Onan diesel generator sets are common on yachts, patrol craft, workboats and RVs, and many of them run for years with nothing more than oil, filters and the odd valve adjustment. When a cylinder head finally does come off, the parts that hold the valve assembly together are small, cheap and easy to lose, but the engine cannot be rebuilt without them. Two of these parts are now approved in the Sea Keepers catalogue: valve spring collet 185-5541 as SK-00022 and inlet valve guide 185-5500 as SK-00065-AV. This guide explains what each one does, which Onan sets they belong to and what to order with them.
SK-00022: valve spring collet 185-5541
SK-00022 is genuine Onan (Cummins) part 185-5541, Collet, Valve Spring. The collet sits in the groove at the top of the valve stem and locks the valve spring retainer to the stem, so the spring stays compressed and pulls the valve shut. The record lists it with valve spring retainer 185-5540 and valve spring 185-5539 in the rocker arm and valve assembly, four per engine. It weighs about 0.01 lb. The record lists it for the Onan MDKAU marine set, the HDKBA and HDKBB RV sets and the DKHA commercial mobile set. No supersession was found in the sources reviewed.
Why collets matter more than their size
A collet that is worn, cracked or not seated fully in the stem groove can let the retainer slip off the stem. When that happens the spring lets go, the valve drops into the cylinder and the piston hits it. On a small two- or three-cylinder generator engine that usually means a damaged piston, a bent valve and often a cracked head. Collets are therefore not reused after a valve job if they show any wear on the inner ridges, any polishing where they meet the retainer, or any sign that they have moved. They are always replaced as a pair on each valve, and the retainer and stem groove are checked at the same time, because a worn groove will damage new collets too.
SK-00065-AV: inlet valve guide 185-5500
SK-00065-AV is genuine Onan (Cummins) part 185-5500, Guide, Inlet Valve, for HDKBB (5.0 / 5.5 kW) and HDKBC diesel generator sets. The guide is pressed into the cylinder head and keeps the inlet valve stem central over its seat. The HDKBB parts manual lists two per cylinder head, alongside exhaust valve guide 185-5501, in both the Spec A-B and the Spec C-and-later cylinder heads. The part also carries NATO stock number 2805-01-619-8005, which helps when a defence or government buyer orders against an NSN. No supersession was found in the sources reviewed.
Signs that the valve guides are worn
Worn inlet guides let oil run down the valve stem into the intake port. On a generator set this shows up as blue smoke on start-up or at light load, oily deposits in the inlet port, higher oil consumption and carbon on the back of the inlet valve. Excess guide clearance also lets the valve rock on its seat, so the seat and valve face wear unevenly and compression falls. Guide wear is checked with the valve fitted, by measuring how far the valve head can move sideways, or by measuring the stem and the guide bore and comparing the clearance with the limit in the Onan service manual.
Replacing guides the right way
Old guides are pressed or driven out with a stepped drift, and new guides are pressed in to the installed height given in the service manual. The head should be clean, and many workshops warm the head and chill the guides so that they go in without scoring the bore. After fitting, the guide bore is checked and reamed if the manual calls for it, and the valve seat is then cut or lapped so that it is concentric with the new guide. Fitting new guides without re-cutting the seats usually leaves the valve sealing badly. Inlet and exhaust guides are different part numbers, so they must not be mixed up; on the HDKBB the exhaust guide is 185-5501.
Parts to order for an Onan valve job
For a complete valve job on one of these generator sets, the parts list normally includes the inlet and exhaust valve guides, new valves if the faces or stems are worn, valve stem seals where fitted, collets, retainers, springs if they are weak or below their free-length limit, a head gasket and the head bolt set if the manual says they are single-use. If the bottom end is being opened too, the approved catalogue also carries Cummins Onan connecting rod bushing 185-5571 (SK-00015) and the MDKBH gearcase ball 181-9023 (SK-00004). For the general method of renewing seats and guides on larger engines, see the valve seat insert and valve guide guide linked below.
What to send with an enquiry
Send the Onan model and spec letter from the generator nameplate (for example MDKAU, HDKBB or DKHA), the serial number, the part numbers you need and the quantity per engine, and whether you want the full set of guides, collets, retainers and springs or only the worn parts. Add the delivery port and any NSN or documentation the buyer needs. Sea Keepers will check the Onan numbers against the serial number and quote the complete valve kit.
For the business buyer
Confirm the intended vessel or facility, operating environment, quantity and required approval before requesting a quotation. Capacity, coverage and service interval should be matched to the risk rather than selected by headline price.
For technical experts
Verify the delivered model against the maker reference, rating, dimensions, materials, operating limits and certification scope. Record the serial or batch number and retain the manufacturer datasheet, declaration, test report and maintenance instructions in the vessel file.
Product comparison
Compare alternatives on certified performance, compatibility, installation requirements, inspection interval, spares availability and total life-cycle cost. A lower unit price is not a like-for-like comparison when approval documents or service support differ.
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