Piston to head clearance- Briggs 5HP

F

Flathead Builder

Member
1972 Briggs 5HP "Koolbore"
Can someone please educate me on what is the minimum clearance needed from the top of the piston to the bottom (underside) of the cylinder head? I just took an engine into inventory (non-running) and I noticed that the piston protrudes slightly above the block ("piston popup"?) is about .016" (the measurement was approximated using a feeler gauge)

I have never seen this before. Typically I see them flush or very slightly below the top of the block/deck. If I install the head w/o a gasket and rotate the flywheel by hand the piston hits on the underside of the head and I cannot complete a full cycle. If I install a new (old style) OEM Briggs "asbestos" type head gasket the piston does NOT hit and I CAN rotate the engine by hand through a full cycle.

While the piston did not hit when rotating it by hand, my concern is that when the engine is running the added momentum (especially at higher RPMs) might "throw" the piston slightly higher and it will hit the head. Also, what about heat- will the internal parts (rod, piston) expand enough as thge engine temp rises to make a difference that it will hit? I thought about using "Plastigauge" to check the clearance, however, I need to know what the minimum clearance needs to be. Is that a legitimate way to check the clearance?

If it matters the maximum RPM this engine will ever see is about 4,200 RPM.
Thank you for any help!
Michael
 
So no rules too contend with ?
.016 would the most pop up i would run with a standard . 043 gasket .
Is there a small recess in the area above the piston on the head ?
If so , It will be fine .
.005 is legal limit per wka .
.015 per ikf .
 
Thank you both for the replies!
No "rules to contend with" as this engine will not be used for racing. There is no recess on the bottom of the cylinder head-it is flat all the way across. The head gasket I'm using is a NOS "old school" Briggs (hard) asbestos gasket- not the (soft) newer graphite type. It measures .055" in thickness (uncompressed). I am not following the math- what does ".011 in the head" refer to?
Michael
 
If I did my math correctly, if the head gasket measures .055" and my piston popup is .016", that "should" give me .039" piston to head clearance, correct? I just do not know how much these old type gaskets actually compress when the head bolts are torqued down- it might end up being a lot thinner and (significantly?) reduce the clearance? These old style Briggs gaskets are metal on both sides with asbestos in the middle, I believe.

That is why I want to actually check/measure the clearance with the head gasket in place and all bolts torqued down to specs. I would prefer to use Plastigauge, if possible, if that is a legitimate way to measure. I have never tried the clay method. After you squish it down with the clay do you need to wait for it to harden so you can then measure the thickness with a micrometer? That being said, what is the absolute minimum clearance I should see to avoid having the piston contact the head?

Just a little bit about me...
I am not a racer and CERTAINLY NOT a professional engine builder. - just a 70 year old garage hack that enjoys restoring and rebuilding the old Briggs flatheads in his garage from the 1960's and 1970s. My engines end up getting used on minibike and fun karts projects from the same time period.

The collective knowledge and experience on this forum is amazing and I am very grateful to be able to tap into it.
Michael
 
flattop1 said:
You Could use soft solder.
Play dough or modeling clay
Put between two pieces of saran wrap or wax paper . No need to wait .
Never saw plastic gauge at .030
Click to expand...
Ok- understood. You have provided me with some options - Thank you.
.030" (and thicker) Plastigauge is made , however, I am finding that it is hard to find anyone stocking or selling it.
Sooo..... Is .030" clearance (quench) about the minimum I would want to see?
Michael
 
I am do not know how to accurately measure piston pop up with a feeler gauge. Place some bar stock across the cyc and measure with a dial or digital indicator. It is often said that having the head too close to the area above the valves can limit flow.
 
