
http://www.mgexp.com/phorum/read.php?68,2657584
Be sure to check my engine rebuild kit thread in the Vendors forum for weekly tips. http://www.mgexp.com/phorum/read.php?68,1828263

Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Tech Advice - Alloy wheels for MGB/TR6
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Jeff Schlemmer
Minnesota, USA   usa
1950 Willys Jeep Pickup "Ratrod"
1955 Chevrolet Thriftmaster
1958 MG magnette
1971 MG MGB "POS"
1973 MG Midget MkII   → more

Funny. I do roughly the same thing, only I just fold the paper in half to find center. smiling smiley




jeff@advanceddistributors.com
NitroRustlerDriver Avatar
Andrew Bird
Bend, Oregon, USA   usa
1970 MG MGB GT "Giget"

As far as a piston stop tool goes, couldn't you just use a long 14mm bolt? Those are only a couple bucks at the hardware store.



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Speedracer Avatar
Hap Waldrop
Greenville, SC, USA   usa
1967 MG MGB "The Biscuit"

NitroRustlerDriver Wrote:
Quote: As far as a piston stop tool goes, couldn't you just use a long 14mm bolt? Those are only a couple bucks at the hardware store.

Yep you can, just not that easy to find a 14mm bolt, not a common application, the fastner shop I go to never has stocked them, and they stock everything, McMaster Carr or someone like that, but then it probably comes doen to buying more than one and the tool being cheaper. You can make one out of a spark plug, the luxury of the tool or a bolt is that it is adjustable if you work on more than one type of engine, which I do.

Wait for it, Derek will very shortly put up a link to the best deal you can find on one of these smiling smiley You know, while we're (well me) talking about Derek, I think I just came up with a nickname for him, we can call him "The Linkmaster" smiling smiley



Hap Waldrop
Acme Speed Shop
864-370-3000
Performance Street/Race engines- modified heads, and DIY engine rebuilt kits
New alloy wheels options for MGBs, see vendors forum for details.
http://www.mgexp.com/phorum/read.php?68,2657584
Be sure to check my engine rebuild kit thread in the Vendors forum for weekly tips. http://www.mgexp.com/phorum/read.php?68,1828263

Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Tech Advice - Alloy wheels for MGB/TR6
Blake Sonnier Avatar
lake charles, Louisiana, USA   usa
1956 MG MGA "Maggie"
1963 MG MGB
1972 MG MGB "Sweet Thing"
1974 MG MGB GT "None Yet"
1977 MG MGB "DIXIE"

http://www.restockit.com/Piston-Stop-Tool-(IPA7876).html?source=froogle&Bvar5=100F1&Bvar6=100F1&Bvar7=100F1

Piston stop
A piston stop is any device that prevents the piston from going all the way to top dead center.
Although ready made piston stop tools are available, I made due with a spark plug that had very long threads (a Champion RE14PLP5). But, any 14 mm bolt with a 1.25 mm thread pitch will work. Here's how it works: with the piston NOT at top dead center (just feel with a screwdriver), screw the piston stop (spark plug, in my case) into the spark plug hole and tighten. Next, rotate the crankshaft clockwise VERY SLOWLY AND CAREFULLY until the piston comes into contact with the piston stop. Write down the value that is shown on the degree wheel. Then, repeat the procedure by rotating the crankshaft counterclockwise. Again, write down the value that is shown on the degree wheel. Top dead center is the mid point between these two values.










Edited 1 time(s). Last edit at 05/04/2009 03:20PM by Blake Sonnier.
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