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Converting SOHC to DOHC

A Dual Over Head Cam (DOHC) head is a wonderful thing. This is especially true if you've been trying to make power unsuccessfully with a Single Over Head Cam (SOHC) head. The 4G63's heads have been modified to make well over 1000hp. They are quite a nice piece to work with. It is a belt driven twin cam head with four valves per cylinder. It has a great pent roof combustion chamber with good quench areas on the sides.

This page describes what is necessary to convert a SOHC engine to DOHC. This info is based on a 6 bolt 4G6x and may not apply to 7 bolt or Evo engines unless otherwise specified. We will only cover the longblock (from oil pan to timing cover). All other parts depend on the specific application. If you are doing a RWD swap we do have some pointers for the engine and its plumbing.

Bolting the Head to the Block

Any 4G6x head will bolt to any 4G6x block. The head bolt / stud locations are the same on the entire Sirius family as are the bore spacing. For 6 bolt blocks and heads, there is no need to drill any holes in the head, block or gasket. If you are putting a 1G head on a 7 bolt or Evo block, you will need to block some oil drain holes. It is possible to put an EvoIV-IX head on a 7 bolt engine, but will be difficult to do on a 6 bolt due to the lack of oil drains. Below is a picture you can use to compare the 6 bolt head, gasket and block. All holes line up.

Timing Components

Obviously, going SOHC to DOHC changes the timing belt configuration a bit. Lets start by listing what parts are different.

Name	Part Number
	4G63		4G64
	SOHC	DOHC	SOHC	DOHC
Oil Pump Cover	MD128569	MD175762	MD128569	MD175762
Oil Pump Gear Drive	MD126658	MD174581	MD126658	MD174581
Oil Pump Driven	MD125361	MD174582	MD125361	MD174582
Oil Pump Sprocket	MD122692	MD135005	MD122692	MD135005
Crank Timing Sprocket	MD074433	MD326852	MD074433	MD326852
Timing Mark Plate	MD040751	MD131241	MD040751	MD131241
Timing Belt	MD140228	MD326059	MD140229	MD182292
Idler Pulley	N/A	MD156604	N/A	MD156604
Tensioner Pulley	MD011536	MD129355	MD011536	MD129355
Hydraulic Tensioner	N/A	MD164533	N/A	MD164533
Spring Tensioner	MD041925	N/A	MD041925	N/A
Upper Timing Cover	MD041003	MD141457	MD086696	MD141457
Lower Timing Cover	MD129003	MD141454	MD129007	None (6 bolt)
Cast Timing Part	N/A	MD130032	N/A	MD130032
*Small variations may exist for parts, but overall, this part is compatible with the 6 bolt block.

As you can see on the chart, many parts are shared, but some are not. One of the few parts that is needed, but does not exist is a 2.4L DOHC 6 bolt timing cover. Everyone uses a regular 4G63 DOHC timing cover without any bolts on the top half.

Oil Pump Cover

The Oil Pump covers look similar but upon closer inspection the timing markers and tensioner mounts differ. Below is a picture of a SOHC (top) compared to a DOHC (bottom). You have to look really closely at the details, but they are different. Alternater bracket different thicknesses, some oil passages different sizes, even the lengths of the oil pump gear shafts differ between SOHC and DOHC.

Important Detail

There is also a small detail that could become very bad news if you don't pay attention to it. The SOHC block's tensioner spring is mounted on a stud coming from the block. If you retain the stud to mount the front cast timing part to, it will rub the timing belt when it is tensioned. The solution is as simple as removing the stud and replaceing it with an appropriate bolt. Here is a before and after.