Last edited:
Chipg56 said:
I am do not know how to accurately measure piston pop up with a feeler gauge. Place some bar stock across the cyc and measure with a dial or digital indicator. It is often said that having the head to close to the area above the valves can limit flow.
Click to expand...
I simply laid a feeler gauge on top of the block / deck right next to the piston popup. I kept trying different ones until the top of the feeler gauge "felt" level with the top of the piston. Admittedly, my method was a rough estimate with what I had to work with at the time. I would still like to measure the actual quench with the head bolted on, however, I need to know what you all believe to be the absolute minimum clearance I need to see. If it turns out to be acceptable, I'll be OK with what I have. If not, I'll explore one of the solutions that have been suggested.
Thank you for the reply.
Michael
 
.035" is normal "quench" (distance between the piston and underside of the head.) .030" is still plenty safe at your rpm.
If the gasket surface has been milled completely off the head, then you are working with a combustion chamber depth above the piston of .000." WKA/IKF has been .011" minimum in this area. Leaving a bit of gasket surface on the head helps keep the gasket sealed.
Modeling clay with a piece of Saran Wrap on each side works well for measuring quench. It also means torquing and retorquing the cylinder head and gasket.
If you want to measure piston pop-up with a feelers gauge, then take a straight edge (bar stock) and lay parallel to the wrist pin so that it overhangs the gasket surface of the cylinder deck. Rotate the engine to TDC and use your feelers gauges to measure the distance between the deck and the underside of your straight edge. A dial indicator would be much more accurate, but I feel for what you are doing, feelers gauges will get you by.
All good advice from folks above.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
www.youtube.com
34 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
CarlsonMotorsports said:
.035" is normal "quench" (distance between the piston and underside of the head.) .030" is still plenty safe at your rpm.
If the gasket surface has been milled completely off the head, then you are working with a combustion chamber depth above the piston of .000." WKA/IKF has been .011" minimum in this area. Leaving a bit of gasket surface on the head helps keep the gasket sealed.
Modeling clay with a piece of Saran Wrap on each side works well for measuring quench. It also means torquing and retorquing the cylinder head and gasket.
If you want to measure piston pop-up with a feelers gauge, then take a straight edge (bar stock) and lay parallel to the wrist pin so that it overhangs the gasket surface of the cylinder deck. Rotate the engine to TDC and use your feelers gauges to measure the distance between the deck and the underside of your straight edge. A dial indicator would be much more accurate, but I feel for what you are doing, feelers gauges will get you by.
All good advice from folks above.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
www.youtube.com
34 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
Click to expand...
Thank you very much Brian- All very good information! I will try your (much improved) technique with my feeler gauge. While a more accurate measurement of piston popup would be educational and interesting to know, I believe that it takes a back seat in importance to finding out what is the actual quench I am working with. What I do not understand is, after I squish down the clay, how do I actually and accurately measure this (soft) clay laying on top of the piston? Do I just pick it up and put a micrometer on it? It would seem to me it would just fall apart and/or the micrometer jaws would just compress into this soft clay? What am I missing?
I am humbled (and appreciative!) by the gracious help of the forum members that have taken an interest in helping this old timer with his project...
Michael
 
Flathead Builder said:
Thank you very much Brian- All very good information! I will try your (much improved) technique with my feeler gauge. While a more accurate measurement of piston popup would be educational and interesting to know, I believe that it takes a back seat in importance to finding out what is the actual quench I am working with. What I do not understand is, after I squish down the clay, how do I actually and accurately measure this (soft) clay laying on top of the piston? Do I just pick it up and put a micrometer on it? It would seem to me it would just fall apart and/or the micrometer jaws would just compress into this soft clay? What am I missing?
I am humbled (and appreciative!) by the gracious help of the forum members that have taken an interest in helping this old timer with his project...
Michael
Click to expand...
Just use a mic (or dial calipers if you don't have mics.) Don't "blue thumb" the gauge and you can measure the clay just fine. Modeling clay is soft and moist and will stay compacted (same with fresh play-dough. If you use old clay, you can mix a small amount of water with it to get it back to soft and malleable again so it will not crumble apart.
 
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