AP/Norvel 1/2A Cylinder Heads and Plugs

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sgilkey
Posts: 2351
Joined: Mon Mar 19, 2001 8:26 am

AP/Norvel 1/2A Cylinder Heads and Plugs

Post by sgilkey »

If you come across an older Norvel 1/2A engine you should be aware that Norvels have come with several different styles of heads and plugs, which are only interchangeable in sets.

The "current" (?) Norvel is the "revlite" version. This engine has wider cylinder fins and a dark gray-green color to the cylinder. the cylinder is base-bolted but the bolts are hidden from view by clearance holes thru the fins. The older, pre-revlite Norvel has a silver-grey colored cylinder with smaller fins, and the bolts at the base which retain it to the case are clearly visible. Look at a photo of the AP .061 on Tower's site for a view of what the pre-revlite Norvel looks like, the AP appears to be a knock-off of that engine. To my knowledge there is little or no performance difference between the revlite and pre-revlite Norvel.

All versions of the Norvel use a two-piece head. A glow plug/button is retained by a screw-in head/clamp. However, it is important to know about the different head/plug designs, especially if you have/buy an older engine or are scrounging for replacement plugs.

The pre-revlite engine came with at least two different glow plugs. The older versions use the so-called "cold" plug, which I do not believe is serviced by Norvel any longer. The cold plug has an OD of the body portion of .314", and is distinguished by a square intersection between the sealing flange and the plug body, and one broad notch with a square top on the electrode.
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The corresponding clamp for the cold plug has a .316" i.d. and a square or only slightly beveled edge in the corner where its ID meets the clamping surface.
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The "cold" plug apparently ran OK at WOT but was not great for throttled applications. Norvel therefore introduced the "Freedom XL" plug, which is still the current production plug and carries a part number of 110138. The Freedom plug has a body OD of .334", and there is a radius at the intersection of the sealing flange and plug body. Also, the electrode is distinguished by 2 ribs and a rounded top.
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The corresponding clamp is p/n 110131. It has an ID of .339" and has a noticeable chamfer between the ID and clamping surface, in order to clear the corresponding radius at the base of the Freedom plug.
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Engines equipped with the older "cold" plug cannot use the replacement Freeedom XL plug because that plug will not fit in the small hole in the clamp. TO convert to the Freedom XL plug, you must use the corresponding 110131 clamp. Or machine your old clamp to the proper configuration.

Another plug that was available for the pre-revlite engine used a Glo-Bee style coiled element.
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I don't even know where I got these parts from, and don't know if they are currently manufactured. The glo-bee plug had a reputation for increasing power so I would assume this type of plug is intended for hi-po applications. I have some of them but have never run them. The plug has a radiused flange, and a body diameter of .308. The electrode is skinny at the bottom, transitioning to a broad notch and a flat top.
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The clamp for this glo-bee type plug is very small and has a large chamfer at the base of the ID.
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The clamp/plug assembly has a unique appearance, with no fins.
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The Revlite version of the engine uses the same 110138 Freedon XL plug and a head clamp with p/n 110131. However the clamp has been changed to make it larger in diameter. Here is a photo of a Revlite clamp next to a pre-revlite clamp, same part number:
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Undersides look similar, with chamfer:
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It appears to me that the Revlite version of the clamp can be used on the pre-revlite engine, it just makes the engine look a little top heavy.
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THerefore if you have or obtain a pre-revlite engine that has the old "cold" plug and head, you can convert to the currently-available Freedom XL plug IF you get the 110131 head clamp which is a current service part for the Revlite engine. It's only $4.29 at Sig (if they have any).

I looked at the Tower website and their photo of the AP replacement plug looks like it has a square flange/body junction. I have read on other forums that the AP plug is not interchangeable with the Norvel. HOWEVER i have a sneaking suspicion that the AP plug config MAY be copied from the old "cold" Norvel plug- MAYBE the AP clamp and plug can interchange as an assembly. I request someone with an AP to measure their plug body OD and clamp ID and inspect the chamfer (if any) on the base of the head clamp to help answer this question.

Also keep in mind, if you need replacement plugs for your Norvel and can't find any, that Cox heads bolt right up (though they do not perform as well). Also the Galbreath head, which uses a Nelson plug, is a bolt on. The Galbreath head is a bargain at only $11 including postage (last time i checked) and Nelson plugs are only about $4.50 i think (i haven't bought one in years and i have scads). They take high rpm well and are very durable but in my experience (on larger engines) don't throttle well, though I have never run one on a throttled 1/2A. Lou has one for his Norvels and perhaps can comment on its effect, if any, on throttling.
sgilkey
Posts: 2351
Joined: Mon Mar 19, 2001 8:26 am

Post by sgilkey »

Dr. Evil sent me one of his APs for examination. As suspected, the head (an it appears virtually the entire engine) is a copy of the "old" pre-Revlite Norvel .061. This includes the head and plug, which have the same OD/ID/flange thickness/square flange/non-beveled clamp as the "old cold" Norvel plug and clamp.

This means those who have a Norvel with the old head clamp which takes the (no longer serviced?) "cold" plug can use the AP plug as a replacement- it fits the old Norvel clamp. Don't know if the AP plug will change the performance attributes of the Norvel, this was not tested.

It also means the AP can be retrofitted with the Norvel Freedom XL clamp and plug, or a Cox or Galbreath head.

The AP had a single .009 thick aluminum head shim. As with the Norvel, the shim came out stuck to the glow plug, which made it hard to see since it is the same color.
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